Hey Sparkies: Circuit sizing help -

[wb4rt]

I’ve seen this type question posted here several times and with various circumstances but I can’t find them using the search.

I want to run a circuit underground from the house to a small shed. It is 70’ to the shed. This is not a shop requiring lots of power. Basically 3 led lights and a couple of receptacles for charging tool batteries or a lawnmower battery. Of course once it’s there someone will want to run a saw or drill or something but not a welder.

I assume something like a 30 amp 220v breaker to a small sub panel with two 110v circuits. Is that about right? Would 10g wire be adequate or would I run a larger wire due to amp loss? Would the two circuits be 15 or 20 amp? How deep for direct bury cable?

Thank you for your advice! Any specifics on sub panel are appreciated.

[Oscar]

Would 10g wire be adequate or would I run a larger wire due to voltage loss? .

At 70' length, I'd play it safe and do 8awg, for the reason you stated regarding losses. For the receptacles, no reason why they can't be 20A each. On a 30A breaker, with appropriate wire, each leg can carry 30A into the neutral, so you can obtain up to 60A for 120V loads into the neutral if you needed to. Of course on 240V, the leg ampacities don't add up, and you're still limited to 30A @ 240V.

[Chad86tsi]

I’ve seen this type question posted here several times and with various circumstances but I can’t find them using the search.

I want to run a circuit underground from the house to a small shed. It is 70’ to the shed. This is not a shop requiring lots of power. Basically 3 led lights and a couple of receptacles for charging tool batteries or a lawnmower battery. Of course once it’s there someone will want to run a saw or drill or something but not a welder.

I assume something like a 30 amp 220v breaker to a small sub panel with two 110v circuits. Is that about right? Would 10g wire be adequate or would I run a larger wire due to amp loss? Would the two circuits be 15 or 20 amp? How deep for direct bury cable?

Thank you for your advice! Any specifics on sub panel are appreciated.

If the actual path end to end is 70', then yes, #10 will carry 30 amps. Often people find the wire path is longer than the straight line distance by a lot more than they expect. I ran a sub feeder to my pool shed and it's only 40' from the corner of the house, but the actual path to the panel was 155'.

Depth can vary by jurisdiction and a slew of other rules. consider 24" minimum, I had to put mine 30" deep (60 amp 240), and it had to be a certain distance from gas mains and the pool.

Run a conduit the whole if at all possible, it is often cheaper than direct bury wire, and you can change wire conductor size if you needs ever change. It's also better protection. I used 1-1/2" conduit, never go cheap on conduit, the added size may only be few bucks but makes install easier and you can upside conductors later. No more than 4 90* bends or a max of 360* total between pull points, this is often overlooked and it's not just a rule, it become nearly impossible to get the wire in if you pass 360* in total bends.

If you want 240 volts, you'll need 2 hots, a neutral, and a ground. A sub panel will also need a ground rod. If it's just some 120v outlets, you can skip the sub panel, ground rod and extra hot. If you use a conduit, you can change your mind later. It will need to be a GFCI breaker either way.

Will it be inspected? Some of these rules can be bent (though they really shouldn't be). It's good to know what level of code compliance a guy needs to meet in order to save some headache.

[wb4rt]

At 70' length, I'd play it safe and do 8awg, for the reason you stated regarding losses. For the receptacles, no reason why they can't be 20A each. On a 30A breaker, with appropriate wire, each leg can carry 30A into the neutral, so you can obtain up to 60A for 120V loads into the neutral if you needed to. Of course on 240V, the leg ampacities don't add up, and you're still limited to 30A @ 240V.

So Oscar, I can upsize the wire to compensate for amp loss? For instance then I could run 10-3 with ground on a 20 amp 220v GFCI breaker to a sub panel and split to two 110v circuits?

And Chad - it is 70’ house to shed, but breaker box is fairly close to the outside wall so I’m sure I would be less than 100’. I agree on conduit. And should only need two 90* bends. So the sub panel requires a ground rod, that is ok. I thought it did but thanks for confirming.

[ronsii]

So Oscar, I can upsize the wire to compensate for amp loss? For instance then I could run 10-3 with ground on a 20 amp 220v GFCI breaker to a sub panel and split to two 110v circuits?

And Chad - it is 70’ house to shed, but breaker box is fairly close to the outside wall so I’m sure I would be less than 100’. I agree on conduit. And should only need two 90* bends. So the sub panel requires a ground rod, that is ok. I thought it did but thanks for confirming.

Does your code there require a GFCI on sub panels?

[wb4rt]

I think we may be grandfathered and not have to use GFCI but I’ll confirm.

[Oscar]

So Oscar, I can upsize the wire to compensate for amp loss? For instance then I could run 10-3 with ground on a 20 amp 220v GFCI breaker to a sub panel and split to two 110v circuits?

And Chad - it is 70’ house to shed, but breaker box is fairly close to the outside wall so I’m sure I would be less than 100’. I agree on conduit. And should only need two 90* bends. So the sub panel requires a ground rod, that is ok. I thought it did but thanks for confirming.

It's not amperage loss. Look at the way I changed your quote, I purposely changed it to "voltage". You get voltage loss along the wire just from the length, but also as a function of (aka depending on) how much amperage you pull. The more amps you try to pull, the more the voltage loss compiles. You always get voltage loss no matter how much you upsize, it's just a matter of you deciding how much is acceptable for your needs/preference. There are online calculators where you can input your numbers to see anticipated voltage drop calculations.

A 20A breaker will indeed protect 10/3 w/gnd, and since the 10ga hots only need to carry 20A into 120V circuits. Whether or not you need a GFCI I would not be able to answer.

[MetalMan23]

10/3 is perfect, just do it, it'll work fine.

[danielplace]

The main feeding it would maybe not need GFCI/AFCI but the circuits feeding receptacles would probably need protection.

Say you go 30amp. Get even a 5hp 240 volt compressor and your already down to less than 15 amps on either leg for 120 volt stuff to be able to be run if compressor is on. Maybe shed is smaller and it would never be needed if you already have provisions at main garage or some thing especially.

I would take 60 amps if I was going to do a job for myself and go that far. Put in 1" PVC and pull two #4's a #6 and a #8 and small 6/12 panel and you will be happy you did.

[scsmith42]

WW member @Willie B is a licensed commercial electrician. Hopefully he will jump in with his advice.

[danielplace]

So Oscar, I can upsize the wire to compensate for amp loss? For instance then I could run 10-3 with ground on a 20 amp 220v GFCI breaker to a sub panel and split to two 110v circuits?

And Chad - it is 70’ house to shed, but breaker box is fairly close to the outside wall so I’m sure I would be less than 100’. I agree on conduit. And should only need two 90* bends. So the sub panel requires a ground rod, that is ok. I thought it did but thanks for confirming.

No 10/3 with ground. You can't pull romex in the ground/outdoors in a pipe. You would want loose wire only.

[ronsii]

No 10/3 with ground. You can't pull romex in the ground/outdoors in a pipe. You would want loose wire only.

Yeah, stick with loose wires and do it right, I know plenty of homeowners that put romex and even UF wire in conduits and it can be a pain! Especially when they are too cheap to use a decent size conduit...

[bigb]

We're talking a lawn mower shed here and we're now up to 60 amps? 30 amps at 240 is already overkill! For what the OP says will be used and the distance I would run 20 amps and call it done! If you have spare cash and want to go overkill sure use 10 awg to run those LEDs and charge those batteries. I wouldn't even be afraid to run a saw off 100 feet of 12 awg, it's done all day every day on construction sites.

[Louie1961]

If you do install a sub panel, it will need a ground rod as previously mentioned and do not bond ground to neutral at the sub panel.

[danielplace]

We're talking a lawn mower shed here and we're now up to 60 amps? 30 amps at 240 is already overkill! For what the OP says will be used and the distance I would run 20 amps and call it done! If you have spare cash and want to go overkill sure use 10 awg to run those LEDs and charge those batteries. I wouldn't even be afraid to run a saw off 100 feet of 12 awg, it's done all day every day on construction sites.

If your going to do it right might as well do it RIGHT !!! If putting breaker in panel and subpanel anyhow.

If your going to bury a pipe and run it to the main panel and install a sub panel and take two legs of 30 my point was why go through all that and only take 30. For a slight bit more you might as well go 60.
Breaker cost the same and so does the panel. It would ONLY be the price of the wire and one size bigger PVC.


Yea if just lights and a convenience outlet you could just drill out the back of a receptacle box on the house and LB outside into the ground and hit a box with a switch and outlet in it and call it done.

[William McCormick]

It's not amperage loss. Look at the way I changed your quote, I purposely changed it to "voltage". You get voltage loss along the wire just from the length, but also as a function of (aka depending on) how much amperage you pull. The more amps you try to pull, the more the voltage loss compiles. You always get voltage loss no matter how much you upsize, it's just a matter of you deciding how much is acceptable for your needs/preference. There are online calculators where you can input your numbers to see anticipated voltage drop calculations.

A 20A breaker will indeed protect 10/3 w/gnd, and since the 10ga hots only need to carry 20A into 120V circuits. Whether or not you need a GFCI I would not be able to answer.

You get voltage drop due to resistance, and amperage, the more amperage the more voltage drop. You cannot actually calculate voltage drop without knowing all the resistance and the amperage.

it is interesting to note that if you run a twelve-foot piece of solid number 10AWG wire to an induction device like a wall air conditioner that requires a 15 amp breaker or similar protection, on a 15 amp breaker it will pop the breaker, by actually having this problem presented to me and checking it out. We could put it on a 30-foot 14AWG extension cord and it worked, haha.

I knew what it was from having similar problems. We had the same problem using a very large 3/0 wire to power a welder from a 100 amp breaker. The welder that drew under 70 amps and was fed by a 70 amp breaker was popping a one hundred amp breaker, same distance same welder same wire just smaller breaker. We had removed a 125 amp breaker box that served as the protection for one half of the building. Until we got a new service for that half we were going to run off a one hundred amp breaker right from the main panel and then just remove the run of 3/0 wire to the other side of the building after the new service was put in. The welder was drawing 145 amps when it normally drew around 50 amps, and kept popping the breaker. It is pretty wild stuff.



Sincerely,

William McCormick

[danielplace]

You get voltage drop due to resistance, and amperage, the more amperage the more voltage drop. You cannot actually calculate voltage drop without knowing all the resistance and the amperage.

it is interesting to note that if you run a twelve-foot piece of solid number 10AWG wire to an induction device like a wall air conditioner that requires a 15 amp breaker or similar protection, on a 15 amp breaker it will pop the breaker, by actually having this problem presented to me and checking it out. We could put it on a 30-foot 14AWG extension cord and it worked, haha.

I knew what it was from having similar problems. We had the same problem using a very large 3/0 wire to power a welder from a 100 amp breaker. The welder that drew under 70 amps and was fed by a 70 amp breaker was popping a one hundred amp breaker, same distance same welder same wire just smaller breaker. We had removed a 125 amp breaker box that served as the protection for one half of the building. Until we got a new service for that half we were going to run off a one hundred amp breaker right from the main panel and then just remove the run of 3/0 wire to the other side of the building after the new service was put in. The welder was drawing 145 amps when it normally drew around 50 amps, and kept popping the breaker. It is pretty wild stuff.
Sincerely,

William McCormick

William,
I see you using this example often and you have some things a little screwy.

You never downsize the wiring to make something not trip the breaker.
The only reason it doesn't trip on the smaller wire it because of the resistance in the wire and the breaker doesn't get the hit from the load coming on so it doesn't trip. It never means that this is the way to fix the problem. You fix the problem by using the proper size wire along with the proper overcurrent protection.

I know you were just using an example with the extension cords but in the past you posted you fixed a AC condenser that was tripping breaker by pulling out #10's and using #12's instead.

Been doing this for 43 years and I have never downsized a wire or had a single person suggest or talk of downsizing a wire to allow the overcurrent device to hold. It is crazy to talk about this wire downsize and suggest it be a proper fix for a tripping breaker. You scare me every time you post that ridiculous thought.

Yes of course it may allow something to start without tripping the breaker but you would never utilize that with the wiring as a fix. You leave the wire alone or upsize it and get the proper overcurrent protection it needs to start dependably.

You don't intentionally downsize the wire to reduce the hit the breaker takes on startup. You put big enough wire and overcurrent to hold or use the proper time delay fuses in a disconnect at the equipment.

Yea sure. Hey man I know what is wrong. The wire is too big. NO you will never hear someone say that. Yea keep downsizing until it holds. Nope that isn't usually the fix.

I know I mentioned it before and you told me my problem was I didn't understand capacitance.

I understand perfectly how a smaller wire will surely reduce what the breaker gets hit with at the surge of startup.

Merrily the resistance of the wire is actually why and the same exact thing as causes voltage drop. It isn't a normal practice to troubleshoot a tripping breaker condition and fix it by reducing the size of the wire between the overcurrent protection and the load. You talk about it like it is the first thing you might consider is that wire might be too big. Surely you jest.

Reducing the wire size from #10 to #12 also means to be legal and proper then when the #10's came out then the #12's went in the over current device would have needed to be reduced from a 30 amp to a 20 amp.

So now we have a 20 amp feeding the unit with #12's and that is going to be better than a 30 amp feeding it with #10's. I don't think so. Others variations would not be proving anything if the wire size wasn't match to the over current device or better to start with.

[Chad86tsi]

And Chad - it is 70’ house to shed, but breaker box is fairly close to the outside wall so I’m sure I would be less than 100’. I agree on conduit. And should only need two 90* bends. So the sub panel requires a ground rod, that is ok. I thought it did but thanks for confirming.

Yah, at this distance I'd run #10's if all I wanted was 2@ 20A - 120V. You breaker the 2 circuits at 20 amps with #12 feeding the outlets, or 15 amps on #14. It will likely never sag enough to notice.

The new ground rod will still need to be tied to the new panel with a #6 (physically protected) or #4 (unprotected). The tie between panels can be #12 or #10. The ground ties can be de-rated, there is a table in the code book that tells you by how much.

You can use GFI outlets, don't have to be GFCI'd in the panel. Do what works best for your situation. Being a shed, I'm sure it will be treated like a wet environment, or the fact that the outlets are close to a door where it's likely an extension cord will be taken outside of it. Garages have this rule, I'd expect a shed to be treated like that. Inspectors don't all read the book the same way, and legally they can add whatever rules they want, they just cant' take any away.

You get voltage drop due to resistance, and amperage, the more amperage the more voltage drop. You cannot actually calculate voltage drop without knowing all the resistance and the amperage.

Yep, Current is equal in all series devices, voltage drop differs as load changes. The wire is one of the loads. You certainly don't want your wire to be one of the bigger loads on the system, and it changes at a rapid rate realitive to current flow.

I find it's easier for non-electrically trained people to view voltage like water pressure in a pipe. It's the potential for work, but is dictated by how much work is taking place and how big the pipe is. Everyone knows with small pipes, the pressure drops for everyone on that pipe when someone else is using it. Smaller pipes increase that effect.

it is interesting to note that if you run a twelve-foot piece of solid number 10AWG wire to an induction device like a wall air conditioner that requires a 15 amp breaker or similar protection, on a 15 amp breaker it will pop the breaker, by actually having this problem presented to me and checking it out. We could put it on a 30-foot 14AWG extension cord and it worked, haha.

Inrush is an important spec to check out when you have access to such info. Starving a device for current flow with undersized wire can mitigate it for sure, but at the end of the day the breaker has to match the wire so if you down size wire, so too the breakers need down sized. If the load can't be carried by that setup, you have to upsize both. The code cares little about the load from a safety perspective, only protecting the wire from overheating. Wire size determines breaker. Load determines wire size. This is where generous sized conduit pays, you can re-conductor a circuit in a conduit pretty easily.

[Oscar]

William,
I see you using this example often and you have some things a little screwy.

You never downsize the wiring to make something not trip the breaker.
The only reason it doesn't trip on the smaller wire it because of the resistance in the wire and the breaker doesn't get the hit from the load coming on so it doesn't trip. It never means that this is the way to fix the problem. You fix the problem by using the proper size wire along with the proper overcurrent protection.

I know you were just using an example with the extension cords but in the past you posted you fixed a AC condenser that was tripping breaker by pulling out #10's and using #12's instead.

Been doing this for 43 years and I have never downsized a wire or had a single person suggest or talk of downsizing a wire to allow the overcurrent device to hold. It is crazy to talk about this wire downsize and suggest it be a proper fix for a tripping breaker. You scare me every time you post that ridiculous thought.

Yes of course it may allow something to start without tripping the breaker but you would never utilize that with the wiring as a fix. You leave the wire alone or upsize it and get the proper overcurrent protection it needs to start dependably.

You don't intentionally downsize the wire to reduce the hit the breaker takes on startup. You put big enough wire and overcurrent to hold or use the proper time delay fuses in a disconnect at the equipment.

Yea sure. Hey man I know what is wrong. The wire is too big. NO you will never hear someone say that. Yea keep downsizing until it holds. Nope that isn't usually the fix.

I know I mentioned it before and you told me my problem was I didn't understand capacitance.

I understand perfectly how a smaller wire will surely reduce what the breaker gets hit with at the surge of startup.

Merrily the resistance of the wire is actually why and the same exact thing as causes voltage drop. It isn't a normal practice to troubleshoot a tripping breaker condition and fix it by reducing the size of the wire between the overcurrent protection and the load. You talk about it like it is the first thing you might consider is that wire might be too big. Surely you jest.

Reducing the wire size from #10 to #12 also means to be legal and proper then when the #10's came out then the #12's went in the over current device would have needed to be reduced from a 30 amp to a 20 amp.

So now we have a 20 amp feeding the unit with #12's and that is going to be better than a 30 amp feeding it with #10's. I don't think so. Others variations would not be proving anything if the wire size wasn't match to the over current device or better to start with.

Excellent points/advice! Don't know if you saw my reply on the other thread about the phase converters, but hopefully you can advise on my RPC when it gets here. I should start my own thread for it.

[William McCormick]

William,
I see you using this example often and you have some things a little screwy.

You never downsize the wiring to make something not trip the breaker.
The only reason it doesn't trip on the smaller wire it because of the resistance in the wire and the breaker doesn't get the hit from the load coming on so it doesn't trip. It never means that this is the way to fix the problem. You fix the problem by using the proper size wire along with the proper overcurrent protection.

I know you were just using an example with the extension cords but in the past you posted you fixed a AC condenser that was tripping breaker by pulling out #10's and using #12's instead.

Been doing this for 43 years and I have never downsized a wire or had a single person suggest or talk of downsizing a wire to allow the overcurrent device to hold. It is crazy to talk about this wire downsize and suggest it be a proper fix for a tripping breaker. You scare me every time you post that ridiculous thought.

Yes of course it may allow something to start without tripping the breaker but you would never utilize that with the wiring as a fix. You leave the wire alone or upsize it and get the proper overcurrent protection it needs to start dependably.

You don't intentionally downsize the wire to reduce the hit the breaker takes on startup. You put big enough wire and overcurrent to hold or use the proper time delay fuses in a disconnect at the equipment.

Yea sure. Hey man I know what is wrong. The wire is too big. NO you will never hear someone say that. Yea keep downsizing until it holds. Nope that isn't usually the fix.

I know I mentioned it before and you told me my problem was I didn't understand capacitance.

I understand perfectly how a smaller wire will surely reduce what the breaker gets hit with at the surge of startup.

Merrily the resistance of the wire is actually why and the same exact thing as causes voltage drop. It isn't a normal practice to troubleshoot a tripping breaker condition and fix it by reducing the size of the wire between the overcurrent protection and the load. You talk about it like it is the first thing you might consider is that wire might be too big. Surely you jest.

Reducing the wire size from #10 to #12 also means to be legal and proper then when the #10's came out then the #12's went in the over current device would have needed to be reduced from a 30 amp to a 20 amp.

So now we have a 20 amp feeding the unit with #12's and that is going to be better than a 30 amp feeding it with #10's. I don't think so. Others variations would not be proving anything if the wire size wasn't match to the over current device or better to start with.

Before you go on you should do some checking. A circuit breaker limits inrush current according to its size, a wire is a capacitor and gets charged before the device it is feeding. So when you have a large AWG wire it is also a capacitor, and it also allows the induction device to draw a significant amount of starting amps that a smaller wire will not allow. This action blows the smaller breaker, the same breaker that does function with a smaller proper AWG wire and no increased amperage draw. I am not making this up and these are not the only times I have seen this. Of course, too small a wire is no good as well it causes the same thing more amps drawn from induction devices. Most just never have gone both ways, haha, because most are cheap or never had the larger wire to test it on. But you could hook up a self inducting heating element to that 3/0 wire even though they draw high current on start and it will not take out the breaker like an induction device will.

Secondly, I have never stated that I fixed an AC condenser by putting in a number twelve wire. I am always right on or one size up on AWG. The wall unit air conditioner in question was a new modern unit and had a maximum overcurrent protection rating of 15 amps, the old unit was a 220 unit with number ten solid wire run. When we connected that two a 15amp breaker it took out the breaker. Yet there was nothing wrong with anything involved, it was just missized. I have run 10 AWG long-distance from 20 amp breakers with no problem. But for a short distance, on solid 10AWG wire on a 15 amp breaker, it takes out the breaker.

Too small a wire run to an induction device raises the amperage it draws by lowering the voltage. If you do not know these things do some experimenting, I have, and lots of it.

Sincerely,

William McCormick

[Oscar]

a wire is a capacitor and gets charged before the device it is feeding. So when you have a large AWG wire it is also a capacitor,

Can you show the equation that derives the capacitance based on cross sectional area, and do the math with units? I'd be very interested in seeing this.

[Willie B]

I didn't notice this thread until everything that could be said was said. LED lights, battery chargers will work in this case with #14 underground. The impossible prediction of what will I want in a few years from now comes into play. No such thing as too big, it only costs more. How far do you want to go?

If it were mine, I'd go 3/4" PVC with either #12 THWN, or #10 THWN 4 conductors: Green, White, Black & Red. I'd protect #12 at 15 amps, #10 at 20. This gives you 3600 Watts, or 4800 watts of functional power with acceptable voltage. I'm not good at predicting my future, less informed as to your future needs. I think #8 would fit later.

Remember, it is irrelevant how long your feeder is. Voltage loss is cumulative beginning at the utility transformer. if you have already lost 12 volts getting to the house, additional loss may become unworkable. Conversely, if you already have 235 at the house under heavy load you'll tolerate some more loss to the shed.

[William McCormick]

Can you show the equation that derives the capacitance based on cross sectional area, and do the math with units? I'd be very interested in seeing this.

That is hard to say exactly as the actual calculation would be the area in contact with insulation, and perhaps what the insulation is up against. It would be hard to say how much of each wire is up against the steel conduit, up against the neutral, up against the other two hot legs and up against the air. But there is capacitance, just hook up a length of isolated 4/0 wire to the bus bar in a dark area, and watch it spark as you connect or disconnect. Also if you touch both you can feel it nicely.

I do a lot of fan and compressor motors some with start relays, and I know if you use the wrong capacitor you can get some funky effects, the best size is the right size.

Sincerely,

William McCormick

[danielplace]

Before you go on you should do some checking. A circuit breaker limits inrush current according to its size, a wire is a capacitor and gets charged before the device it is feeding. So when you have a large AWG wire it is also a capacitor, and it also allows the induction device to draw a significant amount of starting amps that a smaller wire will not allow. This action blows the smaller breaker, the same breaker that does function with a smaller proper AWG wire and no increased amperage draw. I am not making this up and these are not the only times I have seen this. Of course, too small a wire is no good as well it causes the same thing more amps drawn from induction devices. Most just never have gone both ways, haha, because most are cheap or never had the larger wire to test it on. But you could hook up a self inducting heating element to that 3/0 wire even though they draw high current on start and it will not take out the breaker like an induction device will.

Secondly, I have never stated that I fixed an AC condenser by putting in a number twelve wire. I am always right on or one size up on AWG. The wall unit air conditioner in question was a new modern unit and had a maximum overcurrent protection rating of 15 amps, the old unit was a 220 unit with number ten solid wire run. When we connected that two a 15amp breaker it took out the breaker. Yet there was nothing wrong with anything involved, it was just missized. I have run 10 AWG long-distance from 20 amp breakers with no problem. But for a short distance, on solid 10AWG wire on a 15 amp breaker, it takes out the breaker.

Too small a wire run to an induction device raises the amperage it draws by lowering the voltage. If you do not know these things do some experimenting, I have, and lots of it.

Sincerely,

William McCormick

Did you try a new 15 amp breaker ?

So if there was 20 amp feeding it that worked perfectly then you installed the 15(max overcurrent ??) only to have it trip.

Your next step would be

pull out #10's and replace with number 12's so you could use a 15 amp MAYBE ?

or

put the 20 amp back on the #10's ?



Yes I realize what you are saying is true about the smaller wire fixing that but it was possibly never really the problem. It was the fast acting breaker kicking it out. But irrelevant if the #12 was plenty fine for load and with only a 15 amp breaker I will give you that it was a fix. A situation such as this is 99.9 % of the time fixed by another method. More than likely the breaker was actually defective. Too fast acting or out of spec.

So it had 10's so it wasn't suffering voltage drop and you say 15 was max and it had a 15 and it was tripping.
Problem. Yes.
Needed the #10's pulled out and smaller wire run instead. Unlikely is all I am saying.

What did a amprobe say it was pulling at startup and running ? Did you try a new breaker ?


If it is sized at max allowed ampacity breaker you should have had PLENTY of amps to start no matter if it had #6's from the breaker to the A/C unit. If it is taking more than the max over current allowed then there is a equipment or voltage issue or they put the wrong overcurrent on the nameplate.
Never realized a wall a/c has a overcurrent max especially at 15 amp.

Sounds like about a 24,500 They draw about 12 amps running but should start with a 15 amp.

The max over should be enough that is required to start that it should not have been happening. Rather than pull out the wire and since the 15 was just too fast acting and tripping you should have just put it on a 20 amp. Wall units are cord and plug connected usually. The overcurrent protection on the wire feeding one is not really where they normally get there protection from. Nice to have the least it requires for better protection but a good 15 amp or if it even took a 20 so be it. Just for start up and wire size is more than there and you know the load. No danger.

I am guessing something more was going on and not working 100%. Like the breaker just wore out from supplying power to the previous unit for last 25 years.


Of course if the volts go down the amps go up and if if are going up the volts are going down some more. Lol.
Long cord going to pull more amps with same load. It is lost to heat from too small of a wire. It is what expected voltage loss is based on and why we must tolerate some amoubt of losses.

Must have needed to be CODE compliant. Lol.

Just not normally how tripping breakers are rectified by reducing wire size is my point. I do understand it allowed you situation to work but similar situation few and FAR between. Betting it just needed a GOOD breaker.

They are +- 10% often so 10% under when you need it all can be a problem I suppose. Usually when a breaker trips that fast during startup it is actually a defective one.

And yes I think you are going to need the right start or run capacitor on anything you work on or they would only sell one. Just changed out the main board in a big Miller engine drive and it had some big ole' cans on that sucker. Had to take them off the old board and bolt on the new one. You would not wanna get bit by one that size. It might leave a mark. Lol.

Yes wire has capacitance.

It is like the magnetic field that is produced when you energize a circuit and hear the wire slap around inside the EMT.

[William McCormick]

To calculate voltage drop at the maximum current draw of 20 amps, you can take the voltage 120 volts and divide it by the amps 20, to get 6 ohms, that is what any device drawing 20 amps will create in resistance through the device.

So for 8 AWG wire that has a resistance of 0.6282 ohms per thousand feet, you would need to find how many ohms there are in the length of wire you wish to run. The original poster said 70 feet. I would assume 100 feet minimum, and I would probably buy 120 feet; you can eat up five feet in the panel box, not to mention the ups and downs. So let us say 100 feet. So you divide 100/1000 = 0.1 or one-tenth of 0.6282 ohms or 0.06282; you move the decimal over one place, making it ten times smaller, or multiply 0.01 times 0.6282 on a calculator and you get the same thing. But if it were 120-feet, you would do the same thing 120 feet divided by 1000 feet to get 0.12 and then multiply 0.12 times 0.6282 to get 0.075384 ohms. Now you can use the voltage drop formula. To get the top of the voltage drop formula, we multiply 0.075384 the ohms in the wire times 6, the number of ohms in the device drawing 20 amps, and we get 0.452304 Then for the bottom of the formula we add 0.075384 to 6 and get 6.075384 then we divide 0.452304 by 6.075384 to get 0.074448627444784 ohms then we multiply that by the 20 amps. We get 1.488972548895675 volts, voltage drop. But that is at a full 20 amps.

If you run a number twelve AWG wire instead, the voltage drop would be 3.0941 volts. But and this is important if we are using a power tool a circular saw or table saw and turning it on and off, you will notice the lights dim because as the saw starts, the amps are much higher than 20 amps. That is the only problem a lot of redundant cutting starting and stoping of induction devices.

But for your lights a bug zapper, a little heater you would be fine. Ideally, a number 10 AWG wire on a twenty amp breaker in the main panel, and use 20 amp rated receptacles and switches. Because what happens is that you end up plugging in a 75 foot 14 AWG extension cord and then the 15 amp breaker just doesn't make it. Going way, way back I used to build homes and dormers, and I just started bringing my own generator because of problems just like this. I never had another issue.

But if you are going with a panel in the shed then go with eights and put in whatever breakers you want, you could always throw in a little Diakan mini-split heat pump and Air conditioning unit, they are 220 volts, that would probably be the next step up. But get the schedule 80 PVC and get it large so it is an easy pull. I would get 1" PVC. Use a shop vac to pull a small plastic bag and attached string through the pipe. Or you can snake it if you have a snake that long. Use LB's, going into the house and shed.


Sincerely,

William McCormick