Sizing circuit breaker??

[chingon]

For logistical reasons I'm running a welder rated at 18 amps max load on a 30 (maybe 50) amp outlet. Obviously I'm not worried about the 30 amp breaker setting off, but am worried about inrush current damaging my welder.

The question is should I install a breaker that works at 18 amps or one that trips at 18 amps?

I understand that breakers are designed to carry max operating current for a period of an hour or 2 before tripping, so I suspect it will be safe as long as I don't weld for 2 hours continuously (duty cycle will take care of that) at max output.

I don't want to trip every time I'm trying to weld at max, but should I worry that the 18 amp breaker trips @ 24 A (maybe damaging the welder in case of inrush)?

[DSW]

The welder will only draw 18 amps, so the breaker will never come into the equasion unless there is an issue ( bad breaker or bad machine). Running a machine with a smaller draw on a larger breaker will not hurt, as long as the breaker is propperly sized to protect the wire in the walls.

I run my 26 amp draw mig on a 50 amp breaker all the time with no issues. Infact, it comes with a 50 amp plug on the machine from the factory.

[chingon]

maybe I should've been more specific. The idea is to run the breaker at the machine to protect it from inrush/overcurrent the same way that many devices have a fuse on their chassis.

[Rick V]

I may be wrong but...

While the purpose of the breaker is to protect the wire feeding the device, the breaker appears to be sized according to the maximum amps drawn by the device and can be 100% to 200% of that plated rating. e.g. Welder plate says max draw is 50 amps, then breaker must be a minimum of 50 amps to a maximum of 100 amps.

I know that many folks do run 50 amp input welders off of 30 amp breaker and 30 amp-rated wire (#10). While welder duty cycle can make the small wire acceptable, it appears that the breaker should still be 50 amps... but then that doesn't protect the 30 amp rated wire. So, I'm not too clear on this point myself. Maybe the difference comes from a dedicated breaker/wire to service just one device (like a welder - that has it's own code rules) versus a regular circuit that may have several different and changing devices/loads on it

I thought the electrical codes said...
Wire size for welders may be reduced from the maximum amps drawn by the device according to duty cycle. e.g. Welder plate says max draw is 50 amps and max output is 225 amps at 20% duty cycle. For a 20% duty cycle, the reduction factor is 0.45 and the ampacity of the wire may be reduced from 50 amps (#8 wire) to 0.45 x 50 amps = 22.5 amps (#10 wire).

[tiny]

If I'm not mistaken the inrush/overcurrent you're talking about wont happen. Your welder will not receive 50 amps of current just because it is on a 50 amp circuit. It will only draw the amperage that it needs and will have up to 50 amps available to it, but not forced upon it. So running a welder that draws 18 amps on a 50 amp circuit will do no damage to the welder, if I understand what you're asking for here.

[welds4d]

Just curious, chingon, but how did you come to be concerned about protecting your machine from this "inrush current"?

[chingon]

Originally Posted by welds4d

Just curious, chingon, but how did you come to be concerned about protecting your machine from this "inrush current"?

Newer machines installed a switch/breaker as an on/off switch. It just seemed like a good idea since the outlet i'm drawing off has more current capacity.

Originally Posted by tiny

If I'm not mistaken the inrush/overcurrent you're talking about wont happen. Your welder will not receive 50 amps of current just because it is on a 50 amp circuit. It will only draw the amperage that it needs and will have up to 50 amps available to it, but not forced upon it. So running a welder that draws 18 amps on a 50 amp circuit will do no damage to the welder, if I understand what you're asking for here.

http://en.wikipedia.org/wiki/Inrush_current

then again maybe like the article says the welder may need to see some inrush current on start up.

I think it would also take care of some pesky sparking every time I plug the thing on.

[KGIOR]

Maybe some of the electricians can respond to this thread. I had this very same conversation with a young ( new ) electrician who was telling me that a larger breaker would SUPPLY to many amps to the equipment that I was plugging into it. I was arguing that the breaker acts a limit to the equipment draw. After a few minutes I gave up and told him just to install it, but he was convinced that it was risky. I believe that his desire to install a closer rated breaker is probably a good idea, but I wanted the flexibility of being able to plug in a variety of equipment that had amp draws varying from 18-60 amps (all were 240 volts of course). I have used it now for about 5 years without any issues.

I would like to know if a breaker can limit the supply of power of some kind of surge but I don't think so, that's what surge protectors or power conditioners do.

Ken

[tiny]

Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on.

Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.

I would expect the welder to be designed for this, consider the fact that it is very common to see old buzz boxes that have been in use 50+ years and are functioning fine. I'm no electrician, not even an internet electrician, so don't take my word for it.

You could also consider the reverse of your thinking. Having a circuit that doesn't properly allow for the inrush might defeat the original engineered design of the welder causing it some harm of some sort. lol, I dunno and with my old beat up welder that continues to work I'm not worried about it.

[stiffman]

I'm no electrician, but this is how I understand circuit breakers
its pretty much been said, but a circuit breaker does just that, It breaks the circuit if there is too many amps being drawn. Its a safety device to protect the installed wiring in the building.

Equipment with a circuit breaker installed internally would have it there, not for inrush protection, but as an easily resetable fuse. In the event of a fault with the device (welder), the circuit trips (or the fuse blows)

[bigb]

Inrush is what is drawn at initial startup such as a motor, at startup it is not yet moving and is very nearly a short circuit for a split second. The load (motor) will typically draw around six times it's running current during this momentary inrush period. Circuit breakers and time delay fuses will allow this inrush current, if they didn't you'd have a heck of a time starting most motor loads. Even a light bulb has inrush, a cold tungsten filament has a very low resistance. Low resistance=high current flow.

So if you try to limit this inrush current with an overcurrent protective device (breaker, fuse) you will never start the load.

You are not going to hurt the machine, as said before it will only draw what it needs, nothing will be "forced" into it. This is Ohm's law and unlike man made laws Ohm's law cannot be cheated.

If you are concerned about electronic circuit board damage install a good quality surge protector, like a whole house surge protector, not a cheapie power strip. Many cheap surge protectors use MOV's which simply shunt the surge to the grounding conductor where it can still harm components that are grounded.

Hope this helps

[Sandy]

I some how get the impression this welder is lacking an onboard overcurrent device? If that is the case, this inrush isn't what those that have built in circuit breakers/fuses are guarding against. They are guarding against actual fault conditions such as an internal short circuit and ensuing overcurrent situations.

That is a valid concern I suppose and probably the reason most have added something of this nature. If you have a small ampacity device hooked to a (let's say) 50 amp breaker there could be some problems there under certain conditions.

[assassin_works]

chingon

everyone is right or partly right a breaker is only a resettable fuse that will trip if the the load that is put on it is greater that the wiring can handle or rated .... the inrush current is more for like motors , compressors , refrigerators that has a start cap and or a run cap .... so lets say if you got an air compressor and the it uses for example 60 amp @ 220 vac 8000 watts to start but as the motor builds speed and stabilizes the rpm to proper running speed the watts dropped to lets say 6000 with 35 amp @ 220vac so if you had a breaker smaller than 60 it would trip it every time it tries turning on doesn't mean that the equipment is faulty just that the load is greater than the breaker or wiring but if you want to put a breaker or fuse for your machine try one of these .they run around $ 6 and up depending on the brand
http://tncscooters.com/product.php?sku=101260

[chingon]

Originally Posted by Sandy

I some how get the impression this welder is lacking an onboard overcurrent device? If that is the case, this inrush isn't what those that have built in circuit breakers/fuses are guarding against. They are guarding against actual fault conditions such as an internal short circuit and ensuing overcurrent situations.

That is a valid concern I suppose and probably the reason most have added something of this nature. If you have a small ampacity device hooked to a (let's say) 50 amp breaker there could be some problems there under certain conditions.

would the device be in the shape of breaker/fuse? If so, I didn't see one. It's an inverter tig welder.

[Sandy]

Originally Posted by chingon

would the device be in the shape of breaker/fuse? If so, I didn't see one. It's an inverter tig welder.

It could look like a typical breaker, or even some fuse mounted inside or on a board inside but more commonly it's something like the link assassin works posted. Take a look at his link. If you don't have a built in over current device and/or are worried about it that link shows you what you're looking for. Low cost easy install. Easy visual to see if it's been tripped or not.

On another note someone may have experienced a voltage spike (surge) and simply called it inrush because that sounds like a cool name. Surges are a whole different deal. Keep it unplugged while you're not using it if it is a real concern.

[Broccoli1]

Originally Posted by chingon

would the device be in the shape of breaker/fuse? If so, I didn't see one. It's an inverter tig welder.

That IS a Circuit Breaker- a Push To Reset style of circuit breaker, just different than the ones used in the wall Panel.

I understand your concern but as others have mentioned you don't really need to worry about it- you're fine using the machine on the Circuit you have.

[Joker11]

This question has been asked many times. It is one of the recurring themes here.

Sometimes electrical theory is hard to explain to people. I work on single phase stuff all the time. Put me into 3 phase and I am lost. I figure I just shouldn't be messing with 3 phase. And that's what I would say here. if you don't understand circuit breakers, and there are many types, slow, fast, adjustable, and now they have some that are GFCI as well, maybe you shouldn't be messing with it.

[chingon]

Originally Posted by Sandy

It could look like a typical breaker, or even some fuse mounted inside or on a board inside but more commonly it's something like the link assassin works posted. Take a look at his link. If you don't have a built in over current device and/or are worried about it that link shows you what you're looking for. Low cost easy install. Easy visual to see if it's been tripped or not.

On another note someone may have experienced a voltage spike (surge) and simply called it inrush because that sounds like a cool name. Surges are a whole different deal. Keep it unplugged while you're not using it if it is a real concern.

I was leaning more towards this one:
http://www.ebay.com/itm/18-AMP-Circu...#ht_2461wt_940

replace the switch while at it.

[76GMC1500]

You size the breaker to protect the wiring. The gauge determines the breaker. You run time-delay fuses at the disconnect for the welder to protect the welder. Those are sized to the transformer's rating. Of course, 3 time delay fuses, a disconnect switch, and a box can easily cost $1000 so you have to decide if protecting your welder is really worth that much money.

[Sandy]

Originally Posted by chingon

I was leaning more towards this one:
http://www.ebay.com/itm/18-AMP-Circu...#ht_2461wt_940

replace the switch while at it.

Well I'll get accused of making quick judgements based on a internet photo, but that doesn't look like something I'd want on a welder. Doesn't look very rugged to me. Just doesn't fit my mental picture of an equipment device.

[chingon]

Originally Posted by Sandy

Well I'll get accused of making quick judgements based on a internet photo, but that doesn't look like something I'd want on a welder. Doesn't look very rugged to me. Just doesn't fit my mental picture of an equipment device.

you should see the stock one....

[roughneck266]

Inrush probably will never be a problem for your welder. It isnt for any small draw machines that i have come across. We had an issue on my rig with it after we installed Variable frequency drives for our thrusters. Mind you these machines were connected to 11KV step down transformers which dropped the voltage down to 690. LOTS of windings. Out solution was premag transformers. They keep a current running through the windings and therefore eliminate the inrush. We also had to adjust the settings on our breakers that were supplying the 11KV to the circuits.
As has been mentioned already here. The breaker size is determined by the gauge of the wire.

[76GMC1500]

Also know that many breakers are a combination thermal and magnetic trip. A 30 amp breaker will not trip at 30 amps. At any amperage above, you start heating the thermal trip element. The greater the current is in excess of 30 amps, the faster the thermal trip will occur. There are standards but I can't remember any off the top of my head but think something along the lines of a 10% overload will cause a trip within 30 seconds. The other trip mechanism is the magnetic trip. Current in the breaker flows through a coil which generates a magnetic field If the current is sufficiently high enough, the magnetic field will be strong enough to pull the contacts open. This is the instantaneous trip that opens a breaker in the event of a dead short. Again, I don't remember any numbers, there are standards, but think something like a 30 amp breaker will trip instantaneously at 150 amps.

A good rule of thumb for electricians is to double the current capacity of the wiring for the expected load, size the breaker to protect the wire, and run fuses matched to the load locally at the load controller.

[Broccoli1]

all this for a Harbor Freight welder

[Joker11]

Good point Broccoli1!