NEMA 6-50 Question

[morgaj1]

How is it that welders that draw more than 50 amps still use a NEMA 6-50 plug/receptacle? I thought the 6-50 was rated at 50 amps?

[Stick-man]

What I've been told, was that a 50A plug will handle more. But because of the chance of it arcing, if you were to plug/unplug it while under a greater load, that is the limiting factor.

My Shopmaster has a rating of 91A input @ rated output. I have used 50A plugs on a 50' 6-3 extension cord, running .035 ER70S-6 wire, without a problem. I always make sure the machine is off, before plug/unplugging it.

[aandy767]

OK, I was a hammerhead, my buddy told me to hard wire my syncrowave to the breaker box, but I wanted to be able to switch welders and plasma cutter.

I put my hand on the plug the first several time I used it to see if it was hot. It was warm, but I was not concerned. Well after a couple of months the heat caught up. running AC 280 amp out put on AL

I have since added a breaker and hard wired it.

[aandy767]

that was a 50 amp plug

[Rick V]

Very illustrative picture there aandy767!
Looks rather 'cooked' to me!
Great Picture!

[Rick V]

How is it that welders that draw more than 50 amps still use a NEMA 6-50 plug/receptacle? I thought the 6-50 was rated at 50 amps?
This is really a Good Question morgaj1!

Yep the 6-50 is rated at 50 amps. However... as aandy767 has pointed out, if your welder is plated for more power, say 76 amps, then you got to weigh in a few more factors - like convenience and the electrical code.

Convenience
1 - It's great to be able to switch welders; unplug one and plug in the new one - 6-50 plugs.
2 - There is no other plug/receptable available; no standard NEMA 70 amp or 100 amp plug/receptacle! So you have to hard wire the welder to a breaker box. That is not very convenient, so look to the electrical code for some relief...

Electrical Code
1 - Breaker size for a welder must be rated at 1 to a max of 2x the plated amperage of the welder. Suppose your welder power plate says 76 amps max input @240 volts for 250 amps DC output at 30 arc volts @ 20% duty cycle. Breaker size in not affected by duty cycle so in this example the breaker must be a minimum of 76 amps to a max of 152 amps. What is conveniently available? Perhaps a 100 amp breaker.

2 - Wire size for a welder may be downsized according to the duty cycle.
Duty Cycle - Downsize Factor Multiplier
20%..............0.45
30%..............0.55
40%..............0.63
60%..............0.78
100%............1.0.
Our example welder had a duty cycle of 20% when drawing 76 amps input. So the Downsize Factor Multiplier is 0.45. So, instead of needing electrical wire/cable to handle 76 amps (#3 AWG gauge), we need to handle only 76 x 0.45 = 34.2 amps continuous... more like #8 AWG gauge.

However, and I could use some help here, the code does not appear to be clear on the plug/receptacle... is the plug/receptacle treated like a breaker or like a wire? Seems to be more like the wire to me.
If so, then in our example welder, we could use a 6-50 plug/receptacle since the most current we will push through it would be 76 amps at 20@ duty cycle or about 34 amps contniuous.

So morgaj1, that's why so many folks use the 6-50 plug/receptacle!
- There is nothing else available, and
- It can be used at over 50 amps for reduced duty cycles - often the case.

Where trouble arises is when (like with aandy676) the max-amperage draw X duty-cycle-reduction factor is considerably greater than 50 amps and ya continue to try to use the 50 amp rated plug.

Got to use a little common sense too.
In our example welder, at 20% duty cycle I could draw 76 amps for 2 minutes maximum through a 6-50 plug/receptacle. Do I really want to do that? I mean that a lot of over-amps (50%) through a plug receptacle and 2 minutes is quite a while. I wouldn't feel comfortable doing that.

[morgaj1]

Rick, that it the most comprehensive explanation I have seen. Thank you!

[Rick V]

Thanks morgaj1... just hope it's right - need confirmation from some certified electricians/welders.

[denrep]

I don't want to reinterpret the NEC or second guess NEMA or UL, but fwiw I'll mention that RickV's #2 formula is for installations were actual numbers are "known and remain unchanged." Otherwise, for guesstimates NEC puts a limit of 50% (manual) and 70% (semiautomatic} on the multiplier.

And no, a plug/receptacle is definitely not considered an overcurrent device.
Or shouldn't be, at least.
http://i229.photobucket.com/albums/ee66/karmainsantoro/muttley.gif

Anyway, I would guess that with a 6-50, the rating spec has as much to do with limits on the physical wire capacity of the terminal clamps and strain relief, as it does with blade ampacity limits and air space.

For example, from here, the plug seen in reply # 3 appears to have failed and flashed at the overcrowded strain relief. Even though it's reported as having been used at high amps and "warm" for months, the terminals and blades don't appear to show any signs of overheating.
Had the strain relief been large enough (or the cable small enough) so that the strain relief would squeeze a fully jacketed cable, then, the insulation breakdown that ultimately caused the flashover at the strain relief probably wouldn't have occurred. So at least in reply-#3's case, it appears that 6-50's blade/terminal ampacity wasn't an issue at all.

Good Luck

[Rick V]

... it appears that 6-50's blade/terminal ampacity wasn't an issue at all.
??? At high over-currents, the terminal-to-wire contact conductivity is going to be real important - not the time nor place for a loosely screwed down terminal screw! :nono:

[denrep]

. . .terminal-to-wire contact conductivity is going to be real important. . .

No doubt.
I'm just saying that with the plug seen in reply # 3, which was used at high currents, it doesn't look like blades or their terminal connections were an issue. What I see as having gone wrong is a flash at the short near the strain relief; that's where the "carbon spray" stems from.

Maybe Aandy767 has a postmortem up-close view of the strain relief?

Good Luck

[aandy767]

The plug got hotter the any thing else, I assume the heat came from a restriction in current flow. The plug housing slowly deformed from heat. The wires to welder lost insulation and shorted out on the band that keeps them from pulling out of the plug.

The blades seems to carry the load but I think, not sure, the screw to wire connection is where the heat came from.

I think my machine can pull 70-90 amps out of the wall when operating 60-40% duty cycle. It calls for a 125 amp breaker, but I put a 100 amp instead, I would rather throw a breaker in stead of running that much load in my panel.

I had # 6 suppling the plug but have added a circuit with # 4 wire, and put in a splice brass nut type deal.

[that'll_buff_out]

Rick great initial response. And the plug is treated like wire and not designed to be a controlled failure to protect from too much current like a breaker is.

I realize some of this is just regurgitated to help explain, but:

??? At high over-currents, the terminal-to-wire contact conductivity is going to be real important

Agreed but it does not appear to factor in the failed receptacle picture provided above.

What I see as having gone wrong is a flash at the short near the strain relief; that's where the "carbon spray" stems from.
Good Luck

Yep


The blades seems to carry the load but I think, not sure, the screw to wire connection is where the heat came from.


The blades, the screw, and the wire all carry the load. But with regards to the heat coming from blades, not true. If so, the insulation covering the wire closest to the blade connection would have failed/burned first.

The wire in that image failed. But it did so due to the clamp on the plug (if the wire was rated to handle the 76a of the machine to begin with and not rated to match the plug.)

I don't remember the wire size in the pic. If it's less than #4 than it's not rated for 76 amp load and would get warmer than it should. Even if it didn't feel it on the wire to the quick touch. The air would cool it and the steel strap in the clamp would not and get warmer and warmer and hotter and then poof. Burned insulation right at the strap.