NEMA 6-50 Question

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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?
Reply: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."Where's Stick man????????" - 7A749"SHHHHHH!! I sent him over to snag that MIC-4 while tbone wasn't looking!" - duaneb55"I have bought a few of Tbone's things unlike Stick-Man who helps himself" - TozziWelding"Stick-man"
Reply: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 ALI have since added a breaker and hard wired it. Attached ImagesLast edited by aandy767; 03-02-2012 at 08:37 PM.
Reply:that was a 50 amp plug
Reply:Very illustrative picture there aandy767!Looks rather 'cooked' to me!Great Picture!Rick V 1 Airco Heliwelder 3A/DDR3 CTC 70/90 amp Stick/Tig Inverters in Parallel1 Lincoln MIG PAK 151 Oxy-Acet
Reply:Originally Posted by morgaj1How 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?
Reply:Rick, that it the most comprehensive explanation I have seen.  Thank you!
Reply:Thanks morgaj1... just hope it's right - need confirmation from some certified electricians/welders.Rick V 1 Airco Heliwelder 3A/DDR3 CTC 70/90 amp Stick/Tig Inverters in Parallel1 Lincoln MIG PAK 151 Oxy-Acet
Reply: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.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 LuckLast edited by denrep; 03-03-2012 at 01:36 PM.
Reply:Originally Posted by denrep...  it appears that 6-50's blade/terminal ampacity wasn't an issue at all.
Reply:Originally Posted by Rick V. . .terminal-to-wire contact conductivity is going to be real important. . .
Reply: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.
Reply: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:  Originally Posted by Rick V??? At high over-currents, the terminal-to-wire contact conductivity is going to be real important