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#201673 06/15/11 09:05 PM
Joined: Jul 2005
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jay8 Offline OP
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I have been given a mig welder to supply power for. Rated input is 230volt 20 amp single phase, but plug supplied is 50 amp 250 volt - I guess that although oversized this is just the standard welding plug, but does one wire the receptacle for the demand of the welder or the 50 amp plug supplied?

jay8 #201675 06/15/11 10:39 PM
Joined: Jul 2007
Posts: 265
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I would wire for the demand of the welder. I have frequently run into the same thing with welders, seems there is no standard type of plug used on welders.


Jimmy

Life is tough, Life is tougher when you are stupid
jay8 #201676 06/15/11 10:41 PM
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twh Offline
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Demand of the welder. It's like a range that has a 50 amp receptacle but is wired for 40 amps.

Is there a demand calculation for a welder circuit, or is it just the 80% rule?

jay8 #201677 06/16/11 12:32 AM
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As a non Electrician, I would ask the customer if they plan to upgrade to a larger power supply in the NEAR future. If so, I would use a larger conduit to allow for an easier upgrade for the next electrician, which may be you.

jay8 #201678 06/16/11 04:32 AM
Joined: Jul 2002
Posts: 8,407
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Originally Posted by jay8
I have been given a mig welder to supply power for. Rated input is 230volt 20 amp single phase, but plug supplied is 50 amp 250 volt - I guess that although oversized this is just the standard welding plug, but does one wire the receptacle for the demand of the welder or the 50 amp plug supplied?

Jay,
Welders are pretty hard on a power supply.
Is this in a residential or commercial workshop?
One thing I would note for a start is the nameplate on the welder itself, especially the Duty Cycle.
Welders with high Duty Cycles (of 60-70%) require heavier supply wiring, this is due to the weldor being able to use the welder for longer periods at a time and therefore increasing the amount of heating in the supply cabling.
If however, the welder only has a Duty Cycle of say 30-40% you can pretty much get away with a circuit that matches the current draw of the welder.
Another thing, welders and circuit breakers don't tend to go together that well, unless the CB has magnetic protection only, using a CB with thermal protection added in, will end in tears.
I've seen CB's bypassed by those that don't know any better in the past, because the wrong type of CB was installed in the first place.

jay8 #201681 06/16/11 12:07 PM
Joined: Aug 2002
Posts: 402
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26-700(2) say receptacle shall only be connected to voltage and ampere rating corresponding to the receptcle.

Is there some sort of exception for welders?

jay8 #201682 06/16/11 12:26 PM
Joined: Jul 2005
Posts: 183
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jay8 Offline OP
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The welder is a pretty common Miller mig welder in a small commercial shop. Did'nt notice a duty cycle on the nameplate.

Section 42 does not seem to address this other than (42-004) letting the plug on the welder be less than the O/C rating but not less than conductor ampacity. With the price of copper these days I would want to justify the 50amp conductors.

jay8 #201697 06/17/11 09:52 AM
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twh Offline
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42-004 says that the plug can't be less than the "conductors required". That's different than the ampacity of the conductors because you can always oversize the conductors without changing plugs. If you couldn't, engineers wouldn't be able to inflate the cost of the job by going "above code".

If the size of the cord end bothers you that much, change it. Why would you run #6 wires to a load that needs a maximum 24 amp circuit ampacity?

jay8 #201703 06/17/11 11:17 AM
Joined: Aug 2002
Posts: 402
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If you put the 50 amp receptacle on a 20 amp breaker what is to prevent the user from getting a full size welder that needs 40amps and plugging it into that receptacle.

I think you either need to change the plug on the welder or put in the full 50 amp circuit.

jay8 #201704 06/17/11 11:23 AM
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Posts: 183
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jay8 Offline OP
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Just out of curiosity I had a look at a welder from same manufacturer with twice the rated input - and same 50 amp 250 volt plug on it. And I agree, why run the #6?

So it comes out of the factory as "listed and approved" equipment with one oversized plug that fits all. I could go and change out the plug on this approved and listed piece of equipment but then I am interfering with the approval - ironic.

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