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Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
I'm installing a ciruit for a Lincoln Mig Welder which has a 240v 50a recept on it from the manufacturer. In the manual it calls out for a 125a breaker because of the duty cycle of the welder and to prevent nuisance tripping. How can I put a #8 THHN wire rated at 55amps on a 125amp breaker?


The Golden Rule - "The man with the gold makes the rule"
Joined: Jul 2002
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E
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I am not sure about the NEC, but most codes limit the OCPD for a transformer arc welder circuit to 200% of the ampacity of the circuit conductor, to allow for starting inrush current.

Ed

Joined: Oct 2000
Posts: 2,749
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See the requirements in Article 630, for welders, they are clear and will provide an answer to your questions.

Did the equipment come with instructions for the supply?


Joe Tedesco, NEC Consultant
Joined: Aug 2001
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aldav53 Offline OP
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Yes, the welder came with a manual and called out for a 125a breaker.
In Art. 630-12 (a) it says welders shall have overcurrent protection rated or set at not more than 200% of the maximum current. So 200% x 50amps = 100 + the 50amps = 150amp breaker. umm, am I wrong?


The Golden Rule - "The man with the gold makes the rule"
Joined: Nov 2000
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200% = 2 times or 100A...

An additional 200% would make it 150A


-Virgil
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Hello, you need to find out what the welder is rated at, not what the plug is rated, although you wouldn't think the plug would be rated under the primary. I'm guessing the rated primary current is just over 50 amps, let's say 56amps. For an arc welder, the ocpd can be no higher than 200 of the rated primary or rounded up to the next standard size (2x56=112 rnd=125). Your wire will need to be #6THHN, as the OCPD is also allowed to be 200% of conductor ampacity for arc welders.

If this is a resistance welder, see 630.32, you can go 300% of the rated primary current.

Hope this helps.

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aldav53 Offline OP
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This is a Tig welder, not sure how you got you number - (2x56=112 rnd=125) but that must be how they got 125a. I think the primary side was over 50a. So, with this welder circuit it is legal to use the 125a breaker with a #6 THHN wire?
Thx for your help..


The Golden Rule - "The man with the gold makes the rule"
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For the 1999 NEC, tig [& mig?] welders are typically classed under Art.630 part B "Nonmotor-Generator." Duty Cycle likely 20-30% if fitted with 240V 50A cord cap. Electrically, {codewise} welders are their own breed when it comes to conductor/overcurrent-device sizing.

If the machine is listed/labeled, UL, etc. are fairly conscientious about reviewing installation instructions to make sure the welder manufacturer is not giving instructions that violate the NEC.

aldav, I'd go with the 6AWG.





[This message has been edited by Bjarney (edited 08-06-2002).]

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aldav53 Offline OP
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Still does'nt seem right because if that #6 wire were to short to ground, it would still get pretty hot before that 125a breaker would blow, raising the chances for a fire.
Am I wrong, or does that make sense?


The Golden Rule - "The man with the gold makes the rule"
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In the event of a short or ground fault, as soon as the fault current exceeds approximately 600% of the breaker rating, the magnetic trip unit will open the breaker very quickly, well ahead of the thermal damage curve of the conductor.
The wire will be adequately protected as long as it can carry the welder's rated primary current multiplied by the welder's duty cycle factor.

Ed


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