Wire size from breaker to Arc welder, please help..


Old 02-03-07, 11:01 PM
Thread Starter
Join Date: Feb 2007
Posts: 3
Wire size from breaker to Arc welder, please help..

I need to run a line about 60' from a 50 amp breaker to a (45.5 Amp 20% duty cycle) arc welder (specs below). I am not clear about the minimum wire size, as this stuff is very expensive these days. I have a 240/50 amp line going part way(10') there, from a non functional hot tub, I would love to make this the dedicated welder line, but it is #8. I know basic code says it should be #6, but given its 20% duty cycle would #8 be legal AND/OR safe??

I was told this by someone "Given the manual operation of this welder, and its rated duty cycle, you can go as small as 50% of the nameplate ampacity in the supply conductors. So 10AWG would be acceptable. See article 630.31 of the NEC. "

Is this acceptable practice? Safe?


Miller Thunderbolt 225V

Single Phase 60 Hz

Max Rated:
Duty Cycle 20%
Max OCV 80

The plug from the welder has only 3 wires, 2 hots and a gnd.

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Old 02-03-07, 11:58 PM
Join Date: Aug 2004
Location: NA
Posts: 1,065
You could go as small as 45% of nameplate input amps... (.45 x 45.5 = 20 amps).. based on NEC 630.11 (A) and duty cycle of 20%.

So you could go as small as #12 awg copper . Your plug is a nema 6-50P. The maximum breaker would be 50 amps... the ampacity of #12 copper is 25 amps so your breaker maximum would be 200% of 25 AMPS.

#8 AWG copper would be just fine for this welder use a 40 or 50 amp breaker. Do not exceed 50 amps for breaker size.

Old 02-04-07, 11:06 PM
Join Date: Jan 2003
Posts: 111
Do you have the owner’s manual?

As Roger said the minimum ampacity for the supply conductors can be determined from the primary input current and the duty cycle. For this welder that would be 12AWG, too small IMO especially at that distance.

The breaker cannot exceed 200% of the primary input current or 200% of the supply conductor ampacity. This would limit you to a 50 amp breaker with 12AWG conductors. With 8AWG conductors you could go up to a 90 amp breaker.

The circuit needs to adequately serve the load. With a 40 amp breaker it would not.

At that distance I would use the 8 AWG and a breaker not less than 60 amps. You may need to go even higher, up to 90 amps.

If you don’t have the manual you can probably download a PDF from Miller, free.

The conductor sizes are based on copper conductors.
Old 02-04-07, 11:35 PM
Thread Starter
Join Date: Feb 2007
Posts: 3
The run I am conveting from a line to a hot tub, to this line to the welder has a 50 amp breaker and #8 wires; I will be continuing the run in #8 for a total run of about 55', will the breaker trip given the smaller wire size? Might it be dangerous to leave the 50 amp breaker?

Thanks sincerely for all your help.
Old 02-05-07, 09:52 AM
Join Date: Jan 2003
Posts: 111
Article 630 of the NEC allows welder circuits to be sized based on the specifications of the welder. These circuits cannot be used for any other purpose.

Here is a link to the owner’s manual for the welder you described.

On page 15 you will see it calls for 8AWG conductors with an 80 or 90 amp breaker. Sometimes the instructions are also based on article 630, sometimes not. For this welder the instructions are based on article 630. We are told to follow instructions.

There are hundreds of details to consider when adding or extending any circuit.

Is the welder going to be in the same building as the panel feeding it?
Old 02-05-07, 10:49 AM
Thread Starter
Join Date: Feb 2007
Posts: 3
The welder is in an attached garage, the entire structure, home and gargae, is fed from this panel. Will that impact the breaker amperage?
Old 02-06-07, 12:23 AM
Join Date: Jan 2003
Posts: 111
A separate structure would have different details to consider. But, your service has to be big enough to handle the added load.

Using this abandoned hot tub panel seems a little goofy. It is best to avoid unnecessary splices or to add unnecessary length to a circuit.

Having a welder likely means other high draw tools like grinders, cut off saws, compressors, etc. IMO the best thing to do would be to add a 100 amp sub panel in the garage. Then, as your needs change you can easily accommodate them. Tripped breakers and disconnecting means will be right there. It sounds like you need to run cable a good distance to the garage anyway. I would find the most direct, accessible path from the main panel to the garage. It can be through an attic space, a crawl space, outside or any combination

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