Maximum breaker for #4 awg copper?

[dslew]

I am setting up a workshop and the electrical is being finalized. I recently ran wire #4awg and was planning to use it for a miller 250dx in the future. I was hoping to run a 100 amp breaker. I tried finding info on the web about maximum amp capacity but there is not a clear cut answer. My run is about 50 feet from the 200 amp service for the workshop.

Thanks in advance,
Doug

[MAC702]

Assuming you are referring to the Miller Syncrowave 250DX, it has a duty cycle of 60% at an output of 200A and 40% at an output of 250A, and linearly less as output increases to its maximum of 310A.

Also, this is assuming you get the model that does NOT have power factor correction.

At 200A output, it will draw less than 77A. At 250A output, it will draw less than 96A, each within their respective duty cycles.

The relatively small duty cycles mean that at its rated output of 250A output, you can multiply the input amperage by 0.63 to determine minimum input conductor requirements. 96A * 0.63 = 60.5A. Your #4AWG is already plenty for this machine. #6AWG would also be legal and Code-compliant. In this installation, either could be connected to the 100A breaker. Please label the receptacle or disconnect "For Welding Machine Use Only."

[dslew]

Thanks for the prompt reply. You answered a question that my electrician didn't know the answer to. What a great resource you are! Anyhow, I initially had #6 awg copper romex installed and I was told that the maximum breaker that I could install was a 60amp unit - to meet code. I wanted a 80 amp breaker. In response, I replaced the wire to 4awg copper romex to allow for this upgradabiliity. Now that my interest has grown more into potential other machines that I my get (i.e. miller synchrowave 250 dx), I want to be sure that I have that option - so I think I should install a 100 amp breaker. It seems that I am better off changing the breaker after the final inspection is done just to allow the installation to pass. In California, #6 awg copper is good for up to 60 amps, and #4 awg is good for up to 80 amps. As I understand your comment ( please correct me if I am wrong), I can actually safely use a larger breaker recommended for the wire than what meets code because the duty cycle of the machine - as long as I understand that it is for the welder only! Not for something that would truly require 100 amps at full duty cycle.

I have one other question. I have been using the 6-50 plugs for my Lincoln pro mig 215, pro cut 55, and 230v compressor (30 amps). Would it be wrong or a hazard to place a 6-50 plug on the synchrowave 250dx? What would be the recommended way to plug the box in? I would rather not use a safety box because then I can't move the machine as easily. Thanks again in advance.

[MAC702]

I did the same thing when I was working in the States. It's too difficult trying to convince an inspector that Article 630 of the NEC is perfectly legitimate, even when they know it's a welder receptacle. So I often have a 40A breaker on #8 wires, and then swap in the 50A for the welder later, etc.

You are absolutely correct that you must make sure these types of derated circuits are only used for welding machines of the appropriate duty cycles. Yours will already be slightly better than actually needed; which is good insurance and even further upgradability.

It is common to see NEMA 6-50 hardware on the cords for these machines, because of the exact same duty cycle allowances, and anything else (except the simple safety switches) starts to get expensive rather quickly. But how far around the shop do you see yourself moving a Syncro 250? You can still have a long length of #6 SJOOW cord between the machine and the safety switch.

[wlbrown]

HELLO TO ALL,
try this.
http://www.csgnetwork.com/wiresizecalc.html

wlbrown