Use care when using the welder

I had the second electrician down at the space this morning to look at the welder.

He mentioned that our current wiring could only really do about 30 Amps total. I seem to recall that our welder has the ability to do more than that.

His suggestion was that we lock it out until we get the cabling fixed. Our first electrician didn’t give me that dire of a warning, but, I wanted to share it with the group.


Check the plate on the welder or the specifications in the manual. The
AC input amperage isn't the same as the welding amperage. Typical 240V
circuits are 15, 20, 30, and 50 Amps. See the chart here:

I took a class, but forgot almost everything: I think the electrician
is supposed to verify that the entire branch circuit and service can
handle the rated amperage of the outlet before installing it.

My interpretation of what Ian is saying is that the outlet (and welder
cord) is 50A, but your wiring is only big enough for 30A.

What gauge wire is used between the service breaker box and the welder outlet?

This page is confusing me, but all my NEC manuals are at work, so I
can't verify it:

There *might* be exemptions for welders that aren't used continously.
In that case the outlet might need to be placarded with time
limitations. If there is an exemption, I'm sure it's frowned upon.

When I was a kid, one of my neighbors was an electrician. He had a
red-box Lincoln welder. On high power welds (like 200-220A), he could
weld maybe ten minutes, then he'd let the wiring cool down for at
least 30 minutes. At 120A or below, he could weld continously and the
wiring just got warm.


I hate to second guess professional electricians. However, I just investigated wiring requirements for the welding equipment we are putting in The Manufactory.

From what I know 30 amps should be adequate to power the welder. The welder should have an input amperage rating on the plate but there is also a rating for duty cycle, which is probably in the 20-50% range. There is a factor in table 630.11 of the NEC that permits the use of smaller wiring for welders based upon the duty cycle. That is to say the plate may say 45 amp with a duty cycle of 40% needs to be wired for 45A*.63(factor)=28.35A.

The reality is that it is extremely rare that a large welder will get cranked all the way up on amps. Also it would be nearly impossible to push the duty cycle to the full limit. A 40% rating means that an arc will need to be struck for 40% of the time. For a handheld welder it would get too hot to hold and nobody has that kind of stamina. Furthermore the ampacity ratings for wiring is based upon continuous usage which is something like 3 hours straight. Machine welding is the only type of welding that could begin to push the limits of a welder unless it is a very old or cheap sputter box running off a 120v plug.

If the welder is fed by a 30 amp circuit with #10 wire, it should be good to go. As long as it has a breaker or fuse sized to protect the wiring there should be no concerns. If it gets overloaded the breaker or fuse will pop.

If you post the model number of the welder I can look up the specs and let you know what it requires for wiring.

The welder is fed by #12 cable, which is then spliced into 10 Gage cable to the welder. It isn’t a good situation – to say the least.

I’m unsure of the model number of the welder, unless it is readily available on the wiki. If I’m feeling any better, I might swing by later tonight and grab that info.


It's a Millermatic 211.

Specs say:

Input Power
230 V, 25 A, 60 Hz, 1-Phase
120 V, 20 A, 60 Hz, 1-Phase

I think 12ga 12/3 is what they spec for 20A outlets in houses?

I found this with some google-fu – not saying that it’s accurate:

80% max load is 16 amps.


It is not clear if it is hooked up to 120v or 230v. As I understand it, at the duty cycle ratings of the welder, according to the NEC the wiring should be:

For 230v you need 25A*.55=13.75A. This requires a 20 amp breaker/fuse and #12 wire.

For 120V you need 20A*.45=9.0A This requires a 20 amp breaker/fuse and #12 wire.

Technically either could operate on a 15A breaker and #14 wire but you are not permitted to use less than #12 in a commercial building.

If it is hooked up to 120v it really should be rewired to run off of 230v, because you are losing a large amount of the capacity of the welder. It is rated at 150A output if powered by 230v but it is only 90A output if hooked up to 120V.

Either way the current wiring appears to be adequate to handle the load. The#12 spliced into a #10 is of no consequence, you only need concern yourself with the smaller wire as long as it is fused for the smaller wire. Rule of thumb, 15A for #14, 20A for #12, 30A for #10

We have this welder:

Currently its not hooked up to 110.