View Full Version : 50 amp welder on 30 amp circuit

March 15th, 2011, 05:46 PM
Cards on the table: I'm out of my depth right now, trying to work my way through this (using the "water in a pipe" analogy).

Often people with a welding machine at home connect into a 30 amp 220v clothes dryer outlet, even though their machine calls for a 50 amp circuit. The welding machine works- why does the machine call for 50 amps?

My guess: the machine works at the lower end of its capability because demand on the circuit is lower, but at the higher end (welding on thicker metal, for instance) it will overtax the circuit...? Heat up the wires?

Am I close?


March 15th, 2011, 06:48 PM
It depends on duty cycle and what you're actually looking at. Does it really require a 50A circuit per the instructions, or is the nameplate amps 50? Code permits a larger circuit breaker than normal for a given wire size on a welder circuit (and it may be required to be labeled "welder use only" because of the too big circuit breaker). The permitted sizes are based on the nameplate amps and the duty cycle. Both of those numbers should be stamped on the nameplate.

So if it has a 50A nameplate, you would never really need 50A unless it had a 100% duty cycle. Many welders have a 20% or 30% duty cycle rating. Run them more than that, and they shut themselves down because the welder is overheating. This auto shutdown protects a circuit from being over taxed.

Many welders have 50A plugs installed just so you can move it around and have one size power oultet for all your welders. Many would probably function just fine on 30A circuits and you could standardize on that. A 50A receptacle will also be more tolerant of the high current surges needed during welding, but I believe the receptacle only needs to be rated the same as the wires.

A true welder circuit may have #10 wire (30A rated) and a 50A breaker. That is because a 30A breaker may actually trip during those welding surges. But most circuit breakers are pretty tolerant of overloads when the duty cycle is low. Code allows you to go higher than normal, but it may be unnecessary.

March 15th, 2011, 07:31 PM
The 50 amp rating is for the highest welding setting the welder can support. Welders also have variable welding capacity. If you weld at 60-70 amps it uses much less power than if you weld at 180 amps.

March 15th, 2011, 11:43 PM
DaveO welding processes change as power requiremnets do. mig is quick VS tig and sustained power draw may influence wire size and/or breaker size.

March 16th, 2011, 11:26 AM
For the machines I'm thinking of (including the one I'm using on a 30 amp dryer outlet) the installation instructions call for a 50 amp circuit.

I'm still trying to muddle through the duty cycle component of this discussion. Max duty cycle of my welder is 20%. Are your posts saying that since 20% of 50 amps is 10 amps, I'd have plenty of power on a 30 amp circuit?

It also sounds like your posts are saying the machine would run well enough on
30 amps at the lower end of its capabilities (welding on thinner metal). What is the affect on the machine, or the circuit itself, of running at the higher end of the machine's capabilities (welding on thicker metal)? I suspect the machine would be underpowered and not perform properly.

Sorry to run on about this- am I close to correct?


March 16th, 2011, 12:42 PM
A 30 amp breaker will hold at 30 amps. It will trip at 35 amps but not instantly. It takes some time for the heat to build. By using the welder 30 seconds out out of 2.5 minutes you have a 20% duty cycle. This gives the breaker and the welder cooling time.

Also welding at 70 amp might only draw 25 amps out of the circuit while using 120 might draw 40 amps.

So you have two components to deal with when deciding whether it will work on 30 amp circuit.

March 16th, 2011, 02:50 PM
From NEC table 630.11A (assuming arc welder), a duty cycle of 20% gives you a multiplier of .45 to determine the ampacity of the wire. So if the nameplate input amps is 50, you would need circuit wires rated to just under 25 amps. So you could actually have a circuit with #12 wire feeding this welded (#12 copper wire is good for 25A even though most think it is only 20A).

If the welder nameplate has an Ieff value, use that for the wire ampacity requirement instead of the calculated one above.

The breaker is permitted to be up to twice the nameplate amp rating (so you could have a 100A breaker on #12 wire). But breakers are usually calibrated for wire thermal characteristics, so if a 25A wire is good enough, a 25A breaker will probably work too. You don't have to go to the max breaker, you just can if it nuisance trips.

From reading the code book, I'm still unclear as to whether the receptacle needs to be rated at the welder nameplate amps or just the Ieff value. It would be best to use one rated per the nameplate amps.

March 16th, 2011, 06:08 PM
Generally you will match the receptacle to the circuits ampacity. For example, use a 50 amp receptacle (no smaller) on a 50 amp circuit. It would not be compliant to have a 30 amp receptacle on a 50 amp circuit. It is OK to use a higher amp receptacle such as a 50 A on a 30 A circuit. Obviously the circuit's ampacity is based on the input rating on the welder. Hope that makes sense.:stupido2:

March 16th, 2011, 08:38 PM
I think you're right. In general, the rating of a circuit is defined by the size of the overcurrent device, and the receptacle would need to equal or exceed that value. However, there is an exception for welders in 210.21(B)(1):

Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.
Exception No. 1: A receptacle installed in accordance with 430.81(B).
Exception No. 2: A receptacle installed exclusively for the use of a cord-and-plug-connected arc welder shall be permitted to have an ampere rating not less than the minimum branch-circuit conductor ampacity determined by 630.11(A) for arc welders.

March 16th, 2011, 11:24 PM
dave o you are getting there but still a shade away. duty cycle is a term that allows use(welding) vs idle or cooling off. the higher the % of duty cycle the more use per. better quality machines run higher % duty cycles. "country code" has allowed us to run a lincoln 225 at 90 or so amps from a 30 amp receptical and breaker for years so long as mom aint drying clothes