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Can I convert a three wire plug into a round four wire plug?

Wiring is for an "AC 225 arc welder". It has a round three wire plug on it now but the only outlets I have available are round four wire.


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No, you can't convert a 3 wire plug to a 4 wire. You can replace the power cord on the welder, or replace the receptacle to match the power cord. That being said, the electrical codes now require the neutral and ground conductors to be separated, which requires the 4 wire configuration.
Details and interpretation of codes depend on the opinions of the Local Authority Having Jurisdiction. They aren't the same in all places, and there may be "grandfathered" exceptions.

If I was doing new wiring for a range I'd have to put in four-wire, but I'm allowed to continue using the three-wire circuit until the wiring has to be altered for other reasons (or until I decide I actually want to have it changed -- which I may do some time this year, just on principle). The drier, on the other hand, had to be switched over to 4-wire long ago. And yes, I checked that distinction with both a local electrician and the inspectors.
You are cortrect, the local AHJ has the final word on apllication of the code. I was merely pointing out the current requirements. More importantly, I wanted to clarify that the 3 wire plug could not be "converted" into a 4 wire plug. It would have to be replaced with a configuration that matched the receptacle or the receptacle would have to be changed to match the cord. 3 wire receptacles are still widely available from most hardware stores and home improvement centers and would probably be the easiest solution. The spirit of the code would indicate that the cord and plug be changed to include a separate neutral conductor and equipment ground. That being said, I doubt that Burt would actually apply for a permit to change the receptacle and have it inspected. I would just change the receptacle to match the appliance I wanted to connect. If he chooses to do so, please make sure the circuit for that receptacle is de-energized before working on it. It's an easy project, but make sure it is safe. Being nicked by 240V is not fun and can be leathal. And yes, I'm an electrical contractor.
PS - Very important! Please also verify that the circuit breaker and wiring is rated for the load of the welder. Most hobby sized welders are about 50 amps. The load should be listed on a data plate on the unit. If the existing receptalce was wired for an electric dryer or air conditioner it may be only a 30A circuit. The plug might be made to fit, but it would trip the circuit breaker when the welder is used. If the wiring is too small for the load, it might even start a fire.
orksecurity6 years ago
If these are the connectors I think they are, the answer is yes.

The difference between the three-wire and four-wire systems that I'm familiar with is that the latter includes adds a safety ground -- much like the difference between two-wire and three-wire 110/120V plugs.

In my area the four-wire version is required for electrically heated clothes driers; electric stoves here can still be plugged into either three-wire or four-wire outlets (and can be ordered from the manufacturer configured either way, or can be switched over in the field by replacing their power cord).

You have two options, that I can see: Replace the power cord (gives you the opportunity to connect the welder's case to safety ground, which is probably a good thing), or buy or build a short adapter cable (make sure it can handle that voltage and current!).