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I'm doing some electrical upgrades to my garage/workshop with upstairs apartment; I'm going to be replacing its 1960s-vintage subpanel which is fed by #4 wire from a 200 amp service entrance panel (on the house, about a 40 foot wire run).

While out shopping for a bench grinder (1970s Craftsman; I'll take 50 years old, a little rust, but Made in USA over brand-new cheap Chinese crap any day) I found that the seller had an old Miller welding machine which he was willing to let go for a reasonable price. It looks to be in decent condition but the power cord is deteriorated and will have to be replaced prior to use. I'll need to install a suitable outlet for it...nameplate specs are 230 volt, single phase, 37 amps. The plug currently on the machine is a NEMA 10-50P.

I'd rather not do any welding actually inside the garage, with perhaps rare exceptions. I was thinking of installing the new outlet on the outside of the garage, at the driveway, just 2 or 3 feet from the subpanel I'm replacing. I don't currently own an RV, but it strikes me that it would be ideal to install a receptacle which could be used either for the welder or for an RV.

Suggestions? (Also, suggestions for "tuning up" and safety checking the old Miller welder prior to first use are welcomed as well.)

  • Are you OK with spending a bit to have a bit of extra functionality at this receptacle location? – ThreePhaseEel Jun 16 at 23:22
  • Yes. I've since found an exterior outlet box with a rain-resistant cover, intended for RV parks, which has both a NEMA 14-50R outlet and a 20A convenience outlet, each with breaker protection right in the box and fed from a single source circuit. I'm considering it. – ehbowen Jun 17 at 0:02
  • Is that #4 wire copper or aluminum, and what sort of loads are in this garage/apartment already? (I ask because a RV can be a pretty big honkin' load.) Also, what duty cycle is the welder capable of handling? – ThreePhaseEel Jun 17 at 0:16
  • It's copper. It feeds a couple of subpanels that I'll be upgrading. The first one, for the garage, is a 125A ITE that has four spaces for receptacles and I'm not sure what else...I've bought a circuit tracer and I'll attempt to identify all circuits before changing the panel. I also had the electrician put in a sub-subfeed breaker (60A) to the apartment panel, which is an 8-space 60A GE panel for the apartment loads...one 220V 20A breaker for the window A/C, then small appliances and lighting. The #4 feed to the first panel I put on a 70 amp breaker, but I have some headroom to go up. – ehbowen Jun 17 at 2:18
  • I take it that #4 copper is in conduit, then? – ThreePhaseEel Jun 17 at 2:35
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I appreciate your effort to try and combine outlets and wiring but it's usually pretty hard to do. Your welder will need a 40 Amp breaker, two hots and a ground. Your future RV would probably need at least a 20 Amp breaker, one hot, a neutral and a ground. Weather proof, water tight outlets and cover plates can run into a lot of money so I'd think about installing your welder outlet in the garage. Put a longer cord on your welder or get longer ground and electrode cables so you can weld outside. If and when you get your RV, then deal with the outside outlet. If you run 3/4" conduit and THHN for the welder circuit, you could add the new RV circuit in the conduit and extend it outside with your weatherproof/watertight fittings. If you're planning on running cable, you'd need two separate runs.

As far as testing your welder, A Megger test would be the best for verifying the insulation is good. You could also remove the access panels and clean everything up and check all the connections. Plug it in and strike a small arc at each setting.

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    Hmm. My understanding is that modern RV outlets are 50 amp, 240 volt (two hots), plus neutral and ground. It seems as though that should be quite compatible with my new-to-me welder, especially since I'm looking at replacing its power cord anyway. – ehbowen Jun 16 at 13:55
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    @ehbowen Sure, but your future RV may end up being just as "modern" as your new-to-you welder and you may have an outlet that does you no good and needs to be rewired anyway. Why not prepare for the future RV hook-up (conduit in place, proper wire that will allow additional wire pulls, etc.), but leave the "doing" until you know exactly what you need? – FreeMan Jun 16 at 13:57
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    @ehbowen Yes, they would be compatible. If you're pretty sure of the future requirements of the future RV, you could install the outlet for the RV outside in a watertight enclosure and cover. Then use the same type plug for your welder but don't hook up the neutral terminal. – JACK Jun 16 at 16:21
  • Looking it up, it seems that what's currently sold as a 50 amp RV outlet is a NEMA 14-50R...the same receptacle used for electric ranges. I'd think that's unlikely to go obsolete any time soon. If I want to keep the welder "standard", I can replace the bad cord with a 6-50P power cord and then buy an adapter for less than $20. Seems like the way to go. – ehbowen Jun 16 at 19:55
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    @ehbowen -- you can even simply have a 14-50P on the welder, with neutral insulated (capped off) at the welder end. (I suspect how the welder got a 10-50P in the first place was because someone couldn't find the right cord, so they simply whacked a 3-prong range cord on it since that's all the home store had) – ThreePhaseEel Jun 16 at 23:21
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I'd use a 50A/20A combination RV/temporary power box with loop feed lugs and GFCI breakers

Your welder is fine on a 50A receptacle/circuit (you can fit it with a NEMA 14-50P, just don't hook the neutral on the plug up to anything, although NEMA 6-50P is technically the correct plug for the job here), so I would provision an appropriately rated RV combination outlet box at where you want this new receptacle to go. This provides you with a NEMA 14-50R (big RV, welder, EV) and a pair of NEMA 5-20Rs (dinky little RV, general convenience receptacle); you could get a NEMA TT-30 for intermediate-sized RVs in this box as well, but since this is a personal install and not an RV park site, that's not particularly needed.

However, just whacking the box in there isn't enough. You'll probably want a box with loop feed lugs; this way, you can simply reroute the existing garage feeder to the box then continue onward to the primary garage subpanel, or if you'd rather run a feeder from the garage subpanel out to it, you could run a 60A breaker and four #6 THHNs in 3/4" ENT headed into the back of the box. You'll also want a version with a GFCI breaker for the 50A receptacle in addition to GFCI protection for the 20A receptacles; this is required under a TIA to NEC 2017 for EV chargers, and will be required for general use come NEC 2020.

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  • Waitaminit...GFCI on a circuit intended for powering an arc welder? That doesn't sound right... – ehbowen Jun 17 at 2:13
  • @ehbowen -- it works because the welder work circuit (electrode and work leads + workpiece and arc) is isolated from the primary circuit. – ThreePhaseEel Jun 17 at 2:31
  • If you say so; GFI protection is a good thing. I just remember from a construction job at a commercial lab facility how the contractor made us remove GFCI protection from the circuits they were using before they could weld. – ehbowen Jun 17 at 5:12

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