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The clothes dryer in the garage is plugged into what appears to be a NEMA 10-30 socket (receptacle).

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Reading the voltages on the socket with a multimeter, the voltage between the two diagonal blades is 240 V.

But the voltage between any diagonal blade, and the L-shaped blade, is 120 V. How come? If the L-shaped blade is connected to ground, shouldn't I read 240 V between it and one of the diagonal blades, and 0 V between it and the other blade?

Or are the diagonal blades in opposite phase with each other, each carrying 120 V relative to ground, and therefore 240 V relative to each other?

I'm asking because I need 240 V for a small Everlast PowerTig 200DX welder - it could be connected either to 110 V from a regular AC socket (through a short passive adapter) or to 240 V. But the 240 V plug on the welder is not 10-30 but 6-50:

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I've found a 10-30 to 6-50 adapter but I need to make sure I won't fry the welder if the wiring is wrong.

The welder can sense whether it's being fed 240 or 110 V and switch automatically (the 110 to 240 adapter that came with it is passive, just a short cable with specific connectors on each end).

EDIT: I should add that the machine, according to the manual, draws less than 30 A continuously even at 100% output settings. Not sure why it has a 6-50 plug - probably because that's pretty standard for welders?

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  • But... I think this will work: I have the same problem: 10-30 receptacle and a 6-50 plug on arc welder. The welder states, at 230v: 28A max, 21A diff. The circuit is on 2 15A breakers - 30A. His welder is asking for 50A, mine only 28A. I think you can do it in my situation, but not yours if its asking for 50A. – Matt Oct 23 '18 at 1:28
  • I know this has an accepted answer however: A 30 amp plug would work on your welder at 230/28 amps for a welder it is fine , not sure why they wired it for 50 . The welder only needs 100% ampacity as it is an intermittent duty device. NEC 630.11.A so if your welder nameplate says 28 amps it will work fine on a 30 amp breaker when turned up to 100% .you could just get a replacement plug and swap between the dryer and welder add a new outlet. The only reason I can see they wired the welder with a 50 amp plug is 630.12.A. Allows 200% OCPD where most cord connected limit to 150% – Ed Beal Oct 24 '18 at 20:30
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In a 120/240V single split-phase system, there's two ungrounded (hot), one grounded (neutral), and in most cases one grounding conductor. Measuring between the two ungrounded (hot) conductors should yield 240 volts +- 5%. Measuring between either hot and neutral, should read 120 volts +- 5%.

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In the case of a NEMA 10-30. Each angled contact is one of the ungrounded (hot) legs, while the "L" shaped contact is grounded (neutral). Therefore, measuring between the angled slots should give you 240V. While measuring between either of the angled slots and the "L" shaped slot, should measure 120V.

As for plugging a NEMA 6-50 plug into a NEMA 10-30 receptacle, forget about it. NEMA 10-30 recepracles do not have a grounding conductor, whereas a NEMA 6-50 requires a grounding conductor. NEMA 10-30 is an nongrounded 120/240V device (hot, hot, neutral), NEMA 6-50 is a grounded 240V device (hot, hot, ground). A 10-30 receptacle will be protected by a 30 ampere breaker, and supplied by wires only rated to carry 30 amperes of current. If you plug a 50 ampere load, into a 30 ampere circuit. You will overload the circuit, and (hopefully) trip the breaker.

If you need a 50 ampere circuit for your welder, have a proper 50 ampere circuit installed by a licensed electrician.

You should NOT plug a 50 ampere device into a 30 ampere circuit!

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  • I see, so the L contact is neutral. But AFAICT, connecting neutral to ground is optional? How do I check if the neutral also goes to ground? I think it's pretty likely it's grounded already, but is there a way to make sure? – Florin Andrei May 30 '16 at 6:08
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    @FlorinAndrei you're missing the point that 30 amps does not supply 50 amps. The wire feeding a 30amp circuit is not big enough. – Tyson May 30 '16 at 11:30
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    @FlorinAndrei connecting neutral to ground is forbidden. There is no ground on your NEMA 10 receptacle. Adding a ground is possible, which would allow you to change that to a NEMA 14-30 which provides both neutral and ground. – Harper - Reinstate Monica May 30 '16 at 19:07
  • Wait - so that means my clothes dryer is not grounded? Isn't that a safety issue? – Florin Andrei May 31 '16 at 8:05
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    @FlorinAndrei That's why newer dryer installations use the NEMA 14-30 receptacle. It's fed by 4 wires, so it has a ground. – Tester101 May 31 '16 at 10:14
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No, 120V to Neutral (Grounded) or Ground (Grounding) is perfectly normal for US/Canadian electricity supply. 240V line to line, 120V line to neutral or line to ground.

Of course, if you plug a 50A welder into a 30A socket, things might not work so great on the high current end.

There is no Ground (grounding) conductor on this socket - just Neutral (Grounded)

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  • "Of course, if you plug a 50A welder into a 30A socket, things might not work so great on the high current end." To be clear, if everything is set up properly, the circuit breaker will trip. If everything is not set up properly, the wires in the wall will melt or catch fire. – Random832 May 30 '16 at 13:48
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The welder needs hot-hot-ground. Remember your welder's metal chassis is grounded, and it grounds the work via the clamp. It has to get that ground from somewhere. It needs ground so that a fault inside the welder doesn't electrify the things you are welding.

Hold on there. Neutral is not ground.

I know you really want ground so you can run your welder. But NEMA-10 does not supply ground, it supplies neutral, and that is not an acceptable substitute.

The link to that adapter just shows anybody will sell you anything on the Internet. It certainly does not have a UL listing, he's just going down to the electrical supply, buying the ends and a few feet of cable, building it, and selling it to you at a big markup. (Probably to cover his insurance; just kidding, his insurer has no idea he's doing this, and would cancel his policy if they knew.)

NEMA 10 is an obsolete way to hook up dryers and ranges that is ungrounded. When the idea of "grounding" came along in the 1960s, they got the hare-brained idea to bootleg ground off the neutral. That's grand, but if the neutral wire has any trouble, this definitely will electrify the chassis of the machine. It's a single point of failure with a body count, and was outlawed in 1989, but they didn't mandate retrofits.

Connecting a welder is right out.

Retrofit ground

However, in 2014 (actually somewhat earlier for dryers) they liberalized the rules to allow retrofitting (just) a ground wire instead of having to replace the entire cable run. You can run a #10 AWG ground wire via any available route, from that receptacle to any of a) the service panel it's powered out of... b) any junction box connected back to the panel by metal conduit or #10 or larger wire... or c) anywhere on the Grounding Electrode System, i.e. The copper braided wire from the panel to your water pipe, ground rods etc. You cannot just tap any water pipe. Note you can retrofit this ground without even opening up the service panel, which makes it easier.

Once you bring a Real Ground to that socket, you can change the socket to a NEMA 14-30, which includes both hots and neutral and ground. If you still have an electric dryer, find its instructions on the web and change its cord to NEMA 14-30 (key thing is, remove the neutral-ground bootleg jumper). Now your dryer is up to modern Code. Side benefit!

Now it's legit to plug in a welder there.

What's the deal with welder amps?

Welders are a strange bird when it comes to amp ratings and wire size. Stuff is allowed with welders that wouldn't be allowed anywhere else.

The math can be pretty gory. But here's the takeaway: if you are plugging a welder with a 50A plug into a 30A circuit wired with 10AWG wire, this is not the end of the world. Most likely if you are doing intermittent duty-cycle welding (as one would assume welding at home), and you ran the gory math, you'd find that 10AWG wire is perfectly legal. Code would advise changing the breaker to 50A, but if the 30A breaker isn't tripping, leave it! Dual-use of the socket for the dryer is good reason to leave it that way.

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