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I have a detached shed that was wired by somebody who evidently thought codes were just a friendly suggestion. I'm slowly trying to correct the electrical sins as I find them.

In this shed is the following 240V receptacle:

240V receptacle

The receptacle is wired as follows:

240V receptacle wiring

The wire that feeds it is 10/2, with this stamped on the side:

E30445 (UL) AWG 10-2 WITH AWG 10 GROUND TYPE UF 600V SUNLIGHT RESISTANT

And that's fed from the following double-pole breaker:

60A double-pole breaker

This receptacle is the only thing on the circuit. Total wire length is roughly 80-100 feet one-way.

As I understand it, I have the following problems:

  1. It looks like the receptacle is a 10-30R, which as I understand is frowned upon today because it doesn't provide a ground prong.
  2. It doesn't seem okay to use the ground wire in a 10/2 cable as the neutral.
  3. The breaker seems way oversized for this application, unless I'm misunderstanding how current works with double-pole breakers. It's not clear to me if "internal common trip" changes anything.
  4. Other problem(s) I'm not aware of?

I'm looking for the most reasonable way to make this safe and code-compliant, bearing in mind that I don't even have a use for this receptacle right now. I know I always have the option of removing the receptacle from the wall, pulling the breaker out of the panel, and capping the wire at both ends. I'm curious what it would take to fix it right, however.

Bonus question: Does anybody have any idea what purpose a 240V receptacle in a shed could serve? My guess is the previous owner was using some kind of heater or heavy-duty tool -- what could it have been?

  • First off, #10 wire is only good for 30 amps. The breaker is way oversized. – Hot Licks Mar 17 at 20:10
  • I've seen older homes wire up 240V outlets like this in sheds/garages when installing a clothes dryer and the house was designed before indoor laundry facilities were common. Installing it in a shed can be a lot easier since you don't have to modify your house, especially when it comes to the exhaust vent (common to vent out a nearby window with a sheet of wood where the glass would normally be). – bta Mar 17 at 20:32
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The problematic receptacle

Use of a NEMA 10 family receptacle is illegal today. It was always illegal with that cable type, which is a "/2 + ground" type, for reasons I go into in comments on George Anderson's answer. Use of SE cable was legal, but only to help dealers use up their remaining stocks. So the NEMA 10 must go; you don't have the right cable for it, and so it's not grandfathered.

Install a universal junction box.

I would fit a 4-11/16" square steel box there, set about 4" lower on the wall. (to give you the legally required wire length. Note that in your installation, wire length is precious; don't CUT wires off the old recep, unscrew them).

For wire length, you need 6-1/4" past the cable clamp; plus 3" beyond the surface of the wall.

Then, on the 4-11/16" square box, add a 1-gang mud ring (not a dome). That will you give you a 1-gang space for any recep, and also LOTS OF ROOM behind the box for GFCI, AFCI, bulky receps etc. You also have space to change to a 2-gang mud ring, to allow GFCI + second recep. The box also has room for extending wiring to other points of use e.g. lights.

Fit the receps you want

Once that universal box is in, fit whatever receps you want:

  • For 240V, a NEMA 6-30. OR
  • For 120V @ 30A, a NEMA TT30, or L5-30
  • For 120V @ 20A, the common 5-20 or dual 5-15.

You must choose 120V or 240V. You can't have both unless you add a subpanel and a proper 120-240--240/480V supply transformer, which will give you both, totaling 7,200W. You can also go "two transformers" which will give you 14,400W or up to 18,000W. The cost of these will rise exponentially with increasing power because of size and scarcity of the transformers.

Now change the breaker (mandatory)

According to Code, the breaker size must match the receptacle size. (with one exception: if the circuit has two or more 15A sockets, a 20A breaker is allowed.)

I recommend a 2-pole, 20A or 30A breaker, because it'll fill the hole completely. A 1-pole breaker is $5 and a 2-pole is $10. A blanking plate is $3 + $50 of your time just running around trying to find somebody who has the darn thing in stock (won't be a big-box store). So I use breakers to fill empty holes.

Also do not under any circumstances buy a 30A, 1-pole breaker. They are completely useless for any application except certain small travel trailers (which take a TT30). If you need one, buy a 30A/2-pole instead, which have many applications. You'll thank me later.

The 60A breaker is perfectly usable and about $10 to buy a new one. They're good for subpanels or bigger EVSE's. Do with it what you will.

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  • Looks like there's a loop of excess cable inside the wall below the receptacle, so the wire length isn't quite as precious as it could be. I'm leaning toward the 6-30 receptacle option, and swapping in the correct breaker ASAP. – smitelli Mar 16 at 23:17
  • @smitelli That's legit. Just keep in mind you'd only be able to power 240V compressors, welders etc. There's no such thing as a 6-30 to 120V outlet adapter cable... at least there shouldn't be; though some crooked sellers on eBay/Amazon Marketplace will cheerfully make you a dangerous thing like that. Also using a cheapie $100 stepdown transformer has a 50/50 chance of being dangerous. I was referring to an isolating 120-240--240/480V service transformer. – Harper - Reinstate Monica Mar 16 at 23:54
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Maybe used for a welder? Kiln? The worst thing right now is the breaker "protecting" the circuit. It's double what it should be. 10ga wire in cable is rated at 30 amps! Not 60!!!

The wiring on your outlet is OK, just dated. It's fine for a 240v only appliance: 2 hots and a ground. Not code legal anymore, but commonly done for clothes dryers in the past.

What are your goals for power in the shed? Just a simple outlet? Multiple outlets? 240v outlet? A sub-panel? The problem with a sub-panel is you don't have a 4 wire feed to the building. I suppose you could do a 120v only sub-panel...not sure. One of the big-three here could weigh in on that.

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    @user3757614 That type of surface mount receptacle is supposed to be its own box. We might complain about the unprotected NM entering the bottom. There is a knock out in the back of the receptacle that is supposed to be used when wall mounting like this, but its not a big deal the way it is. – JPhi1618 Mar 16 at 18:49
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    @GeorgeAnderson You're misunderstanding what I'm saying. Back when 3-wire was legal, you were supposed to use insulated neutrals, except an exception was cut to allow use of (then, common) #10 SE cable. SE cable is for service drops, the conductors are H-H-N. Idiots decided that since SE's neutral is uninsulated, therefore /2+gnd cable must be alright too. Code never said that. The bare-neutral exception was only ever valid for SE, USE and SE-R cable. (all SE cable). And even that was just to let the supply chain use up their stock of #10 SE, which had no use after that. – Harper - Reinstate Monica Mar 16 at 19:43
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    In other words, Code intentionally put you in a bind: 3-wire connections were legal, but only if you used obsolete cable that was now unobtanium. The idiots decided "since that cable type no longer exists in market, that implies it must be legal to substitute other cable types". Code never implied that. Code's goal was to force everyone to 4-wire connections as soon as the supply of SE was exhausted. #10 SE died because of Code changes requiring >40A service drops and grounds on subpanel runs. – Harper - Reinstate Monica Mar 16 at 19:52
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    @GeorgeAnderson Oh, what I'm saying is arcane as heck. I doubt 10% of electrical inspectors even know it. You're absolutely correct, retasking this for a 240V+ground+no neutral is fine. the only thing unacceptable here is the NEMA 10 socket. (well and the breaker lol). – Harper - Reinstate Monica Mar 16 at 19:56
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    That wiring isn't fine - it's not in conduit and runs right between the panelling and the face of the stud!. One little nail tacked through the wall, especially with that 60A 2-pole behind it, could make one hell of a pop. – J... Mar 17 at 13:26
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You had a pretty accurate knowledge of what was wrong with the outlet and George summed it up nicely. One thought to make it safe, and usable, would be to remove the outlet and add a regular metal box, cover and 20 Amp GFCI outlet. At the panel, add a 20 Amp breaker right below the existing 60 as a slot's already available. Remove the black wire from the 60 amp breaker and add install it in the 20 Amp breaker. Remove the white wire from the 60 amp breaker and install it on the neutral buss bar. Check that the ground wire from that circuit is actually connected to the ground buss bar and not the neutral bar. If it is connected to the neutral, remove it and install it on the ground bar.

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It looks like the receptacle is a 10-30R, which as I understand is frowned upon today because it doesn't provide a ground prong.

Indeed, the socket is nominally two hots and a Neutral.

IIRC it used to be allowed to use the neutral as a ground in certain circumstances, but it was never allowed to use a ground as a neutral. So the nema 10 socket has to go. Furthermore tha 60A breaker also has to go.

If you want to keep the existing wiring then I think you have a few options.

  1. Reduce the breaker to 30A and convert it to a 240V only 30A circuit (with a 6-30 socket)
  2. Reduce the breaker to 20A and convert it to a 240V only 20A circuit (with a 6-20 socket)
  3. Reduce the breaker to 20A, turn the white wire into a neutral and convert it to a 120V only circuit.

Personally for now i'd just disconnect it, if/when you have a need for more power in your garage then you can determine whether the wire you have in place is useful for the load you plan to connect and buy the relavent breaker and socket at that time.

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