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I am attempting to replace a 110v outlet with a 240v outlet. This outlet is behind the stove in my kitchen. It was formerly powering a gas range, and this new outlet will be used for an electric range. From what I can tell, the metal junction box that this outlet will go in is not grounded (the box itself), but should be. How should I ground this box, or do I need to ground the box itself?

Details:

Wire is fed to this box through non-metallic conduit. The wire itself is stranded aluminum, with 2 hot lines and 1 uninsulated ground line (see first image below). With the power on, I get 120v between each of the hot lines and the ground line (240v total between the hot lines), but only 30v between each hot line and the metal box itself. Given all this, I suspect the box itself is not grounded.

For other metal boxes with copper wire, I've seen a pigtail used to connect the ground wire to a ground screw on the box. I would do this, but this box lacks a ground screw and lacks a threaded hole for a ground screw. Also, the stranded, uninsulated aluminum ground wire is pretty short and very brittle - I don't think I could connect it directly to the box and still have enough wire to reach the outlet.

What is the typical way to ground this metal box? Is there a way other than replacing the conduit with metal conduit? I've thoroughly searched for information on this, but all the info I can find is in reference to copper wire, which seems easier to deal with than aluminum.

Image below shows the box and the three wires referenced above. The image below that shows the old outlet (only wired for 110v) and the new outlet I'm trying to wire up sitting next to it.

box and the three wires referenced above

old outlet (only wired for 110v) and the new outlet I'm trying to wire up sitting next to it

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    You really should fire the person who connected that up. You will not be able to make that circuit as a NEMA 10. They were banned/made illegal in 96. You could only use that 10-50 if you were replacing another 10-50 receptacle(you are not). We will need to know what the stove requires for power before giving you the right information
    – crip659
    Jul 5, 2023 at 22:12
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    That wire was originally two hots plus neutral, presumably for a previous ungrounded oven or range, which was allowed at the time (both "no ground" and "bare neutral" on that type of circuit). Then someone put in gas and put in an illegal (assuming grounding required by that time, which is likely the case) receptacle using one of the hots and using the bare wire as both neutral and ground. You are no longer grandfathered. (Well, maybe you are, I don't know your family :-) ) and have only two possible solutions: 1 - Get an oven that does not require neutral and that the Jul 5, 2023 at 22:16
  • existing wires are large enough to legally handle - then the two black wires are your hots and the bare wire is ground. 2 - Get an oven that needs neutral or that needs larger wires and run new wires. FYI, from what I can see here, you do not have non-metallic conduit (which would be great - pull out old wires and pull in new wires as needed) but rather non-metallic cable which can only be replaced, not added to. There is a possibility you may be able (if wires large enough and need neutral) to use the existing cable for hot/hot/neutral as originally designed and add a Jul 5, 2023 at 22:17
  • separate ground wire. The catch is that unlike some situations, here you will need a relatively large ground wire (because of the circuit size) so it is very unlikely you will have another suitable wire already in the kitchen, which means another run (could be easy, could be hard) for ground from here to the panel. Jul 5, 2023 at 22:20
  • Thank you, everyone! I'd be far outside my comfort level for wiring to fix this safely, so I think I'm going to call in an electrician and have them bring it up to code. I just moved into this house (built 1977), so no idea how long it's been like this or who originally wired it, unfortunately.
    – Dan Scott
    Jul 6, 2023 at 0:33

1 Answer 1

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Completely illegal from start to finish.

This is very bad work.

To start with, this is SEU cable. The bare mesh wire is allowed to be either ground or neutral but not both at once.

As far as the socket, you are confused because you think all modern circuits are grounded. True, but that is not a modern circuit. That is a legacy non-grounded socket that would have been banned with all the others back in 1966 except for the fact that the appliance industry has a really good lobby and got a stay of execution for awhile.

The fact that you have a shiny new one, that you just bought, ready to be installed, is an outrage. They are not legal to use except for exact replacement of a broken one, and should not be sold except after asking a lot of questions.

Every range made since 1966 has a procedure for changing the cord between 3-prong and 4-prong. The correct socket is NEMA 14-50.

What to do with this circuit, then?

The good news is the SEU cable is reusable. That is 3 conductors you do not need to replace, as long as they are large enough for the load. (And #6 aluminum supports both 40A and 50A). You need to either retrofit a ground wire, or fit a GFCI breaker in the panel and label the socket "GFCI Protected / No Equipment Ground".

A retrofit ground wire must be #10 copper and can go to any junction box with #10 or larger wire back to the same panel, e.g. water heater or A/C.

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  • Thank you so much! I'm very glad I asked before naively wiring this up. This is very helpful.
    – Dan Scott
    Jul 6, 2023 at 0:25

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