I’ve got a red-black-white 220v outlet that I want to use as a junction box to split off into two 110v circuits. The newly created junction box would be 18" away from an existing 110v (grounded) receptacle on a different circuit. I want to run the ground wire from this 110v circuit to ground the 220v metal junction box and to become the ground for the new 110v circuits. So-- one new 110v circuit would be red-white-green and the other new 110v circuit would be black-white-green. (Plus I’ll change the breakers to 20 amps) I’m sure this would work but is it legal?
If I personally saw that someone had done that in my house or garage, I’d have a few choice words for the electrician that had wired it up.
That said, I think from an NEC point of view, you’re ok, provided that you change the breaker to 20A as you mentioned, and also provided that both circuits are fed from the same panel. If the circuits are fed from different panels, using the ground from one on the other is a big no-no.
As always, local codes may vary.
I’d check with an electrician … as you said, it would work … but is it safe … something about sharing a neutral across two different legs sounds fishy … not to mention a ground wire of (assumed) smaller gauge … why do you want three different 20 amp circuits within 18" of each other?
Thanks for your quick responses.
It’s for a basement hobby shop. The shop side of the wall is unfinished and the other side of the wall is finished and has an outlet (on the finished side of the wall). I would like the circuits to be separate. These two boxes would be close but on opposite sides of the wall.
My thought was that the neutral wire is tied to the ground at the box and ground is ground no matter where in the circuit. My only alternative is to rewire from the breaker but I can’t figure out an accessible path to feed new wire so I came up with this Mickey Mouse idea. 
And also I can’t think of why it wouldn’t be safe.
As long as everything works it’s safe. If the 240 neutral breaks for some reason (corrosion, water damage, mice chew through it, etc) then the two 120 circuits will act as a voltage divider with the voltage on each dependent on the loads. You could easily end up with an overvoltage situation on one of the circuits that could cause anything attached to it to fail and possibly catch fire.
It’s also going to confuse anyone who comes along later and makes changes or tries to do maintenance on it. For example, someone could theoretically disconnect the circuit where the ground is attached, thinking that they are only removing that one circuit and not realizing that they are disconnecting the ground to a completely unrelated circuit.
That said, I believe the NEC allows it.
The only reason I question the safety is that it is not common practice … the neutral wire does carry the return current although in your case they would be 180º out of phase …
My own little shop has just one 20 amp circuit and I’ve never had a problem, but I generally only run one power tool at a time … at most I leave the table saw running while I use a worm-drive saw to trim up pieces …
I did something somewhat similar when I replaced the electric cooktop with a natural gas one in the house I bought after my divorce. There was a 220v plug for the electric cooktop where I needed a 110v for the igniter for the gas unit. I disconnected one hot leg, replaced the breaker in the panel and wired everything up using the remaining hot, neutral, and ground. Works fine. I don’t see why what you’re wanting to do wouldn’t work, although I would probably mark the off-color hot leg with some electrical tape of the proper color, just to be nice to the next guy in the box.
Would this be any better?
Instead of a junction box, I could build a sub breaker box (same grounding idea but separate 20 amp breakers). The new outlets and sub breaker box would be on the unfinished and open side anyway so it would have easy access. I think I will do that. And tape a quick explanation to the inside of the new breaker box. Again, I can’t come up with another way to get enough power down there.
Thanks again!
When I renovated my kitchen, my dumb electrician apparently didn’t get the memo that the new wall oven/microwave required a 240v circuit, and ran 12/2 for 120v there instead. Nobody caught the error in time, and by the time the oven was installed, it would have been a real PITA to redo the wiring. So instead, he combined the 120v circuit with a separate receptacle circuit for the fridge onto a two-pole breaker, then ran a short length of wiremold from the fridge receptacle to the oven to provide the other hot leg for 240v.
It’s kind of the opposite of your situation. I don’t know if it’s against code, but it really annoys me. It’s the one thing about that job that still really cheeses me off. (Every other person who worked on it was a true craftsman who did fantastic work.)
Anyway, my thoughts as an amateur who doesn’t know what he’s talking about is that regardless of what code allows, mixing 240v and 120v devices on the same branch circuit is probably bad practice.
I’ve done this myself and it passed inspection back in the early 1990’s … I had a natural gas furnace that only needed 110V to run a little fan … but with the whole climate change problems being so obvious by then I re-wired the outlet for a 220V electric heater and just used one leg for the time being … I sold that house to an electrician and he confirmed that what I did was perfectly fine …
The question is whether we can split the two legs into individual 110V circuits …
@watchwolf49 – why the 180 out of phase?
Maybe I can do away with one of those extra circuits. Seems like the vote here to get reliable 110v is to tape off one of the hot wires so that the neutral is being used in only one circuit?
That’s how 240V split-phase AC works. When one hot wire is high, the other is low, and vice versa. Repeat 60 times a second.
If you imagine a graph of two opposite sine waves, with voltage on the vertical axis, the neutral wire is the horizontal axis at 0v. If you measure the distance from neutral to either of the sine waves, then average[sup]1[/sup] up the absolute values, it comes out to about 120v. If you measure the distance between the two sine waves, it comes out to 240v.
[sub]1. Technically the quadratic mean or root mean square value.[/sub]
Thanx friedo
Try it … if you keep tripping your breaker then we can work from there … ECG is usually right about these things and in fact you may be prettily fine splitting the circuit into two …
It will be safe to use both legs of this circuit with the shared neutral as long as (1) you make sure the legs remain on opposite phases and (2) the handles of the breakers are tied together so that if one leg is shut off or trips both legs shut off. This is most easily done using a 20A two-pole breaker to replace the existing two-pole breaker.
Also, you can share the safety ground with other circuits but make sure you do not accidentally share the neutrals. Neutrals and ground should connect together only at the main circuit box.
You could look into putting in a subpanel, but I think you might run into a problem with the minimum size conductor for the safety ground - if I recall correctly, grabbing a 12 AWG ground from an nearby circuit isn’t going to do it.
ETA: Here’s a linkwhich addresses the minimum size ground wire you need if you put in a subpanel…
I’ve got a somewhat similar problem to the OP or watchwolf49. Mine is just different enough though …
I have two separate 220V feeds to a water heater and an adjacent HVAC air handler unit installed in a utility space. There’s no other power nearby.
I’d like to tap off an ordinary 110V outlet off the 220V supply in the same utility space to install a low power load. Think something like a wall wart for a WiFi. IOW, something consuming well under a half amp continuous load and with no big start-up spike.
Is it kosher *enough *to just tap off one hot leg and the neutral? So unlike watchwolf I’d still have the 220V two-hot feed from the panel and the 220V load is still in use. Plus my small 110 parasite load on one leg.
I can use proper boxes, conduits, wire gauges, etc., so the job meets the mechanical standards of the standard electrical code.
It’s the electrical standards I’m unsure of. I doubt the split-phase breaker would be sensitive to the unbalance; heck I don’t know how well phase-balanced the loads are in either the water heater or the HVAC to begin with.
Any thoughts?
The big potential problem is you’ve now got a 15- or 20-amp rated receptacle sitting on a breaker that’s probably sized for 30 or more amps for your HVAC. It’ll work fine (make sure you take the ground as well as the hot and neutral) but in the event of an overload, you could potentially start a fire in the receptacle before the breaker trips. The risk is low, but who knows what somebody will plug into that thing at a later date.
A subpanel with two branch circuits for the HVAC (240v) and the new receptacle (120v) would be ideal, but obviously isn’t practical in all situations.
It’s also a violation of the NEC, for exactly that reason.
OK, scratch that hare-brained scheme. Time for plan B.
I suppose I could disconnect the feed going to one or the other load, terminate that at the input to a small sub-panel, then run a split-phase breaker in the sub-panel to the original load and a separate 15A single phase breaker to my new outlet box alongside.
The original load would be protected by two, say, 30A breakers in series, one at the base panel and one at the subpanel. Smells kosher but IANA expert.
But that leaves the problem that the feed to the panel is just the original 30A or whatever. I think that a panel’s upstream input breaker and wire doesn’t have to be sized to support 100% load on all output breakers simultaneously. But I bet I’m getting short of headroom versus whatever the requirements are.
Any clever solutions that don’t require pulling additional or larger wires in what’s a very difficult environment?
Thanks Marvin, I did check your link and I can’t imagine anywhere near 20 amps being pulled through the two circuits so I think I’m fine with #12 wire. I am going to try using 2 110v circuits with a sub breaker panel. I’ll let you know if I have trouble.