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I have two 60A (to my knowledge) split-phase outlets (14-50R) in a commercial building, which I'd like to break out semi-permanently to 120V outlets, ideally without having an electrician modify the building.

I took an educated chance and wired up two branches of 120v (hot-neutral + ground) from one of these receptacles to power an LED screen array. The wiring was done inside the plug, so there are two three-wire cables coming out, terminated by "Powercon" connectors (but they could just as easily be 5-15 style). Each branch has an equal number of panels, and so the branches are about as close to balanced as possible (although there is always the possibility of some panels drawing more than others).

This has been working well, but a bit of self-doubt has set in; most importantly, is this safe? Would it be smarter to build a small sub panel which is fed through the 14-50 plug? If it is acceptable, can I safely do it again with the other circuit, without the guarantee of a mostly balanced load?

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You should make a plug-in subpanel. Remember to keep ground and neutral isolated in the subpanel.

You don't need to worry about balancing loads - if the loads are balanced, the current in the neutral is zero, otherwise it's more than zero, in any case it's fine if the circuit is properly wired. This is the way it works into your house/building and at every sub-panel feed.

You want a sub-panel so that there are more-reasonable sized (15-20 amp) breakers on your 120V circuits, rather than a 60 amp - unless your 120V circuit is a 60 amp circuit with all conductors properly sized for that load.

  • Would you mind clarifying what you mean by "keep ground and neutral isolated"? I assume this just means electrically separate (not 'grounding' the neutral) by running them separately back to the receptacle, but I want to be sure I'm not missing an important detail. – Justin Ryan May 17 '15 at 22:08
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    Most panels (which you'll be using one of as a sub-panel) have a means to join ground and neutral for use as a main panel, and a means to not join them for use as a sub-panel. If you have enough wires that you need an additional terminal-bar for neutral connections, that also needs to be isolated/insulated from the (grounded) metal case (and you need overcome any habitual tendency to put ground and grounded (neutral) on the same terminal bar, which is OK in a main panel but nowhere else.) – Ecnerwal May 17 '15 at 23:29
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I vote for installing a small subpanel, which you could feed from your 60A outlet.

You could probably surface-mount the small panel near the 14-50R receptacle and feed it by hard-wiring a length of 6/3 MC cable into it, clamp that to the box with a jake clamp and fasten it to the wall as many times as code calls for, put a 14-50R plug on the other end, and just plug it in.

Then you could use some 14/2 MC connected to a 15A breaker, or 12/2 MC cable connected to a 20A breaker in the subpanel, to feed a surface-mounted outlet box or two. Make sure the neutral and grounding buses are isolated in the subpanel, that the grounding bus is bonded to the subpanel housing, and that you have 4 wires running all the way back to the main panel (2 hot, neutral, grounding).

EDIT/Correction: Those LED screens are not 60A devices. You'd have to be creative to attach those devices to the 14-50R receptacle, and it isn't legal to put 15A or 20A receptacles on a 60A circuit because the receptacles themselves aren't protected by the circuit breaker. They have to be on branch circuits with their own 15A or 20A circuit breakers.

Hypothetically, let's say you invent a machine that requires 40A or 60A split phase service. The machine has something (or things) with a big draw, like motors and/or heating elements. It also has a number of small-draw devices built in, such as small light bulbs and/or LED lights on a panel, a couple of small LCD or LED readouts, and a 14" color LED screen. That's all good, you can feed all those small devices off of one or the other leg, fed through a 60A breaker. But you're also going to design the machine with built-in circuit breakers or fuses on the internal branch circuits to protect those smaller components.

Those Powerconn connectors are slick, but you're putting them on what amount to extension cords that are hard-wired into a 60A circuit and I'm guessing the cords aren't made with #6 wires? So you still have a loose, undersized extension cord that can serve up to 60A for a short period before it overheats and starts a fire. And that's probably also not legal.

Balancing the branch circuits isn't going to be an issue (unless your neutral comes loose). If your hot wires are connected correctly (connected to different legs in the panel, using a double-pole breaker), and the neutral is good, then the load on the neutral will never be bigger than the capacity of the breaker(s). The neutral (grounded) conductor only carries the imbalance of the loads on the two hot conductors. The neutral can't be overloaded, because the power from each of the hot legs cancels out the power from the other. If one leg is fully loaded and the other has no load, the neutral will only carry as much power as the loaded leg. Add any load to the other leg, and the load on the neutral goes down (not up).

  • It's legal to plug a 1A device into a 14-50R, it's just not legal to have 15/20A receptacles on heavier circuits as the outlets themselves then wouldn't be properly protected by the breaker. However, the overall conclusion is the same -- he needs a subpanel. – ThreePhaseEel May 17 '15 at 19:49
  • @ThreePhaseEel Thanks for the clarification. But what about the legality of branching a 120v circuit straight out of a 14-50R receptacle? – Craig May 17 '15 at 19:56
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    that, however, is illegal -- you need a subpanel for that to protect the downstream branch circuit wiring. – ThreePhaseEel May 17 '15 at 19:57
  • @ThreePhaseEel -- Looks like I'm gonna re-write my answer to do a better job of explaining what I was thinking... ;-) – Craig May 17 '15 at 20:12

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