My garage has very limited power with only one outlet that I share with 3 bathrooms and every outdoor outlet, 4 total, on a 20 amp breaker. (House hastily thrown together in 97' for your reference) I trip the GFCI outlet (only one on the circuit) constantly while trying to work. Since I have to replace multiple sections of sheetrock along what coincidentally happens to be the most direct route from my only breaker box clear across the house, now would be a phenomenal time to add an additional circuit or two. I had an idea to run a single 4 wire (hot, hot, neutral, ground) 240v circuit with 40(+) amps and feed a subpanel where I would split it into 2 circuits of 110v at 20 amperes each. Now before I go and do something dumb, I have a few questions. I'm no stranger to electricity, in fact it happens to be my favorite skilled labor, but I am not an electrician and have no plans to stop learning especially about something so dangerous. So here it goes:

  1. If I have two hot legs running to two individual outlets, do they share the single neutral wire?
  2. If each leg is using all 20 amps, would a single neutral wire be forced to carry 40 amps?
  3. Whats the best way to add a subpanel containing 2 or more circuits in addition to a 240v circuit for future tools?
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    You realize you are not tripping the GFCI due to overload? If nobody is using power outdoors or in the bathrooms, you have the full 20A to yourself. You are tripping the circuit because of a ground fault either in one of your appliances or the wiring. If it's always one appliance, that appliance is faulty. A garage subpanel will not fix this because those will also be GFCIs and will still trip. – Harper - Reinstate Monica Jan 3 '19 at 16:07
  • Its particularly the pump i use to drain water from the inground pool that trips it. It only seems to trip when the pump has removed all the water and spins without resistance. Makes perfect sense this would cause the ground fault. – BST Love Jan 5 '19 at 0:45

Delta. stop Stop STOP! Wiring a subpanel will not fix your problem. Your problem is a GFCI trip, a subpanel won't help that! Fat wire and dedicated circuits won't help that! GFCI stands for Ground Fault Circuit Interruptor. You have a faulty appliance or possibly bad wiring that has a ground fault, the appliance is leaking to ground, or leaking through you and shocking you. If it's always one appliance, it's the appliance. If any appliance causes it, look in the outlet wiring for a neutral touching ground.

If you still want to wire this subpanel, awesome. You have a serious code violation with bathroom receptacles sharing a circuit with non-bathroom receptacles and it'd be good to be rid of that, and more power in the garage is never bad. But I want to be clear, when you finish your masterwork and plug the same tool in, it's gonna trip the new GFCI.

It depends how you wire your subpanel, but either way, you'll be fine. North American panels have two poles or legs, as shown here. So you are putting in a 40A subpanel, 4-wire (hot hot neutral ground) with #8 wire. Suppose you fully load two 20A circuits, one at 20A the other at 18A.

  • If you put both 120V/20A breakers on the same pole (via a double-stuff breaker, or in two spaces both "black" on the linked illustration), the black hot will load to 38A, the neutral will load to 38A, and there will be no activity on the red wire. You could load that to 40A@120V also.

  • If you put them on opposite poles (via a 2-pole breaker or in 1 black and 1 red space), then you will evenly split the load. The black wire will carry 20A, the red wire will carry 18A, and the difference of 2A will be carried on the neutral. Note that this is uses only half the capacity of your #8 40A wire. You can carry another 20A@240V.

To answer your questions:

  1. Once your subpanel is in, you can wire 2 subcircuits with a shared neutral (aka Multi-Wire Branch Circuit or MWBC), but I don't recommend it, because it does not play well with either GFCI or AFCI. This is largely a deprecated wiring method.

  2. The other problem with this method is the danger of overloading neutral. You must put both subcircuits on a 2-pole or handle-tied breaker for common trip and opposite poles, so neutral carries only difference current. 2-pole breakers are easily confused with double-stuff breakers, and if somebody "tries to save space" bymoving it to a double-stuff, it overloads neutral. /3 cable is the right way to wire the subpanel's feed, but stick with /2 for branches.

  3. The best way is in my second example above, conscientiously putting 120V branch circuits on opposite poles to the extent possible. I would recommend using a size bump to #6 wire, because the cost differential is not large, and this bumps you to 60A capacity, enough so you won't be worrying about power for a good while.

    Keep in mind you are allowed to oversubscribe a panel: if you had a 60A subpanel with twelve 20A 120V breakers evenly balanced on the panel, and also one 20A 240V, that is fine, because you are rather unlikely to max out all of them at once. If you followed along you know we have 140A of breakers on each 60A leg. That is fine.

  • First off, thank you, this is amazing. Secondly, the appliance tripping the ground fault is my pool pump. This is happening because neutral and ground are touching? Much to learn in Alternating current. (Diesel mechanic by trade.) Third, I am fully aware of the code violation. The house was wired this way when I moved in and i am second owner. I do not wish to include this circuit in my shop at all because of the multiple issues i have with it. Thank you again for the invaluable knowledge i will use to keep my home safe. – BST Love Jan 5 '19 at 1:11

When using separate hotts L1 & L2 each leg is 180 out of phase so each circuit drawing 20 amps and returning on the neutral will not overload the neutral. You will need the feeder wire sized for 40 amps for your sub panel but if I was going that far I would probably put in a larger sub panel, by today's code each bathroom would require their own 20 amp circuit and you have the existing outlets. I would probably put in a 100 amp panel but feed it with 60a from your main the difference in cost for the larger panel won't be very much and having some room for future is always a good idea and the wire will cost more because you will be going up in size.


Specifically regarding:

  1. If I have two hot legs running to two individual outlets, do they share the single neutral wire?
  2. If each leg is using all 20 amps, would a single neutral wire be forced to carry 40 amps?

There are two ways to wire multiple 20A 120V circuits. You can wire each one separately, in which case they use separate neutrals (e.g., 12/2 to each string of receptacles) or they can share the neutral. If they share the neutral, that is a Multiwire Branch Circuit or MWBC. An MWBC shares neutral, which saves the cost of one wire, and can do that because the current on the neutral from each leg cancels the other one out.

However, MWBCs are a little more confusing to wire than regular circuits and they don't work well if you need both GFCI and AFCI. Typically (depends on code in your area), GFCI is needed for bathrooms, kitchens and garages, and AFCI is needed in most areas of the house. If you need to have GFCI and AFCI then it gets relatively complicated with MWBCs since you can't get a double-pole breaker for an MWBC that provides both GFCI & AFCI protection. Since you are putting a subpanel in the garage, the distances - and therefore the actual wire costs - won't be so big.

End result - go with a relatively big subpanel (as described elsewhere) and put in as many circuits as you need. Remember that you could have, for example, 5 20A circuits on a 60A feed - as long as you don't max. out everything at the same time (which would not be normal), it will work just fine.

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    I doubt i will ever need that much power in my 2 car garage/shop. The most i can imagine running simultaneously is my 2.5 hp air compressor, 15amp table saw, 1/3hp shop vacuum, perhaps an additional router if someone else is working with me, several low amp computers and corresponding components, and a 12vdc battery charger. Hence why I'd like multiple circuits so that one does not have to even attempt to maintain such a current. Thank you very much! – BST Love Jan 5 '19 at 1:25

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