Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It only takes a minute to sign up.
Sign up to join this community
I think I know the answer to this question however I have been known to be wrong before!
I am using GFI breakers to protect multi outlet circuits. One is 15 amp and the other is 20 amp. I realized that while I am running 12/3 using the black for the 15 amp circuit and the red for the 20 amp circuit will work I only have one white.
Question is: Can you use the neutral for both GFI's or do you have to provide a separate neutral for both circuits?
What you're explaining is called a multiwire branch circuit (and has been discussed here many times before), which is where 2 ungrounded (hot) conductors will share a single grounded (neutral) conductor. There are special requirements for this type of circuit, which must be followed to insure safety. This type of installation is slightly more complicated when dealing with ground fault protection, because of the way ground fault circuit interrupters (GFCI) work.
NEC 2008
210.4 (B) Disconnecting Means Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
This means that the breaker handles must be tied together in the service panel, so that if one breaker trips it will cut power to both circuits. This can be accomplished using handle ties, or a double pole breaker.
In multiwire branch circuits, each ungrounded conductor must terminate to different legs of the system. If this is not done the currents on the grounded (neutral) conductor will add instead of cancelling out, possibly overloading the grounded (neutral) conductor leading to a fire. Again, this can be easily accomplished using a double pole breaker.
Ground fault circuit interrupting devices work by measuring the current on the ungrounded (hot) conductor, and the grounded (neutral) conductor. If the currents on these conductors differs by more than a specific amount, the circuit is opened preventing current from flowing. This can present a problem when using two separate GFCI breakers, because the current on the grounded (neutral) conductor will be the difference between the two ungrounded conductors.
Ungrounded conductor 1 = 13A
Ungrounded conductor 2 = 6A
Grounded conductor = 7A
In this situation, the breakers will detect a potential (false) ground fault and trip. This can easily be avoided by using a double pole GFCI breaker. This is because a double pole GFCI breaker monitors both ungrounded (hot) conductors, and a single shared grounded (neutral) conductor.
The conductors in a multiwire branch circuit (like in all branch circuits), must be appropriately sized based on the load to be served and the overcurrent device protecting the circuit.
NEC article 210.21(B)(3), specifies the receptacle rating based on the circuit rating.
Now that you know more than you've ever wanted to know, here is what you need to know.
If you used a 20A GFCI breaker in the panel, you'll want to use receptacles rated for 20A. However, if you have more than one receptacle on a single circuit, you can use receptacles rated at 15A. In a multiwire branch circuit situation, that means you'll need two receptacles fed by one ungrounded (hot) conductor to be able to use 15A receptacles. It's also important to be aware, that NEC considers a duplex receptacle as two receptacles. This means you can use a single 15A duplex receptacle, and be code compliant. Though Because the overcurrent device is rated at 20A, it may be appropriate to use 20A receptacles throughout the circuit.
If you are going to be feeding multiple receptacles off either leg of the multiwire branch circuit, you have to make sure that the grounded (neutral) conductor is unbroken throughout the entire circuit even when devices are disconnected. This means you won't be able to use the "load" side terminal on a receptacle, to feed the grounded (neutral) wire through to another receptacle.
As always, if you don't feel comfortable with your knowledge or skill level, don't be afraid to contact a qualified Electrician.
With a shared neutral, you would need to be using a 2-pole GFCI breaker, such as the Siemens QF220P, and with that, both circuits would need to use the same amperage breakers. If all if your wiring is 12 gauge copper (and are powering outlets), then you could switch the 15 amp circuit to be 20 amp and use a 2-pole 20 amp GFCI breaker.
The downside of this is that 2-pole GFCI breakers are expensive. It may be cheaper to wire one GFCI outlet on each circuit and wire the rest of the outlets on the load of the corresponding GFCI outlet using 12/2 cable. The 2-pole GFCI breaker is as expensive as about seven individual GFCI outlets. It will also be more convenient to reset the GFCI outlets when they trip since you'll be near them already, and won't have to travel to the breaker panel.
Using two individual breakers does not work because of the way that GFCI breakers are designed. They ensure that current flowing into the load on the hot wire is equal to the current flowing out of the load in the neutral wire. With the shared neutral, the two circuits sum their neutral currents together. The "trick" is that the two lines are 180 degrees out of phase of each other such that when the loads of both lines are equal, the neutral currents cancel each other out so that the neutral wire carries no current (-20 A + 20 A = 0 A). The two GFCI outlets in the breaker box don't know how divvy up this current if you connect the shared neutral to both, and would trip.
The two-pole GFCI breaker has only one incoming neutral line. It is wired to compare the sum of the currents on the two hot lines to the current on the neutral, so the GFCI functions as desired.
Also @BradMace I've run over 50A through a 12 gauge conductor. It's all in the rating. Most stranded 12 is rated at 30A but generally you just throw a 20A breaker on there because 12 solid isn't. And mind you these ratings are at an ambient temp of 95 degrees Celsius. So open conductors not in a raceway can carry more amperage, or if you're in Alaska like me good luck finding 95 degrees Celsius anywhere.
In single phase electrical wiring there are actually two phases each to ground carry 120V hence your red & black being your 240. If you share a neutral between 2 phases you actually cancel out the current between the two, so the neutral will only be carrying the difference between the two currents.
You could always get a harbor freight multimeter, wire you're GFCI the way you want and test the results when you trip it with the test button. My kitchen and dinning area wired in 1970 has a shared neutral on two different twin breakers not in the same slot. Fortunately they were on opposite phases of power so I didn't have to play in the main panel. One conductor was feeding the fridge, one counter outlet and a dinning outlet. I added a separate 15a circuit for the fridge (off a sub panel I fed with the 220 cook-top circuit I made when I converted to gas) and I am wiring the first GFCI with pigtails to avoid running the neutral through two GFCIs, but the last circuit in the line will trip with the second GFCI that I run through the load. I made a note in the main panel so then next time I'm in there I can tie the two hots into the same twin and connect the breaker together.
