# Can I share the neutral in 12/3 cable between two GFI circuits with different current ratings?

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?

• I am confused, is it 12/3 with Black, White, Bare(Earth) or 12/3 Red, Black, White? Also is the circuit 15 or 20 amps? You shouldn't have a 20 amp outlet on a 15 amp circuit. Oct 30, 2012 at 23:35
• You should check out this answer, it just about sums up everything you would want to know. diy.stackexchange.com/a/14031/6086 It should also be noted that you don't need a GFI outlet wired to another GFI outlet. Any outlet on the GFI load is protected. I feel you are making this more complicated than it needs to be. Oct 30, 2012 at 23:40
• If you share the neutral you'd have potentially 35 amps on a wire rated for 20. Sounds like a terrible idea to me. Oct 31, 2012 at 3:55
• @BradMace, the two lines are 180 degrees out of phase with each other, so if you are drawing 20 and 15 A, the neutral will only carry 5 A. Oct 31, 2012 at 4:12
• @maple_shaft 12/3 typically means 12/3 with ground. So you would have Black, Red, White, Bare/Green (all of which are 12 AWG). The equipment ground conductor is not counted in the conductor count. Oct 31, 2012 at 15:03

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.

# Multiwire Branch Circuits

## Disconnecting Means

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.

## Legs

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.

## GFCI

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.

## Wire Size

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.

## Receptacle Rating

NEC article 210.21(B)(3), specifies the receptacle rating based on the circuit rating.

# Proper Installation

Now that you know more than you've ever wanted to know, here is what you need to know.

## In the service panel

• Install a 20A double pole GFCI breaker in the service panel.
• Connect the bare (equipment ground) wire (from the circuit) to the ground buss bar in the service panel.
• Connect the white grounded (neutral) wire (from the circuit) to the neutral terminal of the GFCI breaker.
• Connect the white grounded (neutral) wire (from the GFCI breaker) to the neutral buss bar.
• Connect the black ungrounded (hot) wire to one terminal of the double pole GFCI breaker.
• Connect the red ungrounded (hot) wire to the other terminal of the double pole GFCI breaker.

## At the first junction

• Connect the black or red ungrounded (hot) wire to the brass screw on the receptacle.
• Connect the white grounded (neutral) wire from the 12/3 cable to the white ungrounded (neutral) wire leading to the next junction point, and to a short length of scrap wire (pigtail) using a twist-on wire cap (or other approved method).
• Connect the pigtail from the previous step to the silver screw on the receptacle.
• Connect a short piece of scrap ground wire to the green screw on the receptacle.
• Connect all equipment ground conductors together, and to the junction box if a metal box is used.
• Connect the red or black ungrounded (hot) wire to the black ungrounded (hot) wire leading to the next junction point.

## At the next junction

• Connect the red or black ungrounded (hot) wire from the previous junction to the brass screw on the receptacle.
• Connect the white grounded (neutral) wire to the silver screw on the receptacle.
• Connect the equipment ground wire to the green screw on the receptacle, and to the junction box if a metal box is used.

## Receptacle Ratings

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.

## Daisy Chaining

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.

# TL;DR

• You'll need a 20A double pole GFCI breaker. Two single pole GFCI breakers will not work.
• If you use a 20A overcurrent device, you must have more than one 15A receptacle on that leg of the circuit. Otherwise, you'll have to use a 20A receptacle.

As always, if you don't feel comfortable with your knowledge or skill level, don't be afraid to contact a qualified Electrician.

• Wow! I don't know about anyone else, but I learned things. Awesome answer! Oct 31, 2012 at 16:30
• +1. Great answer. I'd +2 if I could.
– mac
Oct 31, 2012 at 17:03
• Out of idle curiosity, is it legal to use a three-phase MWBC? May 17, 2017 at 5:57
• great answer, but I have one complaint / observation - using handle ties may not cause both breakers to trip if a single one trips - if I understand correctly the goal is making it easy to turn off all current at once. Nov 28, 2017 at 17:36
• @SomeoneSomewhere -- the Code takes absolutely no issue with a 3ph MWBC in a Wye system; they aren't used in Delta work though, save for 1ph-equivalent MWBCs in "high leg" deltas Apr 28, 2019 at 17:40

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.

• One phase, two legs. Not two phases. May 25, 2015 at 0:48
• Stranded and solid have the same ampacity. Table 300.15b16 allows higher ampacity on 12 gauge at higher temps. But that doesn't help because 240.4(D) limits it to 20A in any case. Apr 28, 2019 at 18:24

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.

• You are speaking as if a GFCI+receptacle combo device is the only kind of GFCI there is. OP is using GFCI+circuit breaker combo devices. Your solution, while valid for GFCI receptacles, will not work for OP. Apr 28, 2019 at 18:13