Is this a "shared neutral" situation?

Question

In 2001, some electricians added two circuits to my house in California. I took a close look at their wiring and had some questions.

At the Service Panel, there are two 20-amp circuit breakers on the left side of the panel. Both breakers are connected to the same 12/3 Romex cable, with the red wire attached to one breaker and a black wire attached to a second breaker. The two breakers are not tied together.

The 12/3 cable feeds into the a junction box in the crawlspace where it connects to two 12/2 Romex cables. The red wire on the 12/3 cable attaches to a black wire on one 12/2 cable which provides power to my living room, and the black wire is connected to the black wire on a second 12/2 cable which provides power to a bedroom. Thus, the two 12/2 share a neutral.

This wiring configuration is confusing. Is this a "Shared neutral" situation? I am pretty sure that this was inspected in 2001, but is this configuration allowed by modern code? Was this allowed in 2001? What are the hazards of this sort of configuration? Does this present a hazard to the receptacles in the living room and bedroom?

Answer 1

Typically the connection of 12/3 to two side by side breakers (black on one, red on other) is used to run a 220v line. A 220v line has two hots - each carrying 110v - pulled from adjacent breakers on the panel. In such cases, the single white line is the 220v Neutral.

Speaking electrically : What this guy did was use 12/3 to carry 220v to a junction box where he divided it back up into 110 - which should be fine because in reality the 12/3 is two parallel 110v lines, with a single neutral. The single neutral isn't really a problem here - ultimately all lines share a single neutral back at the breaker box. In your case, the shared neutral stops at a junction box - and the lines to the outlets are on their own neutral lines.

Are there any hazards to the receptacles? No, as long as they're wired properly its no different than connecting the neutrals back at the neutral bus bar.

Speaking in terms of CODE and what's allowable - I honestly don't know if it's legal or not, but I cannot think of a reason why it wouldn't be. The junction boxes are accessible, and using 12/3 to carry 220v is legit, and each outlet is wired to 110 properly.

Answer 2

A "multi-wire branch circuit" typically has the breakers joined so that if someone turns off one breaker to work on the circuit, they don't miss the second circuit going to the same box. Since you branch off into a 12/2 for most of the circuit, I'm not sure if the code would allow separate breakers (hopefully Tester101 will chime in). But at the very least, I'd put a label on the crawlspace junction box that says "warning: multiple circuits".

Answer 3

When a circuit is complete, the neutral line carries current.

If you were to fully load one of the circuits (originally fed by the black wire) with 20 amps, and, at the same time, you were to load the other circuit (originally fed by the red) with 20 amps, you could have 40 amps returning on the single neutral (at least from the crawlspace junction box back to the panel) under certain circumstances.

While there is a good deal of over-engineering built into the code, you could be creating a risk of overloading that neutral line (basically an overheating risk, but perhaps a voltage drop issue as well) under certain circumstances. If you did, the breaker only senses the load on the hot leg, and that overload would not trip the 20+ draw traveling over the neutral.

SUPPLEMENT: [Please see the comments below. Also modified answer above, shown by italics, in light of comments.] As indicated by Tester101 and Matthew PK, if the two breakers are adjacent, they are on different legs of the panel and therefor should not cause an overload on the neutral. If they were on NON-adjacent slots (e.g., if they were on breaker slots 1 and 5), there could be a problem. Breakers can be shifted to different slots (or wires shifted), but only do so if you are well acquanted with electric work, and be certain to turn off the main breaker (often a large pull switch) before handling any individual circuit breakers.

Answer 4

Please note...the shared neutral line does not carry 40 amps if both 110 circuits are fully loaded. The shared neutral line actually carries 0 amps when both 110 circuits are fully loaded. Why? Because the current from one 110 volt circuit is 180 degrees out of phase with the other 110 volt line, therefore cancelling out in the neutral line. Trust me, the shared neutral line never carries more than 20 amps. If it did, the neutral line in a 12-3 cable would have to be larger than the black or red lines, but it isn't. Just saying...

This picture gets slightly more complicated with inductive loads in which case the currents between phase A and phase B may not necessarily be exactly 180 degrees out of phase, but the currents will still cancel each other out (in the neutral wire) to a degree. In any case, the current in the neutral wire will not exceed 20 amps.

Answer 5

Using a double pole breaker or a handle tie on a multi-wire branch circuit serves two purposes.

1. It ensures that both sides of the circuit are isolated together.
2. It tells people working on the panel that the breakers involved can't be moved around arbitrarily.

Isolating both sides of the circuit together is important because it avoids a very insidious danger. You isolate the circuit, test for dead and all looks well.