Do I need common trip breakers for a house wired with shared neutrals?

Question

My home inspector stated that my home is wired with shared neutrals for most circuits. I guess this makes sense, as long as the safety ground is present. He also said that most of the circuits with shared neutral do not have common trip breakers. He said I should replace them with common trip breakers.

For some reason the installer stripped the insulation on the romex back so far you can't identify what wires are part of what circuit. This complicates things quite a bit.

What type of breakers do I need to buy for my breaker box? What breakers in my box need to be replaced?

Answer 1

The National Electrical Code requires simultaneous disconnection of multiwire branch circuit ungrounded conductors.

It does NOT require common trip breakers.

Here are the pertinent articles:

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.

Informational Note: See 240.15(B) for information on the use of single-pole circuit breakers as the disconnecting means.

240.15(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1), (B)(2), (B)(3), and (B)(4).

(1) Multiwire Branch Circuit. Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multi-wire branch circuits that serve only single-phase line-to-neutral loads.

Notice the last Article 240.15(B)(1) allows you to use identified handle ties on single pole breakers for multiwire branch circuits.

Besides that, any house built under an older Code (like 1983) is normally grandfathered until you make major changes, like replacing the panel, and have to then upgrade to the new code requirements.

As far as determining which circuits are multiwire. You can start with the white and black on the 50 amp breakers those should be paired. Then any red wire would normally be in a multiwire cable with a black. Determining which red goes with which black will be your project.

Good luck and stay safe!

Answer 2

Shared neutral describes a variety of schemes, only one of which is a multi-wire branch circuit. All of them are sensitive to miswiring, because in the US, we don't put circuit breakers on the neutral. The neutral can be overloaded if the wiring isn't perfect.

If it is a multi-wire branch circuit, the two hots must be on opposite poles, so the neutral carries only differential (imbalance) current. Indeed, you need breakers with common shut-off, ** which in a circuit breaker panel usually amounts to a double breaker. A weakness of the Q-line panels is that handle-tying any two single breakers does not guarantee they are on opposite poles. A strength of the Q-line is that 2-pole breakers are quite compact and can go almost anywhere. So the right answer with this panel is use 2-pole breakers with this panel.

2-pole breakers do 2 things for you: 1) they assure you land on opposite poles (so you don't overload the neutral), and 2) they give you common maintenance shut-off. As a side-effect, 3) they do indeed give you common trip. That is a side-effect inherent in using breakers.

If it is not MWBC, then you must be very careful. In particular, all the hots on one pole, acting together at full power, must not be able to overload the neutral wire. There are more Code requirements discussed in NEC 215.4(A) and 225.7(B).

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** the requirement is the normal maintenance shut-off must de-energize all parts of the MWBC. Common trip is not a requirement. That is relevant if the circuit is fed by fuses, a shut-off switch will suffice, no need to assure both fuses blow at once. However in a breaker panel, breakers are the presumptive maintenance shut-off. Effectively this requires a 2-pole breaker or handle ties.

Answer 3

If only one side of a multi-wire branch circuit is disconnected, the devices on that side will appear to be "off", but not in a way that makes any attached wires safe to touch. While the neutral wire should ideally be at ground potential, it may have voltage on it if either side of the branch circuit is live. In most cases where someone would be expecting wires to be safe to touch, it would be because they deliberately switched off the breaker, and so disconnecting both legs when either leg is deliberately switched off would avoid the aforementioned hazard in such cases. If, however, a breaker was tripped, someone who was trying to investigate why might incorrectly assume that any wiring feeding the devices that no longer worked would be safe to touch, thus creating a potential hazard.

Having both sides of a multi-wire branch circuit switch off when either side is tripped is generally safer than having just one side trip(*), but the hazard from having just one side trip is small enough that many jurisdictions will regard it as tolerable. That doesn't mean having both sides trip wouldn't be better, but having one side trip may be acceptable nonetheless. On the other hand, unless there is some particular advantage to having an overcurrent trip just affect one side, having both sides trip would be better (especially if the breakers have a "tripped" indicator that could show which side tripped).

(*) If a region of a building is fed by one MWBC, and has some lighting on each leg, and if both sides trip and unexpectedly kill all the lighting, the resulting darkness could create hazards of its own. If it is understood that whenever anything is live, everything is must be treated as live, having an over-current trips only kill one side might be better than having everything go dark. On the other hand, having battery-backed emergency lighting might be better yet.