Given I have a 10/3 AWG cable coming into the panel, is one two-pole 30 amp breaker safer than two 15 amp tied breakers?

Question

The short story for how I got to where I am is that I bought a house where the main panel was over 55 years old, so I hired someone that I don't intend to hire again to replace it. The new box is in place, but he did a strange mix of repeating old mistakes and adding new ones. A bit of research and a couple trips the box store later, everything is working, my house is still standing, and I'm still alive.

In the course of correcting his mistakes, I found that have two 15 amp breakers feeding one 10/3 cable. One is connected to the red wire and the other to the black wire. They share the same 10 AWG white neutral. Everything I've read tells me that, as far as the math goes, that's okay because the neutral is still only handling 30 amps, which 10 AWG can handle. As I understand it, the right thing to do per code/standards is to tie them together so they are switched together (but I don't think that will make them trip together without a specific kit that turns it into a 2 pole breaker). What I did instead of tying them together was to put a 30 amp 2-pole breaker.

My question is, is that okay for a bunch of typical 120v receptacles? I'm worried that the breakpoint is too high and and that I should be using two 15 amp breakers with a common trip.

Answer 1

so I hired someone that I don't intend to hire again to replace it. .... In the course of correcting his mistakes...

Well it is true we're really scraping the bottom of the barrel for electricians these days. However, honestly, this sounds like something else entirely: it sounds like a person who knows a little bit about electricity and has... well, we used to call it "sophomoric", but a pair of scientists named Dunning and Kruger made a more in-depth study.

I'm afraid I'm not referring to the electrician :)

It is readily apparent that a) you're seeing a lot of stuff that is "New to You"... and b) you're doing a lot of random stuff that seems right, but you don't actually know. At least, you are asking...

Receptacles must match the breaker.

This is NEC 210.21.

15A receptacles belong on 15A circuits.
20A receptacles belong on 20A circuits.
There is an exception allowing 15A receptacles on 20A circuits if there are 2 or more sockets because 15A devices are internally rated for 20A pass-through. Not 30A.
30A receptacles belong on 30A circuits. That is it, no crossovers, no exceptions.

That receptacle you're familiar with that you've seen everywhere, has 2 sockets for purpose of the above.

You will need to change each receptacle and spur on that circuit to a 30A receptacle. You may use a NEMA TT30, NEMA 6-30 or NEMA 14-30.

"I don't want to do that, though!"

Wire is always allowed to be larger

Anytime Code requires wire be a particular size, you are always allowed to use a larger size. There is no penalty for this, except the practical difficulty of fitting the larger wire to terminals.

The exception is that if conductors are enlarged, grounds must be enlarged in proportion, but that works fine with #10.

What this means is you're in perfectly good standing to have #10 wire in a 20A or 15A circuit.

Enlarging wires is often done to compensate for voltage drop on a long cable run, or because old wiring (e.g. from a decommissioned electric dryer or range) is being re-purposed for new circuits.

The circuit size is the smallest wire size in it.

In residential, where the Tap Rules are generally unavailable, the circuit size is defined by the smallest (in-wall) wires in it. If a circuit has a mix of #10, #12 and #14 wire, it must be a 15A circuit.

If a circuit has a mix of #12 and #10 wire, it must be a 20A circuit (15A is acceptable).

It can only be a 30A circuit if every single wire in the walls is 30A (#10 copper).

Generally anything with #10, #14 and #12 is defined by NEC 240.4. It cuts out some really weird exceptions for certain motor and welder loads, but this is generally not something you'll encounter on receptacle and light wiring.

Multi-Wire Branch Circuits are a thing

AKA shared-neutral or "Edison" circuits (for whom we have to 'thank'). These are +110V and -110V (at least in Edison's day) with neutral in the middle. Only 3 wires are needed + ground, but it has the performance of 2 circuits. The neutral handles only differential current.

These must be phased correctly to avoid overloading neutral - the 2 phases must be 240V apart, not 0V apart. They cannot land on a tandem. To protect maintainers, they must have "handle ties" or be a 2-pole breaker.

The difference is that handle-ties do not provide common trip and most 2-pole breakers do. However MWBCs do not require common trip if they serve only 120V loads. (yeah, they can serve 240V loads also, and then, they need common trip).

MWBCs also require that neutral be pigtailed and do not use the device itself for splicing neutral. Neutral must remain continuous even if the device is removed.

Cleaning it up

The only mistake you have described is lack of handle ties on a MWBC. That's a common error, I'm not going to judge the electrician incompetent over that. Hopefully they are on opposite poles (240V between them).

Everything I've read tells me that, as far as the math goes, that's okay because the neutral is still only handling 30 amps, which 10 AWG can handle.

You didn't read that in NEC/CEC. You might get away with it on the #10, but what about the #12 or #14 beyond the #10? You haven't fully mapped the circuit and don't know what all the wire sizes are.

If you have mapped the whole circuit and found nothing smaller than #12, then you can use 20A as long as there are 2 sockets on each leg (or the 1 socket is 20A). If you have 15A on one leg but the other is all 20A wire, then you can use a 15A and 20A breaker with a handle tie. I have one of those!

I'm worried that the breakpoint is too high and and that I should be using two 15 amp breakers with a common trip.

Correct, that is the problem, but common trip is not required for MWBC.

Answer 2

You are a bit confused about Volts vs. Amps.

The rating of the breaker is the current = Amps. 15A and 20A are typical for lighting and receptacles. 30A is typical for a dryer. 40A for an oven. etc.

A breaker panel has two hot wires coming in and a neutral. The hot wires are 120V each from the neutral (i.e., measure with a multimeter and you'll get 120V) and 240V from each other. (Some places 208V, we'll ignore that for now.) Most breaker panels are designed so that the two legs or phases connected to the two incoming hot wires alternate. That has the neat side effect that if you have one breaker below another breaker you will get 240V between the two breaker hot wires. That means you can connect a 240V appliance (water heater, dryer, oven, etc.) to two breakers - but for safety's sake they must be common trip - i.e., they always trip at the same time. The way you do that is a double breaker. That is all pretty standard - single breakers for 120V circuits, double breakers for 240V circuits.

Now where it gets interesting is a Multi-Wire Branch Circuit or MWBC. In an MWBC you use two breakers on opposite hots/legs/phases, so you get 240V between the two hots. But it is still 120V from each hot to the shared neutral. The neat thing is that the neutral gets the difference between the hots in terms of current. If one hot is currently using 15A and the other hot is using 0A then the neutral will have 15A. If both hots have 15A each then the neutral will have no current at all. In other words, a properly wired MWBC will have a neutral with the same effective rating and usage as the hots. If the hots are on a pair of 15A breakers (installed correctly) then the neutral will also only ever have 15A (more and one or both hot breakers will trip) and not the perhaps intuitive 30A.

Now we get to the complication: In order to stay safe, MWBCs must be on breakers that are next to each other (which makes sure they are on opposite hots/legs/phases), must not be on a pair of half-size breakers (properly done quads excepted), and must be either a double-breaker (which takes care of the first two requirements automatically) or have a handle tie to connect them together. These requirements make sure that if you turn off one of the hots to work on a circuit you also turn off the other hot. Otherwise you could run into some very dangerous situations.

Back to your specific situation:

• Assuming these circuits are for standard 15A or 20A receptacles, you can not use a 30A breaker, only a 15A or 20A breaker.
• If all of the wiring involved is indeed 10 AWG, you can use a 20A breaker. If some of the wiring is 12 AWG, 20A is still OK. If any of it is 14 AWG then you must use a 15A breaker.
• The best solution is a 15A or 20A double breaker.
• To make 100% certain everything is right (and no half-size messed up breakers involved), check voltage between the two hots and between each hot and neutral. If you get ~ 240V between hots and ~ 120V between each hot and neutral then you are all set.

Hypothetically (but such an installation would be against code), if you have a 20A MWBC, which would normally be on 12 AWG wire, on 10 AWG wire, and it was installed incorrectly so that the red and black wires were on separate 15A breakers (so each protected against current over 15A) but on the same leg/phase so that the neutral could have up to 30A, then in theory, the 10 AWG wire would work for the neutral, because neutral does not have direct over-current protection. Bumping up to 30A breakers in that situation would actually be dangerous because then the neutral could have up to 60A before the breakers would trip, melting the wire and burning down your house.