Using a three-wire cable across two breakers

Question

I recently had a new panel installed and asked for a couple new runs to be installed at the same time.

The electrician used a 14/3 cable (red-black-white) for each of these new runs, informing me that 1A = Red / 1B = Black and 2A = Red / 2B = Black. I think he did this as a favor to me as I told him I wanted to get my basement fridge on its own circuit. I've wired up the circuit; breaking the tab in the outlet with Red on top, black on bottom and each outlet can be control by their own 15amp breaker from the box.

I've read a few other posts, specifically this answer about this situation and I'm trying to understand about how/why/when the neutral could end up in a situation of 30amps. I was going to take my second run and split it at a box with A (Red) using 2 wire for my overhead lights and B (Black) 2 wired going to the outlets. This isn't terribly different than the first scenario with the split outlet, just that the two loads aren't beside each other in the final outlet. I just want to ensure I'm not wrong to wire it up this way or could get into the situation of the 30amp neutral.

Per requests, here is an image of the breakers in the panel. Bottom left 2 groups. I currently have one off as they aren't hooked up.

Edit/update: Testing with a multimeter shows:

• red/white = 120v
• black/white =120v
• red/black = 0v --per comments this is a problem.

Answer 1

DANGER!

This is totally wrong. You have 2 pairs of 1/2-size breakers - a.k.a., "double stuff" or "tandem". If you measure voltage between black and red (i.e., between a pair of hots) you will find 0V instead of 240V. If this was done by an actual licensed electrician, he should have his license revoked. If it was done by a handyman-acting-as-electrician, this is why most places don't allow that.

Note that this would be perfectly fine with two separate cables - i.e., each breaker getting its own black & white wires. But sharing (MWBC) only works if the pair of breakers are (a) on opposite poles/legs so that you get 240V between the hots and (b) the breakers are physically paired (handle tie or double-breaker) like the 40A pair across from these breakers.

And now for the really crazy part. The usual reason to go for 1/2 size breakers is because you don't have enough space to put full size breakers. But you have more space right below above (picture is upside down, or panel is upside down) the new breakers! You could easily have 2 pairs (i.e., 2 double breakers) of 15A full-size breakers and everything would be just fine.

There is actually one other alternative. A quadplex breaker:

The beauty of a quadplex is that the inner and outer pairs each function like a traditional double breaker. Red and black to the outer breakers + white neutral. Red and black to the inner breakers + white neutral. Done.

Difference in cost between 1 quadplex and 2 tandem breakers at Home Depot? Less than $11. Probably even less of a difference at an electrical supply house.

Answer 2

Multi-wire branch circuit, or MWBC.

The two breakers should be handle-tied - are they? Honestly, for a new install they should be a "double" breaker with only one handle - it's a better approach and costs about the same.

If you are the one wiring things to the circuit, the neutral needs to be pigtailed in any box where there are multiple devices or where the neutral continues on to another box (So if you have a double-gang box with two duplex outlets, the incoming neutral needs to be wirenutted to two pigtails, and the pigtails go to the outlets. If you have a single outlet but the wires go to another box, you need to wirenut one pigtail and the incoming and outgoing wire, and connect the pigtail to the device neutral.)

Due to the way that USA/Canada power works, the neutral will usually carry less than 15A, never more, if the circuit is wired appropriately. The Hots are on different legs, and should have 240V between hots. When 10 amps of load are on each leg, the neutral current is ... 0A. When 12A of load are on one leg and 5 A of load are on the other legs, the neutral current is 7 A. When 15A are on one leg and nothing on the other, the neutral current will be 15A.

The only way you would get 30A on the neutral would be if it was improperly installed with two hots on the same leg (0V between hots.)

Edit to add: Per your picture, without the deep knowledge of how your panel works that some of our members have ingrained, I think you have a serious problem here (and it's evidently not even done that way because of no spaces left, which would still be wrong.) I'm dead serious about reporting this to the licensing board(s). Both the electrician and inspector are at fault for allowing this.

Answer 3

Think about it...

Neutral is the return line. If it's shared, it's doing double duty.

Suppose Joe and Jodi are each enjoying 1.5 GPM showers. How much is going down the drain? Would you have any reason to think anything other than 3.0 GPM?

If Jim and Jana were each operating a 15 GPM hydraulic mill, and they shared a return pipe, would you believe for a minute the return line didn't handle 30 GPM?

So you know, intuitively, that something pretty special would have to be happening for the neutral to be carrying other than 30 amps. So you are exactly right to question this.

What special thing is happening?

Negative voltage

You can't have negative hydraulic pressure (because you can't go below zero PSI or it'll just cavitate). But voltage is a completely different deal. Voltage can be any degree positive or negative.

Well now, suddenly we have an option. We can run Jim's tool on positive pressure, Jana's tool on negative pressure, and the most of the fluid does a "big loop" - through Jim's then Jana's tool, with the return line flowing two ways to even out any glitches and bumps. And now, we can use a return line rated 15 GPM instead of 30. But we have to do the positive/negative thing.

There's a flaw in this metaphor that turns out to be trivial. AC power reverses itself 120 times a second (in places MWBCs are common: this parenthetical is for the nitpickers). So, this positive/negative metaphor is misnamed. However, since all the voltages reverse at the same time, the metaphor is still valid, it's just correct half the time and neatly inverted half the time.

Several big dangers...

There's an obvious and serious blunder: It's very easy to wire up the circuit so that both hot legs are on the same pole. ("positive" in our flawed metaphor). In that case, yes, the neutral would carry 30A. You'd be able to test that by comparing the voltage of the two hots. If there is 0 volts between them (i.e. they are both positive at the same time), then they're pushing in the same direction and driving that neutral up to 30A. If there is 240V between them, then one is pushing while the other pulls, and we have the desired effect: Neutral only handles differential (imbalance) current.

Remember. Neutrals don't have breakers.

There's another danger anytime you have 2 circuits sharing wires or outlets: Imagine a worker needs to repair Jim and Jana's hydraulic return line. The worker carefully Lockout/Tagout's Jim's hydraulic line. Problem? Bigtime. Jana's tool is still live. So Jim and Jana's shutoff valves better be ganged.

... lead to very important rules

First, the breakers must be carefully phased to be opposite phases. This is where it helps to read about how breaker panels are internally phased. The two legs of the MWBC absolutely, positively, need to be 240V apart - never 0V apart. And if you read that whole Q&A you can see where "double-stuff" breakers are a huge problem here.

But the "necessity of phasing" issue intersects another problem: both legs need a ganged maintenance disconnect, such as a handle-tie or just being a 2-pole breaker. And here's the thing... on most panels (not GE), a factory handle-tie will also enforce correct phasing; it won't fit on breakers that are mis-phased for an MWBC. And on every panel, a 2-pole breaker will enforce correct phasing (unless you defeat the keying on a GE breaker, which people have done).

• Factory handle ties are notoriously difficult to find, and they also cost near as much as a breaker. Whereas 2-pole 15A and 20A breakers generally cost the same as 2 singles, so factory handle-ties are a waste of money and time IMO. Just use a 2-pole.

So, for both phasing and maintenance safety reasons, the two hots of any MWBC must be landed on a 2-pole breaker (or two singles with a factory handle-tie placed in the same position as a 2-pole).

2 circuits on the same yoke must also be handle-tied

Even if the above wasn't a rule, it's also a rule that when 2 circuits are on the same yoke (that split receptacle you mentioned), the 2 breakers must be handle-tied (or 2-pole). That's again to protect workers - typically a worker plugs in a radio and throws breakers off until the radio stops (turning all other breakers back on). That action MUST shut off the other circuit on that same yoke as well, so the worker doesn't get nailed by the other circuit.