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So... I tried swapping two outlets in my kitchen, and tripped the entire power panel!

The old outlets were wired with FOUR wires: red, black, white, bare.

old outlet with 4 wires

I wired the new outlets using the same scheme:

  • Red to one of the HOT terminals
  • Black to the other HOT terminal
  • White to one of the terminals labelled WHITE
  • Bare to ground

AFAIK, these were not GFI outlets. Or, at least are no test\reset buttons on the old outlets.

Also, each outlet are on there own circuit, with nothing else on the circuit.

Something else that's probably key to my scenario is that each of these circuits are "spanned" on the power panel: 15 amps + 15 amps

In the picture below, one of the outlets is the only thing on "24 and 22", while the other outlet is the only thing on "20 and 18".

power panel for these 2 outlets

I'm pretty sure I wired the new outlets correctly. But when I turned their circuit breaker back on - POOF - the entire board tripped and all the power in the house went off!

I'm starting to think these 15A+15A circuits require a special outlet capable of handling 30 amps, and that the outlets I tried to installed were "standard" outlets, which I think are rated for 15 amps.

Am I right? Do I need to buy a special outlet?

Or did I indeed wire these incorrectly?

UPDATE: Thanks for the replies! I got 2 new outlets and broke the tab off (hot side only), now everything works. For future visitors, this video explains this situation, and corroborates the advice the responders posted:

https://youtu.be/rapPtMscrI0

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  • If your first picture wasn't too small to be useful and is the old outlet, you could zoom in and see the reason things are different with new outlets.
    – Ecnerwal
    Commented Dec 11, 2022 at 23:24
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    I'm pretty sure I wired the new outlets correctly. But when I turned their circuit breaker back on - POOF - the entire board tripped and all the power in the house went off! well now you know
    – Traveler
    Commented Dec 12, 2022 at 1:36
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    Is your outlet a split outlet? By that, I mean is the top half of the outlet electrically separated from the bottom half, or are they connected?
    – Questor
    Commented Dec 12, 2022 at 20:03
  • Your picture isn't showing 30-amp circuits, it's showing 15-amp circuits with a common trip. Very different things.
    – Mark
    Commented Dec 13, 2022 at 3:34

4 Answers 4

30

Yes, 30A receptacles are different. If you have an electric clothes dryer you can see an example. But you didn't double the current. You doubled the voltage (120 to 240) and created a short circuit.

TL;DR Split Receptacles on MWBC

There is a special type of circuit called a Multi-Wire Branch Circuit. It uses two hot wires and a neutral to provide 2 x 120V power and/or (but not as common) 240V power. One very common usage is in a kitchen, to provide twice as much power with only 50% more wire (3 wires - black/red/white instead of the usual 2 black/white).

It requires pairs of breakers, which you have (that is not always done properly, especially with older installations), in order to make sure that:

  • The two hot wires are on different 120V "legs"
  • The two hot wires get turned off at the same time if you need to work on the circuit

There are a number of ways to use an MWBC. But the very common way is to use a split duplex receptacle. You remove the little metal tab between the two hot side screws and connect black to one and red to the other. Each gets 120V and they share the neutral (so don't remove the tab on the other side). If you don't remove the tab, you create a dead short between the two hot wires at 240V. Which darn well better trip a breaker. With a dead short it will usually trip the branch breaker before the main breaker. However, that is not guaranteed and the main breaker can trip first, the branch breaker can trip first, or both breakers can trip at the same time.

So the very simple fix here is to remove that little tab.

But GFCI is incredibly important, and has been required for many years now in kitchens - for very good reasons. You don't have to add GFCI if everything is OK. But you do have to add it when you change something. I don't know if a straight replacement would require GFCI or not, but in my opinion it makes sense to add it now.

GFCI on MWBC

The catch is that GFCI on an MWBC is tricky.

GFCI/receptacle combinations (the common "receptacles with Test/Reset buttons") can't handle an MWBC and do not have that removable tab and do not have a way to (safely!) connect two hot wires on different legs.

The usual way to add GFCI to an MWBC is to replace the double breaker with a double GFCI/breaker. That is relatively easy to do, but more expensive than a typical GFCI/receptacle.

The other way to add GFCI is to split the MWBC before the receptacles. Cut a bigger hole in the wall. Replace the single-gang box with a double-gang box. Pigtail two extensions onto the white neutral wire. One white + black goes to one GFCI/receptacle and the other white + red goes to another GFCI/receptacle, both in the one box. (You can actually extend one to another location if you want.) After the white wire split, each pair (black/white or red/white) functions just like a regular 120V circuit except for the double-breaker in the panel.

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    @FreeMan As I understand it (Harper can correct me if I got it wrong) an MWBC is technically one circuit, that happens to be a mixed 240V/120V circuit, so it provides twice the power (120V x 15A x 2 or 240V x 15A x 1 - instead of 120V x 15A x 1) of a regular 120V circuit using only one additional wire. If you've got two toaster ovens running at the same time on an MWBC, you will actually get 12A @ 240V if you look at the two hots (each showing 12A at the time) and the neutral (showing 0A). But each appliance will only see 120V. Commented Dec 12, 2022 at 17:40
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    Ah, fair point. I usually think of them as a way of sharing wiring, not increasing power available. OK, TBH, I hadn't ever though of them as a way of increasing power. Learn something new every day!
    – FreeMan
    Commented Dec 12, 2022 at 17:49
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    It's important to note that the two hots have to be on different legs of your power supply, not the same (which is governed by which slots in the panel they are placed). The reason they can share the neutral is that when one hot is going to the positive (wrt to neutral) side of the AC cycle, the other leg is going to the negative (wrt to neutral) side... and vice-versa. This has the canceling effect of making the current on neutral zero when both loads are equal, or at worst, 15A if only one load is in use. If both hots were on the same leg, the neutral could overheat at 30A.
    – user46053
    Commented Dec 12, 2022 at 19:35
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    @manassehkatz-Moving2Codidact thanks for your thorough reply. I got 2 new outlets and broke the tab off (hot side only), now everything works.
    – steeveedee
    Commented Dec 12, 2022 at 20:06
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    @FreeMan Many years ago I had an electrician wire me an extra outlet. He pointed out the small price difference between providing one normal 20A or a pair of 20As. Commented Dec 14, 2022 at 4:33
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There's 240V between red and black. 120V from each to white. So, you neglected to notice the tab between the two hot screws (the brass ones) being broken out on the outlets you replaced, and thus failed to wire them the same by not breaking it out on the new outlets. Makes a lovely dead short.

Snap it out and things should get much less interesting. Leave the one on the neutral side (silver screws) alone.

It's multiwire branch circuit, which means each half of the receptacle can provide the full rated current of the breaker - so if the toaster and the coffeemaker are both running at the same time, you get toast and coffee, not a pre-coffee journey to the breaker panel to reset one. So it's a pair of 15A circuits, not a 30A circuit.

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  • +1 for "pre-coffee journey to the breaker panel" :)
    – Martha
    Commented Dec 12, 2022 at 18:28
  • @Ecnerwal thanks for your thorough reply. I got 2 new outlets and broke the tab off (hot side only), now everything works.
    – steeveedee
    Commented Dec 12, 2022 at 20:06
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An outlet connected to two breakers requires the small metal tab between the two hot(black and red wires) screws to be removed.

This outlet seems to be on a MWBC circuit that uses only one neutral(white) wire.

These circuit breakers should(must) have a handle tie so both breakers can be turned off for safe work.

It is not a 30 amp outlet. It is two single 15 amp outlet/receptacles. You don't trip the the breaker when you turn on/plug in the kettle and toaster at the same time.

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    thanks for your reply. I got 2 new outlets and broke the tab off (hot side only), now everything works.
    – steeveedee
    Commented Dec 12, 2022 at 20:07
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Tab trouble.

This is the most common misadventure when novices try changing receptacles e.g. for aesthetic reasons. Between the 2 side screws, there is an imperceptible little "tab" which can be broken off. With tab present, it connects both screws so they can be used as a convenient splice. With tab removed, it gives independent control/feed of the 2 sockets.

The main place this is used is a split receptacle, where 1 socket is always-on and the other is split by a light switch.

Forgetting to break off the tab causes both sides to be shorted together. E.g. the switch no longer works, or the MWBC pops your main breaker.

A MWBC is a circuit that takes full advantage of Thomas Edison's "split-phase" way of delivering power, to let two quasi-circuits share a neutral for some wire savings. Nobody likes them anymore because it's more difficult to do AFCI and GFCI.

However, rule #1 and #2 of MWBCs require a handle-tied breaker in the main panel, so it is correctly phased, and the maintainer is forced to shut off the whole circuit and not get nailed by the other half-circuit. Rule #3 is pigtail neutral.

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