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In my bedroom (14 gauge copper wiring-15 amp breakers) It takes two circuit breakers to shut it off including the switch. Is this tandem or MWBC and if possible could you explain the difference? Just trying to understand the difference.

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    Tandem and MWBC are not opposite things. { 2 separate circuits | MWBC } are opposite things. {2-pole breaker | tandem=double-stuff breaker } are opposite things. – Harper - Reinstate Monica Feb 20 '17 at 2:09
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To make it a little clearer tandem breakers are on the same leg. MWBC are on opposite legs. Voltage measurements on tandem hot to hot the voltage is 0. The voltage on MWBC hot to hot is 240v.

  • And to make it a touch clearer still, a tandem breaker mounts to just one tab of the hot bus (and therefore occupies only one space). A mwbc breaker uses two. – isherwood Feb 20 '17 at 3:24
  • They should be, yes. There's big trouble if an MWBC is punched down onto a tandem breaker. – Harper - Reinstate Monica Feb 20 '17 at 3:27
  • isherwood, my old GE panel breakers in the branch circuit ("Main") section are all separate 1/2" breakers. But going down a column of breakers the legs alternate each 1". The geometry of the panel and of the two pole breakers allows insertion of a 2-pole breaker (2 x 1/2" = 1") only across different legs. The 240 V breakers for range, a/c dryer are in an upper section and are full size 2 x 1". – Jim Stewart Feb 20 '17 at 11:25
  • I thought my panel alternated every 1/2" and tried to force a 2-pole breaker into a pair of 1/2" slots which were on the same leg. When it resisted I pushed harder. I finally gave up before damage to the panel or even to the breaker, but my initial conclusion was that the particular model of breaker was not designed for my outmoded panel. When I finally realized what was happening, I easily snapped this same breaker right in to two adjacent 1/2" slots which were on different legs. Warning excuse ahead--I was doing it hot so was concentrating on what not to touch. – Jim Stewart Feb 20 '17 at 17:18
  • Are tandem breakers still acceptable according to NEC. Also thank u all for your explanations and clarity – larry pinsky Feb 20 '17 at 22:10
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You are talking about two totally separate issues that have nothing to do with each other.

Simplex vs Duplex circuit breakers

This is a big subject, please look at this separate question/answer.

Separate neutrals vs Multi-wire branch circuit (MWBC)

Either you're using separate neutrals, or an MWBC. This has nothing to do with breakers.

There will be numerous answers on MWBCs. But here, I'll discuss both way of splitting a single receptacle and powering it with 2 circuits, so you can see the difference.

Since we're working in a single receptacle, we'll be doing a lot of knocking out the tabs that normally connect the two screws. Watch for it.

Grounds omitted for clarity. Disregard green receptacles; that's just to give a contrasting color so you can see screws and tabs clearly.

Illustration: MWBC vs 2 circuit receptacle wiring

Error on this drawing. 2-circuit breakers going to the same receptacle must also tie their breakers. Either 2 breakers with a handle-tie, or a 2-pole breaker. They don't need to be opposite poles. They do need to have common maintenance shut-off. That's to protect workers who plug a radio in one socket and flip off breakers until the radio stops, pull out the socket and get nailed by the other circuit.

2-circuit setup

This is straightforward. We have broken off both tabs, to make each socket a totally separate thing. Each socket gets fed its own hot and neutral. Note that the neutrals are rigidly separated.

Back in the service panel, these can be punched down into any two breakers. There is no restriction on which, and they can be on same pole, different poles, same pole, duplex, outside breakers on a quad, whatever. The dedicated neutral means we don't care. Of course, neutrals go to the neutral bus, and grounds go to the ground bus.

Also note that since they're separate circuits, any common, cheap AFCI or GFCI will suffice if you need that protection.

And by the way -- the wires can be different sizes. As long as the breakers are appropriate for the wires. In this illustration, the upper socket is wired with 12 AWG wire (yellow sheath) and the lower socket is wired with 14 AWG wire (white sheath). The 14AWG cable would need to be fed by a 15A breaker. The 12AWG cable could use a 20A breaker under certain circumstances - but again, neutrals must not be shared.

This won't affect 15-20A circuits, but if you were in conduit, there being 4 active conductors would require you to double-check the derating rules in NEC 310.15b2a. That's where a MWBC would be an advantage.

Multi-wire branch circuit setup

First, note the circuit breaker. The two wires must be on opposite poles in the panel, as discussed in the duplex breaker answer. That's because the socket is getting 240V, with neutral only handling the difference in currents. Imagine if the top socket (red, pole L1) draws 15A. The bottom socket (black, pole L1) draws 14A. The neutral only handles 1A, the difference. If both red and black were on L1, (i.e. a duplex breaker), then the neutral would be handling the full return for both circuits, or 29A, which is way too much.

You notice the neutral is pigtailed. That is a requirement on MWBCs. Like the 12AWG cable, it's done on the expectation of extending this circuit later. The rule is that you must be able to remove this receptacle without interrupting the netural for any devices downstream. This pigtailing makes more sense if we had put both sockets on the red wire and passed the black wire through to loads downstream.



Extending the 2-circuit, separate neutrals configuration

I'd like to illustrate how messy and crowded things can get. Here, we extend the 2-circuit split outlet. Notice some things in this drawing (you may want to click to zoom):

enter image description here

All the neutrals are kept rigidly separate. In fact, I got tired of being confused, and tagged the neutrals with the 12AWG circuit with gray tape.

The receptacle in box 2 is split exactly like the receptacle in box 1.

All those pigtails are not mandatory (this isn't an MWBC). They are pigtailed because you can't put 2 wires on a screw! (never use both screw and backstab at once.) However, some better-quality "screw-and-clamp" type receptacles, are designed specifically to clamp 2 wires per screw. They are perfect for this.

Between Box 1 and 2, that is 12/2/2 Romex. That is a special NM cable that has ground and 4 wires: black, red, white, and white with red stripe. It is made for carrying two complete circuits like this.

After the second box, the 2 circuits go their separate ways. Each circuit stops at 2 more receptacles. Note the "tabs" are NOT broken off - for instance the fourth box (gray neutrals) is using the tabs to tie the two screws together, to avoid needing a pigtail.

On the gray/neutral circuit, all the wiring is 12 AWG (yellow sheath). That means it can use a 20A circuit breaker and 20A receptacles such as those illustrated on the right.

The white/neutral circuit uses 14 AWG cable (white sheath) in at least one place. That means it must be a 15A breaker.

What's that receptacle in box 3? Why, that's a GFCI! We couldn't put it in box 1 because you can't get a 2-circuit GFCI like that. Well, we could've put two duplex receptacles in box #2, one on each circuit, since it's a 2-gang and there's room. One of those could be a GFCI (or both).

The last receptacle on the 14 AWG run has a label of GFCI protected, because it is fed off the LOAD terminals of the GFCI.

This is an awful lot of wires and pigtails, and you may need to watch your box fill, the statutory number of wires allowed in a box. Fortunately pigtails don't count.

Extending the multi-wire branch circuit

I didn't draw this, but the extension would work largely the same way. 12/3 cable would suffice between box 1 and 2. The neutrals in box 2 would be shared similar to box 1 in the first MWBC illustration, and would need to be pigtailed.

At box 2, the shared neutral would also serve both branches (to boxes 3-6). Neutral is no longer shared in boxes 3-6.

Because the neutral is not shared anymore, there can be a GFCI in box 3, and that will work.

Because the neutral is not shared anymore, the receptacle in box 4 does not need to have its neutral pigtailed.

The side of the circuit which is all 12 AWG can have a 20A breaker. The side with some 14 AWG must be a 15A breaker. In 2-pole breakers, you must use a 15A breaker because they don't make 15/20s. But with a listed handle-tie, you can "make that" out of a 15A and 20A single breaker (not duplex). The nature of the handle-tie forces you to put them in adjacent spaces so they are on opposite poles.

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