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71 votes
Accepted

What is a tandem breaker (aka duplex, cheater, twin, double-stuff, etc.)?

It starts with spaces in the panel A service panel has a basic unit I call a space. That's apparent when you look at the "knockouts" on the cover. Electrical power in North America delivers split-...
Harper - Reinstate Monica's user avatar
30 votes

I installed a new outlet and tripped the whole power panel... Does a 30 amp circuit need special outlets?

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. ...
manassehkatz-Moving 2 Codidact's user avatar
14 votes

I installed a new outlet and tripped the whole power panel... Does a 30 amp circuit need special outlets?

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 ...
Ecnerwal's user avatar
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13 votes

Why are tandem breakers on shared neutral (MWBC) problematic?

A shared-neutral (two hots sharing the same neutral) is also called a multi-wire branch circuit (MWBC). Now to go any further, first, North American power uses a slick trick. It supplies 120V in two ...
Harper - Reinstate Monica's user avatar
12 votes

Look over father's plan to fix outdoor electrical circuit

With an appropriate breaker feeding it, this is fine. Typical subpanel is fed by a double-pole (240V) breaker in the USA/Canada. The feed would (per circuit descriptions) EITHER need to be a double-...
Ecnerwal's user avatar
  • 211k
12 votes
Accepted

Sharing Neutrals - How To Correct?

First, technically, the breakers are not #8 and #16. They are actually #4B and #11B. This is important because odd numbered breakers are on one leg of 120 volts and even numbered breakers are on the ...
DoxyLover's user avatar
  • 12.6k
12 votes
Accepted

Can you cut the outlet-bridge clip and run a 2-phase MWBC to a single receptacle?

Yes, that's perfectly legal. The handle-tie (or single-handle two-pole breaker, e.g. QO) is critical - when two different hots go to the same yoke, it must not be possible to shut them off separately.
nobody's user avatar
  • 6,641
11 votes
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Neutral pigtails on MWBC

Multiwire branch circuits (MWBCs) share a neutral. There is a code rule that prohibits wiring devices, such as receptacles, in a way that the removing the device would open (i. e., disconnect) that ...
batsplatsterson's user avatar
11 votes

In a multiwire branch circuit, can the two hots be connected to the same phase?

Yes, this does sound like a problem. Assuming your panel is like most, the breakers are on the same phase and you have two problems: No Common Shutoff MWBC is required to have common maintenance ...
manassehkatz-Moving 2 Codidact's user avatar
10 votes

Splitting 240V into two 120V Circuits

10 AWG wires (based on clarification in comments to the question) can handle up to 30A. The "usual" receptacles for most things in the US are 15A or 20A. There are exceptions (welders, EV ...
manassehkatz-Moving 2 Codidact's user avatar
9 votes
Accepted

MWBC with both circuits on the same bus -- any easy fixes?

That should be an easy fix. Swapping wires: Swap garage and laundry in the "Circuit 2" breaker pair. Swap the garage wire from the "Circuit 1" breaker pair with the first wire in ...
manassehkatz-Moving 2 Codidact's user avatar
9 votes

Sharing Neutrals - How To Correct?

The rules of MWBCs I understand this is NOT to code since there is no tie bar between the two breakers (confirm this WOULD be to code if this was the case). Correct. This is called a "Multi-...
Harper - Reinstate Monica's user avatar
9 votes

I installed a new outlet and tripped the whole power panel... Does a 30 amp circuit need special outlets?

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(...
crip659's user avatar
  • 23.8k
9 votes
Accepted

Why are MWBC's permitted by NEC, given the neutral can take up to 2x the rated current?

A properly installed MWBC does not work that way! In any 240V/120V 2-hot + neutral circuit, from your entire house to a subpanel on down to a 15A MWBC, the neutral carries the difference between the ...
manassehkatz-Moving 2 Codidact's user avatar
8 votes

Neutral pigtails on MWBC

No, you really need to fix that. Search this site for "Lost Neutral" for what happens when the neutral wire has a problem. It's bad, and it can set stuff on fire. Look at that neutral in ...
Harper - Reinstate Monica's user avatar
8 votes

Splitting 240V into two 120V Circuits

I know 8 circuits seems like an embarrassment of riches for a kitchen, but Code says you actually do need most of those. The first question on converting either the range or griddle circuit is whether ...
Harper - Reinstate Monica's user avatar
8 votes

Look over father's plan to fix outdoor electrical circuit

As noted in Ecnerwal's answer, this is probably fine, if implemented as a proper MWBC with proper GFCI protection. However, there may be one additional issue: hardwired loads. Normally, a circuit with ...
manassehkatz-Moving 2 Codidact's user avatar
7 votes

Multi-Wire Branched Circuit and GFCI

Land all your MWBCs on a 2-pole breakers NOW. First, big safety problem. If an MWBC has both its legs on the same pole, it will overload the neutral. It's important to search your entire panel for ...
Harper - Reinstate Monica's user avatar
7 votes

is this the right way to ground fault protect both sides of an mwbc?

Close, but not quite. From the point of the two GFCIs onward, the neutrals must be kept separate. Consider: if you had a device drawing power on the black hot circuit from a load farther down the ...
Nate S.'s user avatar
  • 6,372
7 votes

Shop Electrical - MWBC Serving 240V and 120V Outlets

This is fine, provided you obey a few rules While a MWBC serving a mix of 120V and 240V receptacles certainly may look strange to the uninitiated, or those who interact with electrical systems in a ...
ThreePhaseEel's user avatar
7 votes
Accepted

MWBC and very low power factors

Even with your power factor shift it is not that large over all -- most motors have a power factor of 80 or better. The vast majority of loads are resistive or inductive not only in residential but ...
Ed Beal's user avatar
  • 103k
7 votes

Is this a bunch of wrong MWBCs?

See my post on panel design. Here is your panel with phases highlighted. Since you have not chased each red-black pair from their respective cables, we don't know if they're improperly phased. That ...
Harper - Reinstate Monica's user avatar
7 votes

Is a 120V/208V MWBC good enough for 240V loads?

120/208 is stunningly common in multi-unit housing (condo/apartment.) In most cases (particularly all "dumb resistive heating" cases) the reduced voltage will lead to reduced current and ...
Ecnerwal's user avatar
  • 211k
7 votes

Can you cut the outlet-bridge clip and run a 2-phase MWBC to a single receptacle?

Yes, you can do that. However, keep in mind that some of the main uses for MWBCs are: Kitchen - GFCI required for decades. Workshop - GFCI usually required, though depends on a few factors. But ...
manassehkatz-Moving 2 Codidact's user avatar
6 votes

Microwave on 220 volt branch

The electrician is right. The circuit is improperly sized for these loads. It's very likely that the microwave was added as an afterthought without a moment of consideration for circuit loads. This ...
Harper - Reinstate Monica's user avatar
6 votes

Possible to ground-fault protect both legs of a MWBC with two single-pole breakers?

Whoever told you that was referring to ***A***FCI breakers, and only in the context of a GE panel. In every other case, they don't know what they're talking about. A GFCI operates by comparing ...
Harper - Reinstate Monica's user avatar
6 votes

Possible to ground-fault protect both legs of a MWBC with two single-pole breakers?

Never gonna work. Sorry, but even if you might be able to get one at a time to work, GFCI on MWBC is based on the breaker comparing "all hot" to "all neutral". Two electrically separate breakers ...
manassehkatz-Moving 2 Codidact's user avatar
6 votes
Accepted

Shop Electrical - MWBC Serving 240V and 120V Outlets

Yeah, I work a lot with MWBC. That looks fine to me... It should limbo under the NEC 2017 guidelines. If you don't get this permitted by the time your state adopts NEC 2020, then your entire plan ...
Harper - Reinstate Monica's user avatar
6 votes

Multi-Wire Branch Circuit on wrong breakers

You may think there is nothing on that circuit but it needs to be verified. It is a simple fix to turn both breakers and swap the wires. Why is this important? With both hots on the same leg this ...
Ed Beal's user avatar
  • 103k
6 votes
Accepted

Multi-Wire Branch Circuit on wrong breakers

Well as you know, 120/240V power is delivered in 2 opposite poles, with neutral in the middle. Neutral carries only differential current between the 2 poles. The same applies to a multi-wire branch ...
Harper - Reinstate Monica's user avatar

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