66
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-...
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.
...
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 ...
- 175k
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 ...
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-...
- 175k
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 ...
- 11.6k
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.
- 4,862
11
votes
Accepted
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 ...
- 24.7k
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 ...
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 ...
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 ...
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-...
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(...
- 18.9k
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 ...
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 ...
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 ...
8
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 ...
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 ...
- 6,352
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 ...
- 79.1k
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 ...
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 ...
- 175k
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 ...
6
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 ...
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 ...
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 ...
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 ...
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 ...
- 101k
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 ...
6
votes
Accepted
Two Circuits on a Single 12/3 - Shared Neutral
TLDR: Square D makes a 2-pole CAFCI (# QO220CAFIC) that will do what you want. However, this panel needs a lot more attention, as it has more "bomb" circuits like this one, and it's full, ...
6
votes
Is a 120V/208V MWBC good enough for 240V loads?
It's really marginal for European appliances. The standard across Europe has been set at 230V, with a tolerance of ±10%. That wide tolerance is to allow for existing supplies that are really 220V or ...
- 2,603
Only top scored, non community-wiki answers of a minimum length are eligible