Two breakers share the same neutral

[john in la]

I went to my sisters older house today to replace a wall plug for her and saw something I have never seen before.
In the electric box they had 2 single pole 20 amp breakers next to each other that had a piece between them to where if you turned off one breaker you had to turn off both breakers.
Looking at it I could see they had a 12/3 wire going to the breakers. The black wire was hooked to one breaker and the red wire was hooked to the second breaker. They both seemed to be sharing the same neutral white wire.
At first I thought someone was using 2 single pole breakers to supply a 220 volt plug but they were hooked to 110 wall outlets.
So I googled it and found that it was legal for 2 single pole breakers next to one another to share a single neutral white wire.

Has anyone ever seen this before???

 

[BarnyardEngineering]

Seen it personally? No, but I have heard of it many times. The responses will range from, it's legal to it's illegal and expect the Electricity Police to come bursting through your door in full SWAT gear any second, to it's fine, to you'll burn your house down kid, to I do it that way all the time, to I never do it that way and if anyone did it that way in my presence I would fire/beat/kill them...

This is, after all, an electrical question in an online forum.

Honestly if it's been working for X years and the homeowner doesn't have any complaints, I say it's fine. The odds of them having both circuits loaded to the maximum simultaneously is slim to none.

 

[MarkB_MI]

If you have two large 120V loads near each other, the voltage drop will be less with this setup than if their neutrals were separated.

 

[olddavid]

The breakers are on opposite input lines that is the phases are180 degrees apart for a single phase 240 input line. 240 volts between the hots but each only 120 to the neutral. I believe theoretically if both sides have the same load the neutral would have nearly zero current since the currents are 180 degrees out of phase and would cancel out.

 

[NDIHC]

My shop lights are wired that way. At first we didn't have the bar between the breaker toggles so we had to put in a paired breaker to pass inspection. Perfectly legal and pases inspection here.

 

[Texasmark]

I went to my sisters older house today to replace a wall plug for her and saw something I have never seen before.
In the electric box they had 2 single pole 20 amp breakers next to each other that had a piece between them to where if you turned off one breaker you had to turn off both breakers.
Looking at it I could see they had a 12/3 wire going to the breakers. The black wire was hooked to one breaker and the red wire was hooked to the second breaker. They both seemed to be sharing the same neutral white wire.
At first I thought someone was using 2 single pole breakers to supply a 220 volt plug but they were hooked to 110 wall outlets.
So I googled it and found that it was legal for 2 single pole breakers next to one another to share a single neutral white wire.

Has anyone ever seen this before???

Well being 110 v circuits, the current in the ground wire is the combination of current in both circuits. If low current usage, no biggie (excluding any code violation). That could be bad for high current applications that may cause the ground wire to overheat and possibly cause a fire.

 

[cjunrau]

I know here in Manitoba it was 100% legal in 2014. Not sure if codes have changed but my entire house is wired that way. And I did my shop and barn the same way after the electrician did my house.
I do not know the good or bad of it but for me it was good as when I ran it in conduit it saved me 1 wire for every 2 breakers. I wired my shop with no more than 2 outlets on the same breaker. I hate when 2 machines are going and breakers kick.

 

[Gene in Mi]

Was to code and common in the past. Today the two breaker handles must be tied together and the two breakers must be on two DIFFERENT phases! The current in the white NEUTRAL wire,(not bare/green ground) is the sum of the current in the two hot wires. If either of those is overloaded the neutral is overloaded, until the breaker trips, just like any other circuit in your house.

Advantages to this method is it saves wire and time. There is only one white wire running from the breaker to where that cable is split into two circuits. And the wireman is only running one cable that far. That could be 100 feet or more.

Disadvantages are that if one circuit overloads, two circuits are disconnected. And if one circuit needs to be shut off for service, two must be shut off. Its use is hazardous to people and dwellings. If breaker handles are not tied together the neutral wire can still be energized. I someone later rearranges breakers and circuits in the main panel the neutral can become overloaded and unprotected.

D

 

[Stephen Newell]

I went to my sisters older house today to replace a wall plug for her and saw something I have never seen before.
In the electric box they had 2 single pole 20 amp breakers next to each other that had a piece between them to where if you turned off one breaker you had to turn off both breakers.
Looking at it I could see they had a 12/3 wire going to the breakers. The black wire was hooked to one breaker and the red wire was hooked to the second breaker. They both seemed to be sharing the same neutral white wire.
At first I thought someone was using 2 single pole breakers to supply a 220 volt plug but they were hooked to 110 wall outlets.
So I googled it and found that it was legal for 2 single pole breakers next to one another to share a single neutral white wire.

Has anyone ever seen this before???

I've seen that a lot, especially on small boxes where there are fewer neutral connections than breakers.

 

[john in la]

I watched a YouTube video this morning (that can be dangerous) because this has peaked my interest on this subject.
The way the guy in the video explained it if the load on the black and red wire is the same the white neutral wire doesn’t see any load because the black and red wires are on different phases.
For the white wire to see a combination of the black and red wire they would both have to be hooked to the same phase.

 

[Texasmark]

Was to code and common in the past. Today the two breaker handles must be tied together and the two breakers must be on two DIFFERENT phases! The current in the white NEUTRAL wire,(not bare/green ground) is the sum of the current in the two hot wires. If either of those is overloaded the neutral is overloaded, until the breaker trips, just like any other circuit in your house.

Advantages to this method is it saves wire and time. There is only one white wire running from the breaker to where that cable is split into two circuits. And the wireman is only running one cable that far. That could be 100 feet or more.

Disadvantages are that if one circuit overloads, two circuits are disconnected. And if one circuit needs to be shut off for service, two must be shut off. Its use is hazardous to people and dwellings. If breaker handles are not tied together the neutral wire can still be energized. I someone later rearranges breakers and circuits in the main panel the neutral can become overloaded and unprotected.

D

The breaker is on the hot lead not the ground. So you could be running 20 amperes each on two circuits protected with a 20 amp breaker for each and the breakers would maintain current but the common ground (white wire) would have 40 amperes flowing through it and it wouldn't take long for the insulation to burn through and start other things burning and presto....you have a fire. Iterating, would work fine for lamp circuits or other light loads....1/3 HP (3 amperes type thing) motors. No go with a couple of HD (1 HP) single phase motors.

 

[olddavid]

The breaker is on the hot lead not the ground. So you could be running 20 amperes each on two circuits protected with a 20 amp breaker for each and the breakers would maintain current but the common ground (white wire) would have 40 amperes flowing through it and it wouldn't take long for the insulation to burn through and start other things burning and presto....you have a fire. Iterating, would work fine for lamp circuits or other light loads....1/3 HP (3 amperes type thing) motors. No go with a couple of HD (1 HP) single phase motors.

If the breakers are on the same phase yes, but if the breakers are next to each other, they would be on separate phases 180 degrees apart. The 20 amp loads are 180 degrees out of phase with each other and cancel out therefore zero current in the neutral.

 

[Texasmark]

If the breakers are on the same phase yes, but if the breakers are next to each other, they would be on separate phases 180 degrees apart. The 20 amp loads are 180 degrees out of phase with each other and cancel out therefore zero current in the neutral.

Well sir, coming from opposite sides of the transformer where the polarity is the opposite, you are correct....so connecting the breakers to both sides of the AC source solves the problem. One side is sucking while the other side is blowing at the same rate, equals 20 amperes in the ground (white) wires. I guess the electricians that wired things as mentioned herein were savvy to what you said and weren't worried about it. Well done folks!

You know, this is my preferred site for several reasons. One is that every day you either think about something you hadn't before or learn something. I thoroughly enjoy chatting with you folks.

 

[Red Dave]

Well being 110 v circuits, the current in the ground wire is the combination of current in both circuits. If low current usage, no biggie (excluding any code violation). That could be bad for high current applications that may cause the ground wire to overheat and possibly cause a fire.

Neutral wire, not ground. Not the same thing.

The current in that neutral will be only the difference in current between the two legs, NOT additive.

 

[Texasmark]

Neutral wire, not ground. Not the same thing.

The current in that neutral will be only the difference in current between the two legs, NOT additive.

Yes, the white wire as I said, not the green earth ground wire. Agreed on only the difference current flows. If both loads are identical then no current flows.

 

[tech4]

This was commonly used on kitchen circuits in my area. The tab between the receptacles was broken so the top and bottom receptacle were separated. Then the microwave could be plugged into the top receptacle and the toaster oven in the bottom and both could be run at the same time. The neutral wire would be common to both plugs. If the microwave drew 13 amps and the toaster oven drew 10 amps, the neutral current would only be 3 amps. The danger in this is if the neutral opens there will be up to 240 volts applied to that open neutral.
I had a problem when the wires in the crowded box were very close. The red wire got hot and melted the insulation and touched the bare ground. Many sparks and smoke in the kitchen, but the circuit breakers did pop and no other damage. This was after about 30 years of use.

 

[Big 86]

Multiwire branch circuits (MWBC) is the NEC term that the OP is referring to.

 

[John T]

Good Morning John, hope all is well with you. To answer your good question YES I have seen it often.

FYI for others if there's any confusion, as you found indeed the NEC (when I practiced at least) permits and they are used (although I wasn't a fan) what are called MULTI WIRE BRANCH CIRCUITS in which two hot legs (A & B) of 120 VAC each 180 out of phase with the other, share a single common Neutral. If each leg of 120 pulled the same current THE SHARED NEUTRAL CURRENT IS ZERO). If the two 120 legs aren't equal, the Neutral only carries the difference in current.

If two vertically next to each other (in a typical 120/240 Panel) such that they are on opposite phases Single Pole Breakers are tied together, that accomplishes the same as a two pole 240 volt breaker. The voltage of EITHER is 120 VAC to Neutral (but are 180 out of phase) while the Voltage from one breaker to the other is 240 VAC where you would commonly see red to one black to the other to serve a straight 240 volt load.

As noted yes Ive seen two hot legs of 120 that are 180 out of phase with the other share a single common Neutral. Such would be preferred when the loads are resistive as opposed to inductive (see below).

Hope this helps

DISCLAIMER Im long retired and rusty on the latest codes so NO Warranty, when in doubt consult local authority and applicable NEC where appropriate. NOTE I wouldn't be surprised if this practice was no longer NEC approved or there were some exceptions due to the possibility of harmonic currents if the loads were inductive ?????????? Have to check the latest NEC to be safe n sure

John T BSEE, JD

 

[John T]

Well being 110 v circuits, the current in the ground wire is the combination of current in both circuits. If low current usage, no biggie (excluding any code violation). That could be bad for high current applications that may cause the ground wire to overheat and possibly cause a fire.

In common typical and NEC permissible use (such as when a two pole breaker is used or two 120's tied together) the two hot legs of 120 are 180 out of phase, so if both hot legs carried say 20 amps for example NEUTRAL (NOT Ground) CURRENT IS ZERO NOT 40 ! Neutral only carries the difference in current between the two 120 volt legs. The Equipment GroundING Conductor aka Ground only carries fault current NOT normal return current.

Hope this helps see my explanation above

Best wishes nice chatting with you

John T

 

[drsportster]

That has been code Tie both breakers together for a 3 wire circuit