Sharing a Neutral between 4 circuits with mixed voltage.

[600SL]

In the last garage I built in NC, I ran 4 circuits through 2 conduits, all converging at a box's where I could take out either a 240V circuit or 120V circuit or both. The NC configuration used two 240V 20A circuits and two 120V 20A circuits using 10GA wire as required by de rating. I am about to repeat the process in my new garage in CT. In my NC garage, I ran a neutral for each circuit. This system work very well and was very convenient changing configurations. My question is, since all neutrals go to the same point in the CB box can I replace the 4 neutrals with just 1, and what Ga wire would be required for the shared neutrals.

 

[American Locomotive]

In theory it would work. In practice there are safety issues. Mainly that when you turn off a breaker, you expect the entire circuit (hot and neutral) to be dead. In your scenario, you'd have to identify each breaker sharing that neutral and turn it off individually.

The NEC does allow shared neutrals in a limited and specific manner. Multi Wire Branch Circuits (MWBC). MWBCs are two circuits that share a common neutral. To do a MWBC, you are required to use two breakers next to each other (one on each phase), and their handles must be tied together. So you'd need a double breaker, basically. So you could get your 4 neutrals down to 2 neutrals. The downside of a MWBC is if one circuit overloads and trips, it trips both circuits.

 

[PCustoms]

Won't you need GFCI on all those outlets?

Pretty sure shared neutral won't play well in that situation, safety concerns noted above aside.

 

[Bert_]

240v doesn't even need a neutral. You could easily share a neutral for the 120v circuits. GFCI would complicate that if you need them.

Side note, I don't see any reason that you needed #10 in the first example. Each conduit has 4 current carrying conductors, so 80% reduction. #12 thhn you start with 30amps for derating. 80% of 30 is 24, so those 20A circuits would have been good with #12

 

[dave*99]

The downside of a MWBC is if one circuit overloads and trips, it trips both circuits.

Not true if you use a handle tie on a pair of single breakers. That allows the 2 circuits to trip independently.

 

[Zeke]

Not true if you use a handle tie on a pair of single breakers. That allows the 2 circuits to trip independently.

I don't quite follow that.

Anyway, I have learned over the years that neutral are better kept on the circuit that they run to. This is not an absolute, but there are circumstances where it is better. It's not any additional work when pulling and making up.

 

[billconner]

I'm required to have all circuits GFCI protected in garage. You of course can install GFCI breakers, albeit pricy.

 

[dave*99]

I don't quite follow that.

Anyway, I have learned over the years that neutral are better kept on the circuit that they run to. This is not an absolute, but there are circumstances where it is better. It's not any additional work when pulling and making up.

There are 2 ways to supply a MWBC.

Preferred method is to locate 2 single pole breakers on (electrically) opposite busses and tie their handles together with a handle tie from the breaker manufacturer. Breakers trip internally. When one trips, the other stays on. The purpose of the handle tie is to force someone to turn off both breakers when servicing that circuit so no current exists in the neutral.

This is a Square D Homeline handle tie.




The other method is to use a 2 pole breaker with common trip. if the 2 pole breaker has 2 handles, there is often an internal mechanism that forces both breakers to trip in common. Plus the handles are tied together with a handle tie. This is the type used in a typical 240V circuit in a residential panel. If you used this in a MWBC, overloading one of the two circuits will cause BOTH to go off. 2 pole breaker with common trip and a single handle is shown below. I prefer NOT to use these on a MWBC. That way the two circuits trip independently.

 

[sparky 1971]

I don't quite follow that.

The handle ties don't fit tight enough to cause a common trip. It can happen, but that's the exception, not the rule. When tripped, the breaker goes to the middle. It would have to go all the way to off with a tight fitting tie, bringing the other breaker with it to the off position since breaker #2 isn't tripped. Handle ties do require both or all three breakers to go to off in order to reset one though.

 

[sparky 1971]

There are 2 ways to supply a MWBC.

Preferred method is to locate 2 single breakers on opposite busses and tie their handles together with a handle tie from the breaker manufacturer. Breakers trip internally. When one trips, the other stays on. The purpose of the handle tie is to force someone to turn off both breakers when servicing that circuit so no current exists in the neutral.

This is a Siemens handle tie.




The other method is to use a 2 pole breaker with common trip. This is often an internal mechanism that forces both breakers to trip in common. This is the type used in a typical 240V circuit in a residential panel. If you used this in a MWBC, overloading one of the two circuits will cause BOTH to go off.

Those are Square D handle ties and they fit about as well as a saddle on a sow.

This is a Siemens, which has a chance of making common trip, but as I stated, that would be the exception, not the rule.

 

[Zeke]

Got it. My handle ties look more like a cage. Maybe not all, I'll have to look. I have 2 MWBC's in the whole house and sub. Others are tied 240v breakers which I'm sure trip commonly internally.

 

[600SL]

In theory it would work. In practice there are safety issues. Mainly that when you turn off a breaker, you expect the entire circuit (hot and neutral) to be dead. In your scenario, you'd have to identify each breaker sharing that neutral and turn it off individually.

The NEC does allow shared neutrals in a limited and specific manner. Multi Wire Branch Circuits (MWBC). MWBCs are two circuits that share a common neutral. To do a MWBC, you are required to use two breakers next to each other (one on each phase), and their handles must be tied together. So you'd need a double breaker, basically. So you could get your 4 neutrals down to 2 neutrals. The downside of a MWBC is if one circuit overloads and trips, it trips both circuits.

Not sure I understand. In the circuit you show the neutral never gets turned off with the breaker? The goal is to have all breakers independent.

 

[600SL]

Won't you need GFCI on all those outlets?

Pretty sure shared neutral won't play well in that situation, safety concerns noted above aside.

Good question. My inspector didn't know. Same with ark fault.

 

[dave*99]

Not sure I understand. In the circuit you show the neutral never gets turned off with the breaker? The goal is to have all breakers independent.

With both the red and black wires de-energized there will be no current in the neutral. And then it is safe to work on it.

 

[PCustoms]

Not sure I understand. In the circuit you show the neutral never gets turned off with the breaker? The goal is to have all breakers independent.

Neutrals don't get turned off via the breaker.....

 

[dave*99]

Not sure I understand. In the circuit you show the neutral never gets turned off with the breaker? The goal is to have all breakers independent.

Not independent..... Independently trip, but have a common handle so you must turn BOTH off when you flip the handle.

 

[larry4406]

Put a small 240v sub panel there and breakout your circuits. Then they all share the same neutral back to the main panel.

 

[600SL]

240v doesn't even need a neutral. You could easily share a neutral for the 120v circuits. GFCI would complicate that if you need them.

Side note, I don't see any reason that you needed #10 in the first example. Each conduit has 4 current carrying conductors, so 80% reduction. #12 thhn you start with 30amps for derating. 80% of 30 is 24, so those 20A circuits would have been good with #12

I use a neutral with my lift and machine tools so that I can break out 120V for accessories.

I thought #12 thhn was only good for 20A and #10 was 30A. With the neutrals the upper 240V conduit caries 6 conductors. True that the lower conduit caries 4 conductors + ground as planed but I did end up putting a 3 circuit in the lower conduit so I ended up with 6 plus ground as well. That last change was to add a dedicated circuit for a Dehumidifier. So I might as well just be set up for a 3rd 120V circuit just in case.

There are 2 ways to supply a MWBC.

Preferred method is to locate 2 single pole breakers on (electrically) opposite busses and tie their handles together with a handle tie from the breaker manufacturer. Breakers trip internally. When one trips, the other stays on. The purpose of the handle tie is to force someone to turn off both breakers when servicing that circuit so no current exists in the neutral.

This is a Square D Homeline handle tie.




The other method is to use a 2 pole breaker with common trip. if the 2 pole breaker has 2 handles, there is often an internal mechanism that forces both breakers to trip in common. Plus the handles are tied together with a handle tie. This is the type used in a typical 240V circuit in a residential panel. If you used this in a MWBC, overloading one of the two circuits will cause BOTH to go off. 2 pole breaker with common trip and a single handle is shown below. I prefer NOT to use these on a MWBC. That way the two circuits trip independently.

Good to know. I was never aware of this. I always thought if they were tied together they broke together

 

[600SL]

Put a small 240v sub panel there and breakout your circuits. Then they all share the same neutral back to the main panel.

These will be on a sub panel.

 

[Bert_]

I use a neutral with my lift and machine tools so that I can break out 120V for accessories.

I thought #12 thhn was only good for 20A and #10 was 30A. With the neutrals the upper 240V conduit caries 6 conductors. True that the lower conduit caries 4 conductors + ground as planed but I did end up putting a 3 circuit in the lower conduit so I ended up with 6 plus ground as well. That last change was to add a dedicated circuit for a Dehumidifier. So I might as well just be set up for a 3rd 120V circuit just in case.


Good to know. I was never aware of this. I always thought if they were tied together they broke together

When part of a 120/240v circuit the neutral does not count as current carrying. If both hot wires are carrying 20A then the neutral won't carry any current.

Thhn is 90*c insulation. Regular outlets are only good for 60*c so it limits #12 to 20A. But when we derate we can start with the 90* rating of 30A

 

[mike93lx]

These will be on a sub panel.

Putting in another sub at the location is the suggestion. You can feed a sub from another sub.

If I am ever at the point of needing 4 circuits somewhere, I would just add a little 6 space sub

 

[alfredeneuman]

Anyway, I have learned over the years that neutral are better kept on the circuit that they run to. This is not an absolute, but there are circumstances where it is better.

There is a rule that is absolute>
All conductors of a circuit are required to be in the same raceway, cable etc.
It's meant to avoid inductive heating of metal parts.

 

[alfredeneuman]

two 240V 20A circuits and two 120V 20A circuits using 10GA wire as required by de rating.

Derating of #12 wire to #10 begins at 10 conductors.
7 to 9 is 70%. 30A (it's 90 degree rating) X .7 = 21A

 

[mike93lx]

Derating of #12 wire to #10 begins at 10 conductors.
7 to 9 is 70%. 30A (it's 90 degree rating) X .7 = 21A

I love it when someone knows enough to be "dangerous". In this case, it was just a little wasted money, but not having the whole picture does that

I've definitely been there.

 

[wyliesdiesels]

I use a neutral with my lift and machine tools so that I can break out 120V for accessories.

I thought #12 thhn was only good for 20A and #10 was 30A. With the neutrals the upper 240V conduit caries 6 conductors. True that the lower conduit caries 4 conductors + ground as planed but I did end up putting a 3 circuit in the lower conduit so I ended up with 6 plus ground as well. That last change was to add a dedicated circuit for a Dehumidifier. So I might as well just be set up for a 3rd 120V circuit just in case.


Good to know. I was never aware of this. I always thought if they were tied together they broke together

If GFCIs are required, it will be expensive as you will need a double pole GFCI

 

[billconner]

.

Good question. My inspector didn't know. Same with ark fault.

Just saying, in NYS, based on 2017 NEC, garages require GFCI but not arc fault. I dont know if you had an office space, more of a living space, if that would require arc fault.

And as I meant to point out above, GFCI on 220 was not required till 2020 NEC. I think up to 50 amps. Along with that all basements, not just finished ones.

 

[PCustoms]

.
And as I meant to point out above, GFCI on 220 was not required till 2020 NEC. I think up to 50 amps. Along with that all basements, not just finished ones.

And it looks like CT adopted that in October 2022.

 

[American Locomotive]

Not sure I understand. In the circuit you show the neutral never gets turned off with the breaker? The goal is to have all breakers independent.

Current returns through the neutral. If you had 4 circuits sharing a neutral, and you turned 3 of the circuits off and left 1 circuit on, the neutral wire would still have current flowing through it. That means if you were to disconnect the neutral or work on the neutral wire, there would be a chance of you getting a shock.

That is why MWBC require the handle ties. That way if you turn one circuit off to work on it, they all turn off.

 

[600SL]

And it looks like CT adopted that in October 2022.

Thats what I get for procrastinating for a year.

 

[PCustoms]

Thats what I get for procrastinating for a year.

I just did a high level search, you should look into it more.

We "implemented" in April 2022, but there's a ton of exceptions and re-writes.

 

[dave*99]

I use a neutral with my lift and machine tools so that I can break out 120V for accessories.

For the sake of completeness, and since we talked about MWBC a lot…….

When you have a 240V circuit and you add a neutral for 120V loads, it is not a MWBC. And common trip is required because of the 240V loads.

 

[luvtheheat]

Worth watching and relevant to topic....

 

[600SL]

Derating of #12 wire to #10 begins at 10 conductors.
7 to 9 is 70%. 30A (it's 90 degree rating) X .7 = 21A

Not sure if I derated for # of conductors or length or both. The run around the perimeter of the shop was over 100'. At the time I belive I was using NEC 2013.

 

[600SL]

Worth watching and relevant to topic....

Does explain why I was questioning the size of the shared neutral.

 

[600SL]

For the sake of completeness, and since we talked about MWBC a lot…….

When you have a 240V circuit and you add a neutral for 120V loads, it is not a MWBC. And common trip is required because of the 240V loads.

That part was obvious to me. What I still don't understand is 120V CB's that will work interdependently while having some kind of handle tie as discussed in the beginning of this thread. Sort of off topic but my current house uses several MWBC with nothing tying the two breakers together. Maybe I need to look into that as well.

 

[dave*99]

That part was obvious to me. What I still don't understand is 120V CB's that will work interdependently while having some kind of handle tie as discussed in the beginning of this thread. Sort of off topic but my current house uses several MWBC with nothing tying the two breakers together. Maybe I need to look into that as well.

It's possible your house was built before handle ties were required.

Keep in mind the action of turning the handle off and internal tripping are two different conditions.

When most breakers trip, the internal mechanism opens the circuit. The handle usually moves to the middle. But it does not move forcefully. It does not move to the off position. It decouples from the internal mechanism. When tripped, you can push that handle to the on position, but since the internal mechanism has decoupled, the circuit stays off. When it trips it does not move to the middle with sufficient force to move the second breaker it may be handle tied to the off position. (Although occasionally that can happen.)

In order to reset the breaker you must first move the handle to the off position. Then it reengages the internal mechanism and can be turned on.

When one of the two breakers in a MWBC trips it is common to look in the panel and see the handle in the on position yet there is no power in one of the circuits. In that case the "ON" circuit handle it holding the "TRIPPED" circuit handle in the on position. It looks like everything is on, you can't tell the internal mechanism tripped.

 

[600SL]

It's possible your house was built before handle ties were required.

Keep in mind the action of turning the handle off and internal tripping are two different conditions.

When most breakers trip, the internal mechanism opens the circuit. The handle usually moves to the middle. But it does not move forcefully. It does not move to the off position. It decouples from the internal mechanism. When tripped, you can push that handle to the on position, but since the internal mechanism has decoupled, the circuit stays off. When it trips it does not move to the middle with sufficient force to move the second breaker it may be handle tied to the off position. (Although occasionally that can happen.)

In order to reset the breaker you must first move the handle to the off position. Then it reengages the internal mechanism and can be turned on.

When one of the two breakers in a MWBC trips it is common to look in the panel and see the handle in the on position yet there is no power in one of the circuits. In that case the "ON" circuit handle it holding the "TRIPPED" circuit handle in the on position. It looks like everything is on, you can't tell the internal mechanism tripped.

Thanks this is the first house I had with MWBC, so its never been an issue. It was built in 1991.

 

[mike93lx]

Not sure if I derated for # of conductors or length or both. The run around the perimeter of the shop was over 100'. At the time I belive I was using NEC 2013.

Increasing size due to voltage drop does matter for sure, it's just not referred to as derating.

100' is about when voltage drop tends to start mattering.

 

[600SL]

Increasing size due to voltage drop does matter for sure, it's just not referred to as derating.

100' is about when voltage drop tends to start mattering.

Yeah that particular loop was about 120'. My new garage will require about 100'. I will probably opt for the 10 ga again. My conclusion on this after weighing up all the pros and cons is to not try to share neutrals. The solution I had worked well with using colored tie wraps to secure the neutrals to the associated feed lines. The only real pro to sharing the neutrals appears to be a savings of about 300' of 10ga wire ~ $125.00.

 

[dscheidt]

Thanks this is the first house I had with MWBC, so its never been an issue. It was built in 1991.

common disconnect for MWBC came in with the 2008 cycle (I think, it could have been 2005, but it's pretty recent.) But if you want indepenence, you need more wires, or a subpanel.