What size does neutral need to be when multiple conduit runs are used?

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MechEdetour

MechEdetour

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NY, USA
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Design Engineer
I have a 120/240 100A lighting panel with (12) 20A 1P breakers. I need to wire the (12) circuits to a 12P lighting contactor and I am trying to figure out the easiest way to go about it.

I want to avoid an individual conduit run for each circuit. Initially I started with a single conduit run with (12) circuit conductors from the breakers, (1) neutral, and (1) EGC in a single conduit run. Issue I have here is that the 2AWG neutral is derated to 3/0AWG because of 12 CCCs in a single conduit. 3/0AWG is too large to adequately terminate at the panel and the lighting contactor. So rather than doing a single run, and an individual run for each circuit, I want to meet in the middle. I am looking to have 3 conduit runs, (4) circuit conductors in each. Now this is where I need some feedback. I'm fairly certain I need to run a neutral in every conduit per 300.3(B).

My questions is, since I will have to run (3) neutrals between the panel and the lighting contactor, does each neutral have to be sized for the ampacity of all circuits, or only the circuits it is run with (ie. 1/3, or 4 of the 12 circuits)? It'll make derating/running the neutrals a lot easier...
 
Why are you talking about 2awg and 3/0 in the context of 20A circuits? Only two 1pole breakers can share a neutral (MWBC). I agree with the last post, you need an electrician to explain some basics.
 
jaggedben said:
Why are you talking about 2awg and 3/0 in the context of 20A circuits? Only two 1pole breakers can share a neutral (MWBC). I agree with the last post, you need an electrician to explain some basics.
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Actually an MWBC can originate at a 2-pole breaker also. Either way, the two hots need to come from opposite phases.

Sent from my XT1585 using Tapatalk
 
MechEdetour said:
I want to avoid an individual conduit run for each circuit. Initially I started with a single conduit run with (12) circuit conductors from the breakers, (1) neutral, and (1) EGC in a single conduit run. Issue I have here is that the 2AWG neutral is derated to 3/0AWG because of 12 CCCs in a single conduit. 3/0AWG is too large to adequately terminate at the panel and the lighting contactor. So rather than doing a single run, and an individual run for each circuit, I want to meet in the middle. I am looking to have 3 conduit runs, (4) circuit conductors in each. Now this is where I need some feedback. I'm fairly certain I need to run a neutral in every conduit per 300.3(B).
Click to expand...

You need a single neutral for each MWBC and it does not count as a CCC towards derating unless a majority of the load is non-linear.
 
I think there might be some confusion as to what the intent is... Maybe I did a bad job explaining it. Let me try it another way... And FWIW this is just a preliminary design, nothing is being built/installed yet.

I have a 120/240V panel with (12) 1P breakers. This panel is being fed by a 480-240/120V transformer. G-N bond is taking place at the transformer, I have an isolated neutral in the panel.

I also have a 12P lighting contactor in another enclosure that has an isolated neutral bus and a gnd bus. Customer wants us to pre-wire the lighting contactor to the panel so that when the skid arrives in the field they only have to wire their lighting loads to the lighting contactor. This save the hassle of wiring the panel to the lighting contactor in the field since both the lighting panel and lighting contactor are being provided by us, and all circuits in the panel will be used for lighting loads, controlled by the aforementioned lighting contactor.

Each breaker will be wired to a pole on the lighting contactor (line side) and then customer will wire branches to load side of lighting contactor. I am not talking about the wiring between the lighting contactor and the 120V lighting loads. I understand end user will run a neutral for every circuit or choose to run a MWBC etc. With that being said, end user will not terminate L to the lighting contactor, and the N to the neutral bus in the panel. Rather, end user will terminate L to the lighting contactor, and N to the neutral bus in the lighting contactor so the circuits are leaving/entering the same enclosure. I am tasked with "extending" the neutral bus from the panel to the lighting contactor so this can be accomplished. My OP is in reference to this "extension."

So. Since I initially thought about running all the conductors from one enclosure to the other in 1 raceway and it did not work for me, I opted to run the conductors in multiple raceways (2 actually, I initially said 3). This is what my question revolves around. My question is, is it suitable for me to run ONE neutral sized for SIX branches BETWEEN the lighting contactor neutral bus and panel neutral bus, or is this an issue?

The reason I ask this is because if I fall back to the case where I have an individual raceway for each circuit (avoiding anything related to a MWBC), then I would be running a conductor from a neutral bus in the panel, to a neutral bus in the lighting contactor enclosure (where end user would also be terminating 12 wires for the loads) for each circuit. This would mean I have (12) conductors running between 2 neutral buses, which sounds silly to me and I'm trying to avoid this.

I guess I'm looking at this as if I was installing a sub panel and I needed to run a neutral between the sub panel and a main panel that I was feeding it with... Only I need to wire (12) individual circuits between the 2 instead of just A-B... And FWIW not sure if it's clear but the lighting contactor and panel in question are literally adjacent to each other (maybe 2-3ft apart).
 
MechEdetour said:
So. Since I initially thought about running all the conductors from one enclosure to the other in 1 raceway and it did not work for me, I opted to run the conductors in multiple raceways (2 actually, I initially said 3). This is what my question revolves around. My question is, is it suitable for me to run ONE neutral sized for SIX branches BETWEEN the lighting contactor neutral bus and panel neutral bus, or is this an issue?
Click to expand...

The answer is no look at 200.4(A). If your installation fell under 225.7(B) then that would be example of where in the code it is permitted.
 
MechEdetour said:
So. Since I initially thought about running all the conductors from one enclosure to the other in 1 raceway and it did not work for me, I opted to run the conductors in multiple raceways (2 actually, I initially said 3). This is what my question revolves around. My question is, is it suitable for me to run ONE neutral sized for SIX branches BETWEEN the lighting contactor neutral bus and panel neutral bus, or is this an issue?
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Your answer is in 200.4 and 225.7, if you meet the requirements you can use a common neutral, if not you will need to go with individual neutrals or use MWBC's

Ooops, I was typing while Infinity was posting, sorry Rob.

Roger
 
roger said:
Your answer is in 200.4 and 225.7, if you meet the requirements you can use a common neutral, if not you will need to go with individual neutrals or use MWBC's

Ooops, I was typing while Infinity was posting, sorry Rob.

Roger
Click to expand...

:D

Would be nice if the Forum told you that before you hit the Post Reply. :slaphead:
 
Not an answer to your question but I have to ask.

Since you have a dedicated lighting panel, why don’t you use a mechanically held contactor on the line side of panel and have all loads terminate in panel?

Much more reliable in my experience.
 
Russs57 said:
Not an answer to your question but I have to ask.

Since you have a dedicated lighting panel, why don’t you use a mechanically held contactor on the line side of panel and have all loads terminate in panel?

Much more reliable in my experience.
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We had a few buildings like that. One had old GE panels with the contractor in the middle.
Upper section was basic power, lower section fed via contactor, lower section all lights.
 
Even before reading all of the posts, I was going to suggest that a single 100a 2p standard contactor might cost less than a 12p lighting contactor, and would eliminate the entire common-neutral question.

Plus, if any additional lighting circuits are ever added, they could be added more easily, as long as a larger panel is used.
 
LarryFine said:
Even before reading all of the posts, I was going to suggest that a single 100a 2p standard contactor might cost less than a 12p lighting contactor, and would eliminate the entire common-neutral question.

Plus, if any additional lighting circuits are ever added, they could be added more easily, as long as a larger panel is used.
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Yep.
 
If you have a neutral _bus_ in the enclosure with the lighting contactor, then won't any wires between the neutral bus in the panel and the neutral bus in the contactor enclosure be placed in parallel?

It seems to me that with this design you are _forced_ to have a common neutral between the two enclosures, weather permitted by the NEC or not. And if you use reasonable conductor sizes you would be violating the rules about small conductors in parallel.

Finally, with the shared or common neutral, won't the single pole breakers be a problem?

-Jon
 
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