3P Breaker vs 1P Breaker

[quantum]

I have a question and I'm not sure how to frame it. I'm going to give a hypothetical and try to steer myself to it:


Assume I have a 208/120V 3-Phase panel. I want to feed 8 light fixtures. What would be the significant difference of using:

Scenario 1) 3P Breaker, 4W multiwire branch circuit. One conduit run, fixtures hooked L to N and balancing the load.

Scenario 2) 3 x 1P Breakers in Lieu of the 3P, all circuits in one conduit.


In Scenario 2, can I have 1 neutral shared between the 3 separate phase conductors or would each phase conductor require their own neutral since they are supplied by 3 different breakers?

 

[iwire]

Assume I have a 208/120V 3-Phase panel. I want to feed 8 light fixtures. What would be the significant difference of using:

Scenario 1) 3P Breaker, 4W multiwire branch circuit. One conduit run, fixtures hooked L to N and balancing the load.

Scenario 2) 3 x 1P Breakers in Lieu of the 3P, all circuits in one conduit.


In Scenario 2, can I have 1 neutral shared between the 3 separate phase conductors or would each phase conductor require their own neutral since they are supplied by 3 different breakers?

From an electrical standpoint they are the same, and we used to do that. But now from a code standpoint if you want to use single pole breakers with a shared neutral you are required to provide approved handle ties so that when someone goes to work on it they have to deenergize all the conductors.

 

[ActionDave]

In Scenario 2, can I have 1 neutral shared between the 3 separate phase conductors or would each phase conductor require their own neutral since they are supplied by 3 different breakers?

From the dawn of the electrical age till NEC 2008 you could have three hots and one neutral fed from three independent breakers. Now you need handle ties or a three pole breaker to share a neutral.

 

[quantum]

From an electrical standpoint they are the same, and we used to do that. But now from a code standpoint if you want to use single pole breakers with a shared neutral you are required to provide approved handle ties so that when someone goes to work on it they have to deenergize all the conductors.

I was hoping to hear from you! I need to take it a step further now. If I use a shared neutral between the 3 single pole breakers, is that neutral considered a current carrying conductor or "not a current carrying conductor" like a true 4W/3P circuit?

 

[iwire]

If I use a shared neutral between the 3 single pole breakers, is that neutral considered a current carrying conductor or "not a current carrying conductor like a true 4W/3P circuit?

With a wye supply and the circuit consisting of all three phases and a neutral
It comes down to the load it serves.

If the majority of the load is non-linear the neutral must be treated as a CCC.

If the load is linear the neutral is not required to be counted as a CCC



.

 

[quantum]

With a wye supply and the circuit consisting of all three phases and a neutral
It comes down to the load it serves.

If the majority of the load is non-linear the neutral must be treated as a CCC.

If the load is linear the neutral is not required to be counted as a CCC



.

I agree, but I'm specifically inquiring about the neutral being shared between 3 separate single pole breakers. If I apply the handle ties, can I count that neutral as non current carrying? Assume purely resistive loads.

 

[Carultch]

I agree, but I'm specifically inquiring about the neutral being shared between 3 separate single pole breakers. If I apply the handle ties, can I count that neutral as non current carrying? Assume purely resistive loads.

Yes. Because no matter what the imbalance, the total heat will never exceed what it would be, if only the three hots were carrying the full load balanced.

 

[iwire]

Yes. Because no matter what the imbalance, the total heat will never exceed what it would be, if only the three hots were carrying the full load balanced.

Non-linear loads change things.

Also if you have a wye supply and share a neutral between only two phases of it the neutral must be counted as a CCC.

 

[iwire]

I agree, but I'm specifically inquiring about the neutral being shared between 3 separate single pole breakers. If I apply the handle ties, can I count that neutral as non current carrying? Assume purely resistive loads.

With purely resistive loads and all three phases from a wye used with one neutral you are not required to treat the neutral as current carrying.

This applies even if the loading is not balanced.

 

[Carultch]

Non-linear loads change things.

Also if you have a wye supply and share a neutral between only two phases of it the neutral must be counted as a CCC.

Agreed. Non-linear loads because third harmonics accumulate instead of cancel onto the neutral.

And 2-poles/3-wires of a wye system needs to count all three, because the neutral is a mandatory part of the return path current. Even if balanced on the phases, the vectors don't add up to zero without the neutral.

 

[quantum]

Thanks to all who responded. I understand now.

 

[iwire]

Thanks to all who responded. I understand now.

Glad to help.