Statement from a recent thread that was closed

[david]

Ckt 1 phase A - brn
Ckt 1 phase B - org
Ckt 1 phase C - yel
Ckt 2 phase A - blk
Ckt 2 phase B - red
Ckt 2 phase C - blu

N for Ckt 1 & 2 - wht - just one for both. neutral currents will be almost non-existent since the circuits are closely balanced

First it is not my intention to open a closed thread.

The issue of color coding or other effective means of identification was well addressed

But just before the thread was closed I noticed this statement

"N for Ckt 1 & 2 - wht - just one for both. neutral currents will be almost non-existent since the circuits are closely balanced"

Having one neutral for these six 277 volt branch circuits is a bad idea. To say the the loads will be closely balanced makes the assumption that all the lighting will be on at the same time.
There is no consideration given for the lighting on A phase in both what is being identified as circuit 1 &2 being on while loads on phase B and C being turned off

Feel free to close this thread but I think it was important to address that statement

 

[LarryFine]

"N for Ckt 1 & 2 - wht - just one for both. neutral currents will be almost non-existent since the circuits are closely balanced"

Having one neutral for these six 277 volt branch circuits is a bad idea. To say the the loads will be closely balanced makes the assumption that all the lighting will be on at the same time.
There is no consideration given for the lighting on a phase in both what is being identified as circuit 1 &2 being on while loads on phase B and C being turned off

Absolutely, you're correct; each group of 3ph lines must have its own neutral.

A single neutral for two groups can easily be overloaded by imbalance, just as one can in 1ph installs.

 

[augie47]

and a violation of 200.4

 

[david]

problem. and a violation of 200.4

Yes, I once had a well respected Forman who looked at this as electrical theory and not a real proplem.after he could visualize with an amp probe what was going on he understood why it was a violation.

 

[winnie]

This arrangement is called a 'common neutral' circuit and has history in code.

Previously it was explicitly permitted for outside lighting branch circuits, and was not prohibited for other branch circuits. We had long discussions about the code status of common neutral circuits. I believe current code prohibits them for most situations.

When common neutral circuits were permitted, the neutral ampacity had to be sufficient for the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the supply. A common neutral had to be sized for the worst case neutral current, not the normal neutral current.

I don't believe there is any significant benefit to using a common neutral circuit. You might be able to save a tiny bit of copper on the neutral, say by using a single #10 instead of two #12s.

Jonathan

 

[david]

This arrangement is called a 'common neutral' circuit and has history in code.

Previously it was explicitly permitted for outside lighting branch circuits, and was not prohibited for other branch circuits. We had long discussions about the code status of common neutral circuits. I believe current code prohibits them for most situations.

When common neutral circuits were permitted, the neutral ampacity had to be sufficient for the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the supply. A common neutral had to be sized for the worst case neutral current, not the normal neutral current.

I don't believe there is any significant benefit to using a common neutral circuit. You might be able to save a tiny bit of copper on the neutral, say by using a single #10 instead of two #12s.

Jonathan

That's good to know, never came across that in the field, at least not that I am aware of. Good to know it could be out there in some of these older buildings

 

[Dennis Alwon]

Gus gave the section number. For those without a book

(A) Installation.
Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this Code.

 

[GoldDigger]

This arrangement is called a 'common neutral' circuit and has history in code.

Previously it was explicitly permitted for outside lighting branch circuits, and was not prohibited for other branch circuits. We had long discussions about the code status of common neutral circuits. I believe current code prohibits them for most situations.

When common neutral circuits were permitted, the neutral ampacity had to be sufficient for the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the supply. A common neutral had to be sized for the worst case neutral current, not the normal neutral current.

I don't believe there is any significant benefit to using a common neutral circuit. You might be able to save a tiny bit of copper on the neutral, say by using a single #10 instead of two #12s.

Jonathan

The common neutral came into use along with knob and tube wiring, where the installation cost and space requirements of the extra wire run were more important factors than the lower copper cost.
And I doubt that they really considered the nuances of sizing the common neutral. You might have only two or three 15A r 20A circuits, and the wires were in free air.