Neutral Sizing

Status
Not open for further replies.
S

Samardas

Member
Location
Los Angeles, CA, United States
Occupation
Electrical Engineer
Hi,
A 400A, 120/208V, 3 Phase Panel will feed 3 x 120/208V, 1 Phase Sub-panels (panel) along the same line with a furthest distance of the Sub-panel at about 230'. Since each panel is 100A, 120-208V, 1 Phase, the proposal is to run 6 phase conductors with 1 Neutral conductor to the first panel and then continue to the third panel ending with 2 conductors and the same common neutral. The question - is it required to run a neutral for each panel from the 400A Panel, making the total neutral to the first panel to be 3? In other words, is it necessary to run 9 conductors up to the first Sub-panel or 7 is good enough using the same neutral for all the panels? Does the size of the neutral change in the 7-conductor configuration, if it is permitted? Your help will be highly appreciated. Thanks
 
Samardas said:
Hi,
A 400A, 120/208V, 3 Phase Panel will feed 3 x 120/208V, 1 Phase Sub-panels (panel) along the same line with a furthest distance of the Sub-panel at about 230'. Since each panel is 100A, 120-208V, 1 Phase, the proposal is to run 6 phase conductors with 1 Neutral conductor to the first panel and then continue to the third panel ending with 2 conductors and the same common neutral. The question - is it required to run a neutral for each panel from the 400A Panel, making the total neutral to the first panel to be 3? In other words, is it necessary to run 9 conductors up to the first Sub-panel or 7 is good enough using the same neutral for all the panels? Does the size of the neutral change in the 7-conductor configuration, if it is permitted? Your help will be highly appreciated. Thanks
Click to expand...
see 215.4. Looks like you are ok. You will need to derate your conductor due to having 9 CCC.
 
odd... or more likely I'm misreading one of the Sections as :
200.4(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\
maybe 215,4 is one of the "specifically permitted".
 
Obviously doesn't apply to a feeder or service entrance neutral.

Sent from my BE2028 using Tapatalk
 
augie47 said:
odd... or more likely I'm misreading one of the Sections as :
200.4(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\
maybe 215,4 is one of the "specifically permitted".
Click to expand...
IIRC, both of those sections are kinda new. I seem to remember it being ambiguous if you could share a neuter between feeders?
 
electrofelon said:
IIRC, both of those sections are kinda new. I seem to remember it being ambiguous if you could share a neuter between feeders?
Click to expand...
200.4(A) shows as entirely revised in my version of the 2011 NEC. It has not changed since.

215.4 is the same in the 2020 as it was in the 2011, and is not shown as a revision, so must date back to at least the 2008.

(Not what I'd call 'kinda new' but your milaege may vary.)

Also see 220.61. To my mind, the neutral to the first panel needs to be sized for 300A, and the section to the second panel for 200A, unless you want to get into the nitty-gritty of permitted reductions.
 
Samardas said:
... Does the size of the neutral change in the 7-conductor configuration, if it is permitted?
Click to expand...

In the setup you described, each phase will be connected to two of the three 100A 1-phase subpanels. And so the load on neutral conductor in the feeder between the 400A 3-phase panel and subpanel #1 would be the same as that from a 200A 3-phase panel having the same total branch circuit load as in all of the subpanel loads added together. The worst case neutral current would be 200A if the loading was only L-N and on just one of the phases A, B, or C.

The worst case current on the neutral conductor between the #2 and #3 panels would also be 200A if the loading was only L-N and on just the one phase that the two panels have in common.
 
synchro said:
The worst case current on the neutral conductor between the #2 and #3 panels would also be 200A if the loading was only L-N and on just the one phase that the two panels have in common.
Click to expand...
That should have been:
The worst case current on the neutral conductor between panels #1 and #2 would also be 200A if the loading in the #2 and #3 panels was only L-N and on just the one phase that #2 and #3 have in common.
 
synchro said:
In the setup you described, each phase will be connected to two of the three 100A 1-phase subpanels. And so the load on neutral conductor in the feeder between the 400A 3-phase panel and subpanel #1 would be the same as that from a 200A 3-phase panel having the same total branch circuit load as in all of the subpanel loads added together. The worst case neutral current would be 200A if the loading was only L-N and on just one of the phases A, B, or C.

The worst case current on the neutral conductor between the #2 and #3 panels would also be 200A if the loading was only L-N and on just the one phase that the two panels have in common.
Click to expand...
New here is an interesting question: if the panels be are connected A-B, B-C, and C-A, then does the neutral from the supply to the first panel count as a current-carrying conductor?
 
The rule in200.4 was from one of my proposals, but it had a companion proposal that would have defined "common neutral". They took the action to prohibit common neutrals, but did not accept the adding a definition so that the code user could more easily understand what 200.4 is saying.

215.4 and 225.7(B) permit common neutrals.

My proposed definition of a common neutral was: A neutral that serves two or more ungrounded conductors where there is no potential between those ungrounded conductors" (or something like that, I did not go back and look up my actual proposal)
 
If you compare the total circular mil of the 4 wire feeder to the 3 separate 3 wire feeders using the common neutral, the separate feeders come out slightly smaller for the run from origin to first panel. After the first panel the 4 wire feeder ends up much larger unless you add OCPD or can apply tap rules, because you have conductors only carrying 100A but sized for the 200A OCPD.

-Jon
 
electrofelon said:
New here is an interesting question: if the panels be are connected A-B, B-C, and C-A, then does the neutral from the supply to the first panel count as a current-carrying conductor?
Click to expand...

That's an interesting question, but I can envision cases where this would not be a good assumption. For example, if each 100A subpanel was powering a different unit in an apartment complex, then one or two units might be unoccupied and consuming minimal or possibly no power. Or the main breaker in a subpanel might be turned off for electrical work. If such situations were to occur then the currents from the different phases into the neutral will not balance out and cancel to any significant extent.
 
Thank you all, for your interesting analyses. Just to recapture what we are talking about: The Sub-panels (A-B, B-C & C-A configuration) in question are all 120/208V, 1 Phase, (similar to a residential (120/240V) panel) in an outdoor, non residential application. Here is a hand-sketch, if that helps. There are 6 CB's of 100A each in the Main 400A Panel and there are 6x100A Sub-panels serving 120V, 20A receptacles. Thanks again...
 

Attachments

  • 400A Distribution Diagram_112221.jpg
    400A Distribution Diagram_112221.jpg
    305.5 KB · Views: 19
Ahh, I didn't get that this was outside. In this case I believe 'outside taps of unlimited length' applies. This would let you use smaller conductors for some of the segments of the 'single circuit' approach (a 200A 4 wire 3 phase circuit tapped at 3 locations with 100A single phase sub panels).

As has already been noted, you can use a 'common neutral' for this installation. The common neutral probably reduces voltage drop in the installation. At the lengths and voltages you are describing, voltage drop is a concern.

-Jon
 
synchro said:
That's an interesting question, but I can envision cases where this would not be a good assumption. For example, if each 100A subpanel was powering a different unit in an apartment complex, then one or two units might be unoccupied and consuming minimal or possibly no power. Or the main breaker in a subpanel might be turned off for electrical work. If such situations were to occur then the currents from the different phases into the neutral will not balance out and cancel to any significant extent.
Click to expand...
That's normal for any "don't count the neutral as a CCC" situation. If one of the other conductors isn't carrying current, then the current on the neutral goes up, but you can't have all the conductors including the neutral carrying maximal current simultaneously (barring extreme power factors).

As far as I can see, there are only 6 CCCs in the conduit from the supply to the first panel in the 7-wire scenario. Consider the alternate 4-wire version--is the neutral current goes to differ from the 7-wire version, for any given set of loadings from the panels? I don't see why it would be.

Cheers, Wayne
 
Status
Not open for further replies.
Top