Neutral for 208/120 3P 4W PNL from Grounded X0 Sec.

bigbls

Member
Location
canada
Occupation
RSE
I feel like i'm missing something here.

Distribution Transf. 3P 500kVA Delta Prim. Grounded Wye Sec. 208V
Panelboard 100A, 120/208 3P, 4W

Idea is to come off the 500kVA transformers X0 with a #2AWG for the panels neutral. Legnth is about 12FT from panel to X0 bus.
Neutral will be run on it's own..

Any insight?

Thanks,
R.L
 

Mr. Serious

Member
Location
Oklahoma, USA
Occupation
Electrical Contractor
Generally you can't downsize a neutral very much, because you have to build it to withstand just about the full phase current. Because if one of the phases became open somehow, that's the amount of current that would be on the neutral. Also, if you have nonlinear loads on a 3-phase system, the neutral will have extra current from harmonics so sometimes it is up-sized instead of down-sized.

That being said, if all of your loads are large 3-phase motors or something like that, and you really won't have much current on the neutral, then you can down-size it. The NEC specifies a minimum size, I believe it's the same as the required size for your main bonding jumper.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
What do you mean the neutral will be "run on it's own"?
You will also have to look to the transformer secondary conductor rules in 240.21(C).
 

Eddy_Current

Member
Location
Canada
I feel like i'm missing something here.

Distribution Transf. 3P 500kVA Delta Prim. Grounded Wye Sec. 208V
Panelboard 100A, 120/208 3P, 4W

Idea is to come off the 500kVA transformers X0 with a #2AWG for the panels neutral. Legnth is about 12FT from panel to X0 bus.
Neutral will be run on it's own..

Any insight?

Thanks,
R.L
What are you asking?
Neutral sizing is 4-018
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Related question: Would #2 conductors meet the 240.21 tap rules with a 500kva transformer as far as ratio ?
If so, I may also be missing something, but I don't see a problem.
 

kwired

Electron manager
Location
NE Nebraska
Related question: Would #2 conductors meet the 240.21 tap rules with a 500kva transformer as far as ratio ?
If so, I may also be missing something, but I don't see a problem.
Good observation. I think many seen 500 kVA and made assumptions but OP did say this is feeding a 100 amp panelboard. I don't believe this neutral conductor would ever need to be any larger than the ungrounded conductors to this panel, which #2 seems a pretty common size you might find. Did say they are 12 feet so 10 foot tap rule is apparently out but if needing to use 25 foot tap rule the ungrounded conductors potentially need to be much larger than 2 AWG depending on primary protection and factoring in primary to secondary voltage ratio. Quick assumption might mean he needs at least a 455 amp conductor for the ungrounded tap conductors.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I seem to recall tap rules can’t be used on a transformer secondary. I have installed only a few small transformers.
The tap rules are 240.21(B). Transformer secondary conductors follow the rules of 240.21(C).

Separate conductors can be run from the transformer secondary (not tapped from other conductors) to the panel, subject to 240.21(C).
 

kwired

Electron manager
Location
NE Nebraska
240.21 would apply to the ungrounded conductors. Now what size does that need to be needs first consideration here I believe.

If I am wrong correct me, but assuming a 500kVA transformer and 125% OCPD on the primary, then the secondary current is 1388 amps. take that times 1.25 for the overcurrent device and we have a starting base here of 1735 for the 240.21(C) secondary conductor (could vary some depending on exactly what primary volts is and any rounding up for next standard size). OP said the conductor length is 12 feet, that puts you using the 25 foot tap rule and you need a conductor that is at least 1/3 ampacity of the 1735 which is 578. Most probably would use parallel 350's, but 1250 kcmil is acceptable. Good luck landing either of those in a 100 amp panel.

Now sizing the grounded conductor I'm pretty certain it can't be any smaller than either T 250.66 or maybe 250.102(C)(1), either one gives you same result though. for parallel 350 you would need the grounded conductor here to be no less than 2/0 copper.

If you can get the length down to 10 feet or less you only need 174 amp ungrounded conductors and makes life a lot easier, and the grounded conductor likely can be as small as 4 AWG (or whatever calculated neutral load may be if more than #4 can handle). Bit much for many 100 amp loadcenters but can maybe bump it up to 150-225 pretty easily and have plenty space and large enough lugs to land those conductors.
 

Mr. Serious

Member
Location
Oklahoma, USA
Occupation
Electrical Contractor
I don't think I understood the original question before I replied last time. After reading it again, I am envisioning a large service, 800 to 1200 amps, with subpanels in various places in the building, and OP wants to set a new 100A subpanel near the transformer, but doesn't have a neutral wire available in the panel feeding it, so he was asking if a separate neutral can be added directly from the transformer, just for the new 100A subpanel. Of course, the answer is "no." If my revised understanding of the question is correct, then the correct answer here is the very first one:

It must be run with the other conductors
 
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