Missing neutral question

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
electrofelon said:
What size are the conductors? Sounds like a pretty easy task to pull them out and back in with a small neutral.
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Two sets of three 500kcmil in separate UG PVC conduits over 100' long installed 50 or more years ago.

That brings up another couple of questions. We are allowed to reduce the neutral to the size of a bonding jumper per 250.102 but that table directs us to size it at 2/0 because the two 500's sum to between 600 and 1100. Can we split that in half and run two #2's, one per conduit? I assume that we cannot run a neutral in just one of the conduits; is that correct? It doesn't seem reasonable to me that we would have to run a 2/0 neutral in both conduits.
 
synchro

synchro

Senior Member
Location
Chicago, IL
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EE
ggunn said:
Two sets of three 500kcmil in separate UG PVC conduits over 100' long installed 50 or more years ago.

That brings up another couple of questions. We are allowed to reduce the neutral to the size of a bonding jumper per 250.102 but that table directs us to size it at 2/0 because the two 500's sum to between 600 and 1100. Can we split that in half and run two #2's, one per conduit? I assume that we cannot run a neutral in just one of the conduits; is that correct? It doesn't seem reasonable to me that we would have to run a 2/0 neutral in both conduits.
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According to 250.122(F)(1)(b), it seems like you'd have to run a 2/0 in each conduit.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
ggunn said:
Two sets of three 500kcmil in separate UG PVC conduits over 100' long installed 50 or more years ago.

That brings up another couple of questions. We are allowed to reduce the neutral to the size of a bonding jumper per 250.102 but that table directs us to size it at 2/0 because the two 500's sum to between 600 and 1100. Can we split that in half and run two #2's, one per conduit? I assume that we cannot run a neutral in just one of the conduits; is that correct? It doesn't seem reasonable to me that we would have to run a 2/0 neutral in both conduits.
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Funny, just yesterday I pulled 2 100' runs of 500-500-500-4/0-2/0 AL. You must be super jelly of my 4/0 neutral.

Well that is a not insignificant amount of work and cost, would be nice to fine another solution. I would have to review the code book for the minimum neutral size from parallel runs.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
synchro said:
According to 250.122(F)(1)(b), it seems like you'd have to run a 2/0 in each conduit.
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That makes no sense to me. The CCCs are split, why not the neutral? It's not like an ECG that needs to be able to clear the fault for the whole rating of the OCPD.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I cannot tell you about the legality of a zig-zag derived neutral, but I can tell you about the zig-zag itself.

A zig-zag transformer has two equal voltage phase coils on each leg, arranged to derive the neutral voltage of the three connected phases. If you connect a zig-zag transformer to an existing wye system the derived neutral will be at approximately the same voltage as the source neutral, but due to normal system imbalance will likely not be at exactly the same voltage.

If you connect a zig-zag transformer to a wye system and treat the derived neutral as a hot, it will work just fine. If you ground the derived neutral then large circulating currents can flow as the two separate neutrals fight it out.

Question about the PV meter: does it require a neutral or does it require a grounded conductor?

The wye side of a delta:wye transformer can be used to derive a neutral. Nothing gets connected to the delta side; your three phases are connected to the wye, and X0 becomes your derived neutral.

-Jon
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
ggunn said:
The connection to the neutral in the PV meter is just a short piece of #14. I would have thought (hoped) there existed a small transformer I could connect to the phase conductors that would tell the meter where the center of the wye is, but that's just my fantasy.
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I'm wondering if there's some sort of overcurrent protection inside of the meter that allows a #14 neutral wire to be used. I don't think that would affect what you have to use for the neutral conductor on the feeder, but it is kind of a crazy situation.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
winnie said:
The wye side of a delta:wye transformer can be used to derive a neutral. Nothing gets connected to the delta side; your three phases are connected to the wye, and X0 becomes your derived neutral.
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That's something I hadn't thought of: an isolation transformer that only derives the neutral. I wonder how small such a transformer I can find. I wouldn't even have to worry about what the secondary voltage is, would I?
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
RWC/NC. said:
Would 310.10(G)(1) Exception No. 2 : possibly being utilize, under this particular situation (?)

..Curious..
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I would think so, but there's still the issue of the minimum sized neutral being tied to T250.102(C)(1) which makes no provision for splitting it for parallel sets. The table is meant for grounding jumpers which cannot be split like that.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
ggunn said:
That's something I hadn't thought of: an isolation transformer that only derives the neutral. I wonder how small such a transformer I can find. I wouldn't even have to worry about what the secondary voltage is, would I?
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Don't think of it as an isolation transformer so much as an autotransformer that just happens to have an isolated coil inside the box. You are only connecting to the wye coils.

IMHO you would size it and treat it just like the zig-zag. It would only need to supply the expected neutral load....if you were allowed to use it at all.

Jon
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
winnie said:
IMHO you would size it and treat it just like the zig-zag. It would only need to supply the expected neutral load....if you were allowed to use it at all.
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There is no neutral load, really; it's just for instrumentation.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
winnie said:
Don't think of it as an isolation transformer so much as an autotransformer that just happens to have an isolated coil inside the box. You are only connecting to the wye coils.

IMHO you would size it and treat it just like the zig-zag. It would only need to supply the expected neutral load....if you were allowed to use it at all.

Jon
Click to expand...

I believe there's still the issue of 215.11 that Wayne raised earlier.
 
kwired

kwired

Electron manager
Location
NE Nebraska
synchro said:
I think either a zig-zag or a "wye grounded delta" (the one I was mentioning) would be considered as an autotransformer because it does not produce an output that is isolated from the input.
In the application being considered by ggunn, it would not really be a "grounding autotransformer" as in 450.5 because it is not "providing a neutral point for grounding purposes", even though it is providing a neutral.
I agree, though, that 215.11 could be problematical in this case.
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It should create a neutral that is normally in close proximity to same voltage as the source neutral, but you probably do not want to ground it or you can possibly have unwanted current flowing on unwanted paths. And if OP ran a grounding conductor back to the service, he would no longer need the autotransformer. You should only ground at the service or at separately derived systems.

If you used this autotransformer I would think that if the derived neutral is not grounded it is like any other ungrounded conductor and would at least be required to have an overcurrent device on it.
 
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