Neutral size

[augie47]

Rather than "muddying" this thread: http://forums.mikeholt.com/showthread.php?t=120996,
I brought my question here.
When you have a SDS with your system bonding and grounding electrode conductors terminated at the transformer and an equipment ground and neutral routed to your first disconnect, is there a minimum size for the grounded conductor (other than load associated) ?
250.30(A)(8) addressed the situation where the MBJ is not at the transformer.
250.24 (C) addresses minimum size grounded at services, but there we don't have the grounding conductor.
215.2 addresses minimum neutrals for feeders, but this isn't a feeder and the referenced 250.122 wouldn't be applicable either.

So, is there a minimum ?

 

[chris kennedy]

250.66......

 

[augie47]

I may just be "dumb" this morning but I don't see where that address the grounded conductor.
If the GEC came from the 1st disconnect or we didn't have a grounding conductor to the 1st disconnect, I can see that being applicable.

 

[chris kennedy]

I may just be "dumb" this morning

Great, its not just me today.

What does 250.24(C)(1) say?

 

[augie47]

Great, its not just me today.

What does 250.24(C)(1) say?

"Grounding Service Supplied A/C Systems".. I think that should apply but this isn't a service and we have the grounding conductor as our fault path in this case

thus the frustration

 

[chris kennedy]

I see, SDS, missed that part. Good question.

 

[erickench]

The minimum size of the neutral between the disconnect to the transformer would have to be sized in accordance with either NEC 250.66 or NEC 250.122. Even though it's not a feeder and the NEC does not give a rule for this I would size it in accordance with 250.122 because it is on the load side of the system bonding jumper.

 

[chris kennedy]

215.2 addresses minimum neutrals for feeders, but this isn't a feeder

Gus, your a handbook guy;

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

The definition of service includes the statement that electric energy to a service can be supplied only by the serving utility. If electric energy is supplied by other than the serving utility, the supplied conductors and equipment are considered feeders, not a service.

 

[erickench]

Yes but the system bonding jumper and the grounding electrode have to be in the same location. If the SBJ were at the disconnect I would size the neutral in accordance with NEC 250.66 because it's on the supply side. I would treat it as if it were a service.

 

[RUWired]

Augie i don't see a requirement to bring a grounded conductor into the picture at all. 250.8 says where installed.

Rick

 

[augie47]

Augie i don't see a requirement to bring a grounded conductor into the picture at all. 250.8 says where installed.

Rick

I miss the connection to 250.8, but, I agree, since you have a grounding conductor, the grounded is unneeded unless you have a neutral load.
Thus the question, if the neutral is not used for grounding, such as a service neutral is, what would be the minimum size. Chris' reference to the Handbook note probably shows the intent (treat it as a feeder) but I can't enforce "handbook". If someone installed a #8 neutral from the transformer to handle a 40 amp neutral load on a 225 amp panel derived from a SDS and you rejected it, what Article would you quote as your rejection source ?

 

[RUWired]

Augie i think you would be looking for article 215.(A)1


The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.

Rick

 

[augie47]

Chris started me that way with his reference to the handbook and I failed to look up the definition of a feeder. Now that you mentioned it again, I checked "feeder" and sure enough it includes transformer secondary.
Thanks Rick, Thanks Chris

 

[yired29]

Augie i think you would be looking for article 215.(A)1


The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.

Rick

How can we size this conductor based on 250.122 if it's ahead of the OCPD?
I thought it has to be sized based on line side conductors 250.102 C (but this refers to services).

 

[augie47]

How can we size this conductor based on 250.122 if it's ahead of the OCPD?
I thought it has to be sized based on line side conductors 250.102 C (but this refers to services).

I agree and it still leaves some question which the CMPs need to address (a good 2014 proposal for us). But we do have a section that identifies it as a feeder and with 215.2 some justification to sizing the condcutor to at least 250.122 or 250.66

 

[yired29]

I agree and it still leaves some question which the CMPs need to address (a good 2014 proposal for us). But we do have a section that identifies it as a feeder and with 215.2 some justification to sizing the condcutor to at least 250.122 or 250.66

I would always use 250.66 for both grounded and grounding conductor. This is a line side connection. In order to use 250.122 the conductor has to be after the OCPD.

 

[SG-1]

Use of Derived Grounded Conductor

Use of Derived Grounded Conductor

I hope this is what you were asking.

250.30(A)(1) Exception 2, allows an alternative method of bonding that uses the grounded conductor between the source enclosure and the first disconnect, instead of an Equipment Bonding Jumper. A parallel path is not permitted, except earth.

250.142(A)(3) allows the grounded conductor to be used to ground non-current carrying metal parts... where permitted by 250.30(A)(1).

If there are no neutral loads to be served then the grounded conductor serves as the return for ground fault current to the source.

250.30(A)(1) Exception 2 states that the grounded conductor must not be smaller than the size specified for the Equipment Bonding Jumper, which is not present.

The Equipment Bonding Jumper is sized per 250.102(C), (Table 250.66 for Grounding Electrode Conductors).

Thus the miminum size is established.

 

[yired29]

I hope this is what you were asking.

250.30(A)(1) Exception 2, allows an alternative method of bonding that uses the grounded conductor between the source enclosure and the first disconnect, instead of an Equipment Bonding Jumper. A parallel path is not permitted, except earth.

250.142(A)(3) allows the grounded conductor to be used to ground non-current carrying metal parts... where permitted by 250.30(A)(1).

If there are no neutral loads to be served then the grounded conductor serves as the return for ground fault current to the source.

250.30(A)(1) Exception 2 states that the grounded conductor must not be smaller than the size specified for the Equipment Bonding Jumper, which is not present.

The Equipment Bonding Jumper is sized per 250.102(C), (Table 250.66 for Grounding Electrode Conductors).

Thus the miminum size is established.

This execption has a system bonding jumper in XFMR and Disc. this would only work with a nonmetallic raceway or wiring method. With a metallic raceway we would have a parallel path for grounded conductor.

The OP states system bonding is at transformer and there is a grounded and bonding conductor routed to first disc.

But I still agree with 250.102 C and 250.66

 

[RUWired]

How can we size this conductor based on 250.122 if it's ahead of the OCPD?
I thought it has to be sized based on line side conductors 250.102 C (but this refers to services).

The equipment bonding jumper is sized per 250.102(C).

The grounded conductor is sized based on the phase conductors only when a system bonding jumper is placed at the transformer and the first OCPD.

When an equipment bonding jumper is used in a transformer feeder circuit, the grounded neutral conductor is sized per it's use and at minimum, 215.2(A)1 requires it to be sized per table 250.122.

For example the OP mentioned a 250 amp feeder. If a transformer is installed for 3-phase motor loads, convenience outlets and a lighting circuit using a 250 amp main breaker. The conductors would be 250 kcmil (240.21C), the equipment bonding jumper would be a # 2 awg (250.30(A)2), and the grounded neutral conductor would be a # 4 awg (215.2A1).

Rick

 

[augie47]

I can live with that since the NEC defines the conductors from a SDS as "Feeders", and 215,2 is worded as it is, but I think there is a flaw in sizing any grounding conductor AHEAD of the OCP device using 250.122