Service neutral and "grounded conductor" sizing (2024)

I am having a discussion with an engineer regarding sizing the incoming service neutral to a service switch for a fire pump. Code dictates you need to run a neutral to the line side of the service switch, even if you do not need the neutral on the load side (i.e. a motor like a FP). Our disagreement stems on what section/table that should be used to size this neutral.

The engineer is pointing to NEC 250.24(C) which refers to "the grounded conductor brought to service equipment.", which then points to Table 250.66 for grounding electrode conductors.

I always thought the purpose to table 250.66 was to size GECs only, and that the service netural was the same size as the phase-conductors.

1) Is this a wrong assumption?
2) Is "grounded conductor" an AKA for a neutral?

I feel like all the neutrals I ever see are the same as the phase wires, not sized by Table 250.66.

"Grounded conductor" is the neutral and does not need to be the same size as the ungrounded conductors.

For sizing the "grounded conductor" see 220.61

lielec11 said:

I feel like all the neutrals I ever see are the same as the phase wires, not sized by Table 250.66.

Click to expand...

Since there is no EGC ahead of the service disconnect the neutral needs to be large enough to clear a fault. As Roger said it does not need to be full size. In this case the fire pump doesn't require a neutral so it's brought to the service disconnect for fault clearing purposes.

roger said:

"Grounded conductor" is the neutral and does not need to be the same size as the ungrounded conductors.

For sizing the "grounded conductor" see 220.61

Click to expand...

220.61 appears tp only apply to household ranges, etc. 220.61(B)(1) or specific electric systems like 3-wire DC, no? Or am I misreading that?

You also said they are correct, and I am leaning towards that as well, but you're both pointing to two different code sections. This engineer is using 250.66 to size the neutral (or grounded conductor in this case). In our case we have 750s AL so they're using a minimum of 3/0 AL neutral.

lielec11 said:

220.61 appears tp only apply to household ranges, etc. 220.61(B)(1) or specific electric systems like 3-wire DC, no? Or am I misreading that?

Click to expand...

Even though 220.61 comes after 220.55 it is for the service as well as feeders.

220.61 Feeder or Service Neutral Load.

Read 250.24(C)(1) and you will see it takes you to 250.66.

I'm not sure where 220.55 came into play, was that a typo?

I did read 250.24(C)(1) and the reference to the Table 250.66. I guess my main question is where in the NEC does it link neutral conductor and "grounded" conductor? There is nothing in the definitions that states as much.

In regards to 220.61(B)(2) ... (because I don't think (B)(1) applies to fire pumps or any commercial loads) ... it references "...a portion of the unbalanced load in excess to 200A ..." How does that apply to a fire pump and how is the calculation done to prove the unbalanced current?

lielec11 said:

I'm not sure where 220.55 came into play, was that a typo?

Click to expand...

No it wasn't a TYPO you said you thought 220.61 was referring to household ranges and since 220.61 comes right after household ranges (220.55) it seemed as though that had you confused.

As far as the neutral being the grounded conductor, the NEC is not an instruction manual, it does not need to define every item but read the title of article 200.

ARTICLE 200
Use and Identification of Grounded
Conductors

Thanks for the guidance to section 200, definitely an overlooked part of my code book.

The last open item is in regards to 220.61(B)(2). I don't see how this applies to, or how to implement it, for a fire pump load. I still think the proper way is how the engineer in my original post sized it, using 250.24(C)(1) and Table 250.66.

lielec11 said:

Thanks for the guidance to section 200, definitely an overlooked part of my code book.

The last open item is in regards to 220.61(B)(2). I don't see how this applies to, or how to implement it, for a fire pump load. I still think the proper way is how the engineer in my original post sized it, using 250.24(C)(1) and Table 250.66.

Click to expand...

I see my post sending you to 220.61 could be confusing, it was meant to point out that the neutral does not have to always be the same size as the ungrounded conductors. With that said I agree the engineer is on top of things.

roger said:

I see my post sending you to 220.61 could be confusing, it was meant to point out that the neutral does not have to always be the same size as the ungrounded conductors. With that said I agree the engineer is on top of things.

Click to expand...

I understand, thanks for sticking with me through this little discussion!

lielec11 said:

I understand, thanks for sticking with me through this little discussion!

Click to expand...

You're welcome.