Neutral sizing

[Farmfly]

hey all, I was looking at some prints that’s calling for a new service. It is a 600 amp three phase 208. it is calling for 2 to 500 MCM per phase with a 1/0 AWG neutral. Am I wrong to think that you’re supposed to match the neutral size to the ungrounded conductor?

 

[texie]

hey all, I was looking at some prints that’s calling for a new service. It is a 600 amp three phase 208. it is calling for 2 to 500 MCM per phase with a 1/0 AWG neutral. Am I wrong to think that you’re supposed to match the neutral size to the ungrounded conductor?

The neutral does not have to be the same size as the ungrounded conductors. It needs to be large enough for the calculated load or per 250.24(C), whichever is larger. This is to ensure that there is a large enough fault return path.

 

[augie47]

The servive neutral must be:
(a) sized to carry the load (220.61)
(b) meet the requirements of 250.24(C)

In your case the 1/0 would meet (b) IF the 500s are in separate conduits and there is a 1/0 in each conduit.
Beyond that, a load caluclation would need to be performed to see if the 1/0 is adequate for the load.

 

[Farmfly]

The print and saying all three phases in the same 5 inch conduit does that change anything?

 

[augie47]

If he 500s are parallel in the same conduit and you pulled (1) neutral, then Art 250 would required you look at the combined area and your minimum neutral would be a 2/0 (excluding load requirements)


(5" rigid ?..OUCH, my back)

 

[Farmfly]

Schedule 40 PVC

 

[Farmfly]

Wouldn’t three separate runs be better but it is going up a telephone pole

 

[augie47]

Lots of possibilities... suggest you go back to whomever drew the prints,

 

[Farmfly]

Appreciate the advice thanks buddy