Sizing a neutral in 3-phase/4-wire?

[Jon456]

I'm upgrading the service to a pumping station from 100A to 200A. I'm installing a new meter base and service main rated at 200A. The SEC will be 3/0 copper THHN. The hot service feeds from the service main into the pump house's main wireway will also be 3/0 copper THHN.

The primary loads are the two 230VAC 3-phase/3-wire pumps; the two pumps draw 100A and 60A respectively. These will be fed by 1AWG and 4AWG feeder taps off the 3/0 service feed, and protected by fusible disconnects.

There is also a need for some 115/230VAC single phase power (for lighting, control circuits, etc.). I'll be installing a 70A single-phase subpanel for those loads. The subpanel will be fed by 4AWG feeder taps off the 3/0 service feed.

I'm still unsure how to size the neutral conductor for the single-phase service. It seems to me that it need not be larger than 4AWG from the service main (at the meter base)... is this correct?

What about the SEC? Should the neutral conductor (from the utility connection into the meter base) be 3/0 to match the "hots" or can it be sized smaller to match the maximum single-phase load?

 

[Dennis Alwon]

You can size the neutral on the main panel to the actual calculated load but it cannot be smaller than a #4. Art. 250.24(C)(1)

 

[Jon456]

Thanks Dennis. What about the size of the neutral for the SEC (from utility service connection/drip loop to the meter base)?

 

[Dennis Alwon]

Thanks Dennis. What about the size of the neutral for the SEC (from utility service connection/drip loop to the meter base)?

Is this a utility company wire? I usually downsize my grounded conductor one size smaller than the ungrounded conductors for a residence. In your situation, esp. the underground part, you may want to plan for expansion but a #4 is all that is required I beleive unless your calculated load needs more.