Sizing service conductors (other than 1 phase dwelling units).

[Travis31415]

I'm a bit confused as to the correct way to size commercial service conductors. I've always sized them at 100+% of the OCPD. I base this off of 230.90(A) that says "such protection shall be provided by an overcurrent device that has a rating or setting not higher than the ampacity of the conductor".

Exception No 2 makes it seem as though the next size up rule of 240.4(B) applies to service conductors.

So say, based on the rule 230.42, I calculate the maximum load to be served at 150 amps, can I then use 4/0 xhhw (180a) with a 200a breaker?

Am I misunderstanding something?

 

[augie47]

1st off the conductor must be sized to carry the calculated load.
Once selected, the the OCP device is sized to protect the conductor.
In your case, a 4/0, 180 amp rated conductor obviously is sized above the required and you can use the next size up breaker, 200 amp.
IF you load had been 185 amps the you would have had to go with a larger conductor.

 

[Travis31415]

1st off the conductor must be sized to carry the calculated load.
Once selected, the the OCP device is sized to protect the conductor.
In your case, a 4/0, 180 amp rated conductor obviously is sized above the required and you can use the next size up breaker, 200 amp.
IF you load had been 185 amps the you would have had to go with a larger conductor.

That's the conclusion I came to as well. In my current situation though, the location is not built out and has no designated purpose, thus an accurate load calculation would be impossible. I assume that I'm better off using 250s?