Sizing a service question


Say I have 3-phase commercial service that has a calculated maximum load of 685 amps after all correction and adjustment factors have been applied, as well as all 125% continuous load factors added in. I’m using 3 conductors per phase for service conductors, and I choose 4/0 Xhhw-2 copper conductors for a total ampacity of 690 Amps(230A x 3 conductors per phase).

Is it accurate that I may use the provisions of 240.4(B) to provide the Main fuse/breaker protection and go to the next higher standard size of 700 Amps?

It appears that a great deal of commercial calculations has their conductor ampacity exceeding their main fuse/breaker size even though they technically should not have to. I know there is nothing wrong with having a higher conductor ampacity but from a code standpoint, I don’t see anything wrong with having my breaker size exceed my wire ampacity as long I don’t go past another fuse/breaker size or exceed 800 amps.

Is there a code article that supersedes my ability to go to the next standard size or should my service conductor ampacity always match or exceed my fuse/breaker size?

Dennis Alwon

Staff member
Chapel Hill, NC
Retired Electrical Contractor
Yes, if the actual load is not higher than 690 amps then you can go to the next size breaker.

The 125% for continuous loads should not be factored in for a service calculation.


Electron manager
NE Nebraska
In WA we have a state code rule that requires a sign on the service if the service conductor ampacity is smaller than the OCPD size.
Sounds like something that mostly adds confusion to those that aren't electrical professionals.

A sign requiring use of 350 amp fuses in a 400 amp fuse holder makes sense, but if the 400 amp fuse is allowed even though the conductor is only rated 380 amps just going to confuse anyone that doesn't understand if you make any mention of it. If someone is installing more load - they are supposed to be trained and should know if more needs done than just adding the load. JMO.