Conductor sizing for subpanels

[art]

I think I read or heard that feeders to a subpanel have to be sized according to the size of the subpanel disconnect breaker and not the load on the subpanel. Is this correct?

With a 100 A breaker would I size the feeders for 80 A?

Any help and education is appreciated.

 

[texie]

I think I read or heard that feeders to a subpanel have to be sized according to the size of the subpanel disconnect breaker and not the load on the subpanel. Is this correct?

With a 100 A breaker would I size the feeders for 80 A?

Any help and education is appreciated.

If you have a 100 amp source breaker you need to size the feeders for an ampacity of 100 amp per Table 310.16 after any adjustments. The "sub" panel must have an ampacity of 100 amp.

 

[augie47]

The feeder conductors should be sized at a minimum to carry the load and then protected at their ampacity with Art 240.4 in mind.
If you had an 80 amp calculated load you would likely need a #4 Cu which has an amapaity of 85 amps (74° col,.). You could protect that cable with a 90 amp breaker. The subpanel could be any size 80 amps or greater.

 

[infinity]

If you had an 80 amp calculated load you would likely need a #4 Cu which has an amapaity of 85 amps (74° col,.). You could protect that cable with a 90 amp breaker. The subpanel could be any size 80 amps or greater.

Augie, wouldn't the panel rating need to be at least 90 amps?

 

[augie47]

Augie, wouldn't the panel rating need to be at least 90 amps?

Yes, Sir. With a 90 amp beaker feeding it. My error, Thanks.