Feeder size

[Toros]

Hi,
the question is that,
down here in LA, we ust 500 MCM overhead feed to 400A service located outdoor, sun-exposed and it is approved by athorities;

now, why we can not feed 1200A service via under ground with (3) sets of 500 MCM
they say it is not good enough, use 600 MCMs
???????????

Tanx

 

[GoldDigger]

Because the current carrying (ampacity) tables predict that the current carrying capacity of a conductor in free air (regardless of sun heating and high ambient) will be greater than the ampacity of that same conductor in an underground duct with less air circulation to cool the insulation.

 

[JFletcher]

Hi,
the question is that,
down here in LA, we ust 500 MCM overhead feed to 400A service located outdoor, sun-exposed and it is approved by athorities;

now, why we can not feed 1200A service via under ground with (3) sets of 500 MCM
they say it is not good enough, use 600 MCMs
???????????

Tanx

One, POCO rules are different from NEC.

Two, once over 800A, the conductors have to meet the ampacity of the OCPD (or something similarly worded). Dunno the code section oft hand because I dont deal with them often enough to commit it to memory.

 

[Dennis Alwon]

One, POCO rules are different from NEC.

Two, once over 800A, the conductors have to meet the ampacity of the OCPD (or something similarly worded). Dunno the code section oft hand because I dont deal with them often enough to commit it to memory.

My guess is the contractor is responsible for the service drop and laterals not the power company as they would never run 500 mcm for a 400 amp service

 

[augie47]

Hi,
the question is that,
down here in LA, we ust 500 MCM overhead feed to 400A service located outdoor, sun-exposed and it is approved by athorities;

now, why we can not feed 1200A service via under ground with (3) sets of 500 MCM
they say it is not good enough, use 600 MCMs
???????????

Tanx

Because of 240.4(B) (cant go to "next size up once you pass 800 amps) and 110.14(C) (normally, you have to terminate at the 75° rating for 500s.)

 

[curt swartz]

Hi,
the question is that,
down here in LA, we ust 500 MCM overhead feed to 400A service located outdoor, sun-exposed and it is approved by athorities;

now, why we can not feed 1200A service via under ground with (3) sets of 500 MCM
they say it is not good enough, use 600 MCMs
???????????

Tanx

Actually you can't use 500's for a 400 amp service. You can use 500's for a 380 amp service and protect it by a 400 amp breaker. If you want a full 400 amp service you will need 600's.