Ground Fault Current Formula

[anbm]

Is there a formula to calculate 480V, 3-phase ground fault current at specific panel bus, motor bus? I tried to meet this rule below by setting GF level at electronic CB... Is there a minimum ground fault level setting per NEC?

Settings of the ground fault device(s) are limited to a maximum of 1200A pickup and one second delay for ground faults greater than 3000A (NEC 230.95A).

 

[don_resqcapt19]

You need to have a coordination study done. The ground fault trip setting at the service should be high enough that it does not trip before a downstream breaker trips.

 

[zog]

anbm, correct me if I am wrong here, but you only have GF setting on your main breaker? No others?

 

[anbm]

anbm, correct me if I am wrong here, but you only have GF setting on your main breaker? No others?

For healthcare building, (2) levels of ground fault is required at main service per NEC-517.

 

[zog]

For healthcare building, (2) levels of ground fault is required at main service per NEC-517.

OK, thought this was related to your last thread about the GF issues. If thats the case you will need a coordination study. You want to be sure your feeders trip on GF before your main does, in general you will want your feeders set pretty low with short delays.

One thing to keep in mind here, is that unless you have a 4 wire system with an external GF sensor going to your trip unit a phase imbalance can cause a breaker to trip on GF.