Paul the BIG Finn
Member
- Location
- Ashburn, VA
Our telecom client has an inverter with an internal 40kVA, 480V delta to 120/208V wye transformer supplying a bypass circuit for the inverter. The only time the transformer has a load on it is when the inverter is in bypass. Normally DC power is converted to AC power to feed the critical loads. The client tests the generators every Thursday and about 25% of the time when the building returns from generator power to utility power, the primary circuit breaker trips.
As I read it the NEC allows the primary protection to be sized for 250% of the transformer rated current if secondary protection is also provided (Table 450.3(B) which the inverter has. A 40 kVA transformer draws 48A on the primary side so the largest circuit breaker allowed by the NEC is 125A, the next standard circuit breaker size up from 120A (2.5*48).
The panelboard where the 125A circuit breaker is installed is a Square D, NF Series, which will not accept a circuit breaker with adjustable time delay.
Is there a commercial device available that could be added to the circuit to dampen the in rush current?
As I read it the NEC allows the primary protection to be sized for 250% of the transformer rated current if secondary protection is also provided (Table 450.3(B) which the inverter has. A 40 kVA transformer draws 48A on the primary side so the largest circuit breaker allowed by the NEC is 125A, the next standard circuit breaker size up from 120A (2.5*48).
The panelboard where the 125A circuit breaker is installed is a Square D, NF Series, which will not accept a circuit breaker with adjustable time delay.
Is there a commercial device available that could be added to the circuit to dampen the in rush current?