When sizing circuit breakers & fuses for transformers, the primary & secondary currents, as well as any necessary safety considerations, must be taken into account. This is a general guideline based on the National Electrical Code (NEC) standards:
Primary Side
1). Circuit breaker: Typically sized to handle 250% of the transformer’s full-load primary current.
2). Fuse: Typically rated at 125% of the transformer’s full-load primary current.
Secondary Side
1). Circuit breakers are typically sized at 125% of the transformer’s full-load secondary current.
2). Fuse: Also rated at 125% of the transformer’s full-load secondary current.
Solved Example
Ex: How do I determine the circuit breaker size & fuse for the primary and secondary sides?
Data from the transformer plate.
P= 1000 KVA
Primary voltage, VP = 11 KV
Secondary voltage (Vs = 415 V)
Impedance = 4.71%
Transformer Connection: Delta to Star.
You can also follow us on AutomationForum.co, Facebook and Linkedin to receive daily Instrumentation updates.
You can also follow us on ForumElectrical.com , Facebook and Linkedin to receive daily Electrical updates.