NEC User
Senior Member
- Location
- Marlboro, NJ, USA
Per table 450.3B primary protection of a transformer is always required. Note 2 indicates secondary protection is sometimes required.
My 2 questions are as follows:
When is secondary protection for a transformer required?
Why is the primary protection allowed to be increased from 125% to 250% if you are including secondary protection?
I'm installing a 10KVA Single Phase Transformer in between 2 panels. On the secondary side, the transformer feeds a main circuit breaker panel. I am sizing the main circuit breaker per the transformer secondary protection.
Primary: 480V
Secondary: 240V
The configuration is as
Per table 450.3(B) I'm sizing the breakers as follows:
Primary: (2.5)(10000/480)=52 A Max. I am going to use a 50A breaker.
Secondary: (1.25)(10000/240)=52 A Max. I am going to use a 50A breaker. (As the main circuit breaker of the panel).
My 2 questions are as follows:
When is secondary protection for a transformer required?
Why is the primary protection allowed to be increased from 125% to 250% if you are including secondary protection?
I'm installing a 10KVA Single Phase Transformer in between 2 panels. On the secondary side, the transformer feeds a main circuit breaker panel. I am sizing the main circuit breaker per the transformer secondary protection.
Primary: 480V
Secondary: 240V
The configuration is as
Per table 450.3(B) I'm sizing the breakers as follows:
Primary: (2.5)(10000/480)=52 A Max. I am going to use a 50A breaker.
Secondary: (1.25)(10000/240)=52 A Max. I am going to use a 50A breaker. (As the main circuit breaker of the panel).