If you have a 50KVA, single phase, 480V to 120/240V transformer which requires protection at the secondary side. The full load current at the primary side would be around 104 Amps and full load current at the secondary side would be around 208 Amps. Per NEC 450, the maximum setting of the primary protection cannot exceed 250% of the full load current (104A*2.5 = 260A). Also, Per NEC 450, the maximum setting of the primary protection cannot exceed 125% of the full load current (208A*1.25 = 260A).
Since the NEC article 450 talks about (Maximum) and uses the word (Permitted), then is it permissible to use a smaller protection device at the secondary side of the transformer say for feeding a panel? For example, can I use a 200A panel with a 200A main circuit breaker downstream of the referenced 50KVA transformer, even though the 200A rating is less than the 208A which could be available at the transformer?s secondary when fully loaded?
Since the NEC article 450 talks about (Maximum) and uses the word (Permitted), then is it permissible to use a smaller protection device at the secondary side of the transformer say for feeding a panel? For example, can I use a 200A panel with a 200A main circuit breaker downstream of the referenced 50KVA transformer, even though the 200A rating is less than the 208A which could be available at the transformer?s secondary when fully loaded?
