Machiavelli999
Active member
According to Article 240.4(F) the secondary conductors of a delta-wye transformer require a breaker for over current protection even if there is a breaker on the primary side.
What is the technical explanation though?
In the NEC handbook it delves deeper into this and says that a single phase 120V overload could cause sufficient current to cause problems for the secondary conductors but not trip the breaker on the primary (480V) side. However, I don't understand how that could happen.
In a balanced or unbalanced wye system, the phase (Ian) currents are the same as their respective line currents (Ia). So, lets say I had a 75KVA (480 - 208/120V) transformer. I would have a 100A breaker on the primary side. On the secondary side, my conductors would be rated to handle 250A. If I had an overload on one of my single phase loads and it started drawing 300A. The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.
Any thoughts? Thanks
What is the technical explanation though?
In the NEC handbook it delves deeper into this and says that a single phase 120V overload could cause sufficient current to cause problems for the secondary conductors but not trip the breaker on the primary (480V) side. However, I don't understand how that could happen.
In a balanced or unbalanced wye system, the phase (Ian) currents are the same as their respective line currents (Ia). So, lets say I had a 75KVA (480 - 208/120V) transformer. I would have a 100A breaker on the primary side. On the secondary side, my conductors would be rated to handle 250A. If I had an overload on one of my single phase loads and it started drawing 300A. The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.
Any thoughts? Thanks