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Machiavelli999

Transformers & NEC

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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

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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

To find the actual transformer turns ratio you have to use the phase voltage. On the primary side you would use 480 however on the secondary side you would use 120. Thus 300*(120/480) = 75 amps. If this were a delta-delta then your calculation would be correct. Thus the wording in the NEC. I hope this helps.

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