According to NEC 2017 240.21(C)(1) - "Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device."
I have run into a 3-phase delta-wye, iso transformer (feeding a motor) where the neutral is not used. My only question is can the primary OCPD be considered to protect the secondary wires? Or are we required to have overcurrent protection within 25' (or in accordance with the tap rules) of the secondary side?
My understanding of the rule is that the OCP on the primary (when the secondary is a Y) will never see a neutral current so we cannot count on it to open in that case. Any phase current would be reflected back through transformer and thus the primary OCPD would open. Am I missing something?
I have run into a 3-phase delta-wye, iso transformer (feeding a motor) where the neutral is not used. My only question is can the primary OCPD be considered to protect the secondary wires? Or are we required to have overcurrent protection within 25' (or in accordance with the tap rules) of the secondary side?
My understanding of the rule is that the OCP on the primary (when the secondary is a Y) will never see a neutral current so we cannot count on it to open in that case. Any phase current would be reflected back through transformer and thus the primary OCPD would open. Am I missing something?