?
Thursday, October 24, 2019
Question:
I hope this inquiry finds you well. My question is about tap conductors feeding a 35kva 3 phase transformer. 240.21(B)(3) allows a tap conductor for this purpose so long as all condition therein are met. 240.2 generally defines a tap conductor as one that has overcurrent protection above that which is otherwise allowed. Since 450.3 allows overcurrent protection up to 250% of the load where primary and secondary protection are provided, can a #8 Cu THHN, protected by a 100A OCPD, be used for this installation? Can the #8 land directly on the breaker? Would that not meet the definition of a tap? If not, what is the difference in having a #3 land on the breaker only to be spliced to a #8 for the tap?
Aeron Braukman
A
Answer:
Hey Aeron thanks for your question. Yes, If I understand your question correctly. You don't mention the voltages or load but using a 480 volt 3 phase transformer primary the current would be 42 amperes [35000/ (480 X 1.732)]. The transformer primary protection can be 250% as you mention based on 450.3 which would be 105 amperes so using a 100 ampere overcurrent device is acceptable for the transformer. Transformer secondary protection must be provided at 125% as stated in Table 450.3(B) (rounding up is allowed using 240.6) based on the transformer secondary voltage.
The conductor protection rules are separate, which is reminded to us by 450.3 Informational Note No. 1. Also, as you mention if you comply with all of the conditions of 240.21(B)(3) the same overcurrent devices can be used for conductor protection. The 8 AWG copper conductor would be considered a Tap Conductor as that term is defined in 240.2. Keep in mind that the 60 degree column in Table 310.15(B)(16) must be used to determine the conductor ampacity (40 amperes) unless all terminations are rated at 75 degrees as stated in 110.14(C). Also the overcurrent device located at the load end of the secondary conductors must protect the secondary conductors at their ampacity - no rounding up.