?
Friday, December 2, 2016
Question:
Thank you, for the incredible service you provide. Currently I have a 50 hp motor in an air compressor. It is fed with 3 phase 460 volt. The conductors are 3 each THHN #6 and it is coming off an MCC cabinet with from a 70 amp breaker. Does this meet the NEC requirements? Do I need to change the breaker and the wire size?
Anthony Tropea
A
Answer:
Hey Anthony thanks for your question and the kind words. A 50 HP 460 volt motor has a full load current rating of 65 amperes in Table 430.250. The conductor must have an ampacity of 81.25 (125 % times 65) as stated in 430.22 so a 4 AWG copper conductor is the minimum permitted size. The overcurrent protective devices needs to allow the motor to start as stated in 430.52(B) and is sized based on 430. 52(C) so could be a 175 (65 times 250 % = 162.5) ampere circuit breaker. Motor overload protection also needs to be provided as stated in 430.32.