?
Wednesday, May 30, 2018
Question:
Hey Charlie, I have a continuous duty motor that I am feeding from a VFD. The VFD is fused at 60 amps and is fed from a 480 volt panel. I used NEC table 430.250 for the FLA and for a 20 HP motor it has a 27 IFLA. My drawing shows a 60 amp breaker (instantaneous trip) feeding the VFD with # 8 AWG THHN. I know you can oversize your breaker for motors, but where in the NEC does it show that #8 is good for a 60 amp breaker. According to 310.15 (B) (16) , #8 is only good for 55 amps.
Thanks for your help, Michael from New Mexico
A
Answer:
Hey Michael from New Mexico thanks for your question. Let's start with your example without an adjustable speed drive to help explain the NEC rules. Conductors must be protected based their ampacities unless different rules in 240.4(A) through ((G) apply as stated in 240.4. Section 240.4(G) allows the use of Table 240.4(G) which includes a reference to Article 430, Parts II to VII.
Conductors supplying a single motor must have an ampacity at least 125 % of the motor full load current (FLC) rating as stated in 430.22(A) based on the Table Values (430.250 as you indicate) which are referenced in 430.6(A)(1).
The motor and motor branch circuit also need overload protection as sated in 430.32 which is generally either 125% or 115 % of the motor nameplate current value based on 430.6(A)(2).
Instantaneous trip circuit breakers are only allowed if adjustable and part of a listed combination motor controller as stated in 430.52(C)(3). Table 430.52 contains the maximum ratings of overcurrent protective devices based on a percentage of the motor FLC.
The overcurrent device, that is used for short circuit and ground fault protection, can be sized higher (oversized) than the conductor ampacity is because overload protection is provided which often "matches" the 125% required for the motor conductors. If the conductors were protected based on their ampacity it is likely the overcurrent protection device would operate when the motor was started due to inrush current.
Using your 20 hp 460 volt motor with the 27 ampere rating from Table 430.250 times 125% from 430.22 gives us a value of 33.75 amperes for minimum conductor ampacity. Without any adjustment or correction factors and assuming 60 degree terminations for the equipment based on 110.14(C) the allowable ampacity of 40 for an 8 AWG copper conductor sufficient. If you actually have an inverse time circuit breaker, used for short circuit and ground fault protection, it can have a rating of 250% of the motors FLC rating as stated in Table 430.52. The 27 ampere value times 250% gives us 67.5 for the maximum circuit breaker rating without using exception No 1 from 430.52(C)(1) so a 60 ampere circuit breaker is acceptable.
For adjustable speed drive systems (could be a VFD) the conductors must have an ampacity of 125% of the drives input current rating as stated in 430.122(A). There are also specific branch circuit short circuit and ground fault protection rules in 430.130 for adjustable speed drive systems.