IBEW Local Union 701 Electrican
IBEW Local Union 701 Electrican
It is my understanding we use the design letter from the motor to size HP rating of disconnect for design letters F-V. We need to refer to Table 430.7 (B). We will use the locked rotor formula:
LRC = (kVA per HP x 1000 x HP) / (E x 1.73)
LRC = Locked Rotor Current
kVA per HP = Value from Table 430.7 (B) based on design letter of motor
1000 = multiplier to convert from kVA to VA
HP = Horsepower of motor in question
E = Voltage of motor
1.73 = multiplier for 3 phase
For example a 60 HP 460 volt 3 phase design letter N would be as follows. From Table 430.7 (B) kVA per HP for design letter N would be 11.2-12.49 I would use the maximum rating of 12.49. The formula will be:
LRC= (12.49 x 1000 x 60) / (460 x 1.73) = 941.69
Now take 941.69 to Table 430.251 (B) cross reference 460 volts down until you find a amperage rating that meets or exceeds 941.69. The answer would be you need a 150 Amp HP disconnect.
If it were a design letter B it would have been:
LRC = (3.54 x 1000 x 60) / ( 460 x 1.73) = 266.90
Table 430.251 (B) would result in a 40 HP rated disconnect
If we use the LRC formula for design letter A-E we could end up with an HP rating lower than the motor HP.
This would be a violation of 430.111 (A), 430.83 (A) (1)
430.81 requires suitable controllers for all motors
430.82 (A) Shall be capable of interrupting the LRC of the motor
NEC 2005
If I've missed something please set me straight.
Yired29@hotmail.com
LU 701