frank_n
Senior Member
- Location
- Central NJ
I need help sizing motor calculations. The job is to install the electric for a hood in a deli. There are 2 motors three-phase (intake and exhaust). I haven't done any work like this in a very long time.
First, I sized the feeder conductors. The large motor is 1-1/2 HP and the small motor is 1 HP. Using table 430.25 the large motor is 6.9 amps and the small is 4.8 amps. Using 125% of the largest motor plus the sum of the other motors, the total is 13.425 amps. Therefore I can use 14 THHN. (Will probably use #12 because I already have it)
Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, I multiply total amps by 800% (13.425 x 800% = 107.4 amps) 430.52(C)(1) Exception 1 allows the next larger standard size breaker 110 amps. Can this be right? Is a QO breaker considered "instantaneous?"
Next, I need to use figure out the feeder tap conductors. According to 430.28(3) I can use the same size (#14 THHN) as the feeder conductors.
Section 430.28 requires that the feeder taps be protected from an overcurrent device. 430.102 requires an individual disconnecting means for each controller. The disconnect shall be in sight of the controller. 430.52(B) & (C)(1) require the OCPD to carry the starting current for each motor. Using Time-Delay Fuses motor #1 OPCD would be (6.9 x 175% = 12.075a or 15 amp fuse) and motor #2 OCPD would be (4.8 x 175% = 8.4a or a 10 amp fuse).
Section 430.32 requires a Seperate Overload Device. This device depends on the service factor of the motor and is per the manufacturers specs.
The motors will be located on the roof and will require a disconnect. The disconnect does not have to be fused.
All of this looks good to me EXCEPT for the 110 amp circuit breaker.
Any experts out there care to critique me?
Frank
First, I sized the feeder conductors. The large motor is 1-1/2 HP and the small motor is 1 HP. Using table 430.25 the large motor is 6.9 amps and the small is 4.8 amps. Using 125% of the largest motor plus the sum of the other motors, the total is 13.425 amps. Therefore I can use 14 THHN. (Will probably use #12 because I already have it)
Next, I needed to find the circuit breaker size (Square D QO). According to table 430.52, I multiply total amps by 800% (13.425 x 800% = 107.4 amps) 430.52(C)(1) Exception 1 allows the next larger standard size breaker 110 amps. Can this be right? Is a QO breaker considered "instantaneous?"
Next, I need to use figure out the feeder tap conductors. According to 430.28(3) I can use the same size (#14 THHN) as the feeder conductors.
Section 430.28 requires that the feeder taps be protected from an overcurrent device. 430.102 requires an individual disconnecting means for each controller. The disconnect shall be in sight of the controller. 430.52(B) & (C)(1) require the OCPD to carry the starting current for each motor. Using Time-Delay Fuses motor #1 OPCD would be (6.9 x 175% = 12.075a or 15 amp fuse) and motor #2 OCPD would be (4.8 x 175% = 8.4a or a 10 amp fuse).
Section 430.32 requires a Seperate Overload Device. This device depends on the service factor of the motor and is per the manufacturers specs.
The motors will be located on the roof and will require a disconnect. The disconnect does not have to be fused.
All of this looks good to me EXCEPT for the 110 amp circuit breaker.
Any experts out there care to critique me?
Frank
