boost transformers for motors.

[codeunderstanding]

Can you protect the boost transformer from the motor branch circuit device. boosting from 208-240 volts single phase.

 

[iggy2]

Re: boost transformers for motors.

Probably not. The boost transformer OCP is covered by 450.4, whereas protection for the motor is per 430. What is the HP? What is the transformer rating?

Is the motor rated for 208 through 240 volts? A boost transformer may not be req'd.

 

[codeunderstanding]

Re: boost transformers for motors.

To Iggy2. The HP rating is 3 hp. Motor rating is 120-240v single phase. The x-former is rated .5kva, which the manufacturer says will take a 5kva load. The motor will work on 208 but the torque is reduced. The contractor does not want to change the motor out to work on the right system voltage. Its used for a booster pump for lawn sprinklers.

 

[jtester]

Re: boost transformers for motors.

codeunderstanding
I did the following calculation.
OCPD for a motor 430.52
17 amps X 2.5(breaker) = 42.5 amps
a 50 amp breaker is max motor OCPD
17 amps X 1.75(TD fuse) = 29.75 amps
a 30 amp time delay fuse would be OK

OCPD for an autotransformer 450.4(A)
if the boost is 24v 500va/24v = 21 amps flc
21 amps X 1.25 = 26 amps a 30 amp fuse or
breaker would be OK

if the boost is different, change the calculation to 32v or whatever and check.
For a 24 volt boost transformer it seems that you could use a 30 amp fuse as both motor B-C, S-C, & G-F protection and transformer protection.
Look at it and see what you think.
Jim T