Motor Starter

[aelec84]

Hello Forum,
I am confused on when to use non-fused switch vs fused switches for motors. In addition when to use magnetic starters?
Can someone guide me to right NEC code sections?
Pls advise.
Thank you.

 

[Eddie702]

Mag starters are used to start & stop (control) motors specifically motors without internal overload protection and mag starters and VFDs have motor overload protection built in Smaller motors generally have built in overload protection and can use a relay , or contactor for control. The other reason is many motors are 208, 240, 480 or 600 volt. By using relays, contactors or mag starters the control circuit can be 24 volt, or 120 volt (those are the most common) but the control voltage can be different from the motor voltage.

As far as disconnects go most motors are code required to have a disconnect within 50 ' of the motor and in site of the motor. Weather you use a fused or unfused disconnect depends on the wiring. If the motor circuit is protected by a circuit breaker or fuses in the main panel, switchbord or buss duct the proper size for the motor then you could use an unfused switch. If not an unfused switch is used.

Frequently machines and motors get changed and the power feed requirements change so feeding more power than you need to a fused switch is a good idea and provides extra future capacity

 

[don_resqcapt19]

Mag starters are used to start & stop (control) motors specifically motors without internal overload protection and mag starters and VFDs have motor overload protection built in Smaller motors generally have built in overload protection and can use a relay , or contactor for control. The other reason is many motors are 208, 240, 480 or 600 volt. By using relays, contactors or mag starters the control circuit can be 24 volt, or 120 volt (those are the most common) but the control voltage can be different from the motor voltage.

As far as disconnects go most motors are code required to have a disconnect within 50 ' of the motor and in site of the motor. Weather you use a fused or unfused disconnect depends on the wiring. If the motor circuit is protected by a circuit breaker or fuses in the main panel, switchbord or buss duct the proper size for the motor then you could use an unfused switch. If not an unfused switch is used.

Frequently machines and motors get changed and the power feed requirements change so feeding more power than you need to a fused switch is a good idea and provides extra future capacity

Just be careful with large motors...unfused switches can typically only be used where the fault current at the location of the disconnect does not exceed 10kA. I have done a few where we had to use fused disconnects because the fault current exceeded 10kA.

 

[Eddie702]

My mistake. Last sentence should read, " If not a fused switch should be used" in addition to what @don_resqcapt19 posted.

 

[kwired]

Just be careful with large motors...unfused switches can typically only be used where the fault current at the location of the disconnect does not exceed 10kA. I have done a few where we had to use fused disconnects because the fault current exceeded 10kA.

Most if not all non fused Square D safety switches (not certain about other brands) have higher ratings if there is certain fuse types ahead of them. Basically they end up with same rating as the same series switch in a fused disconnect unit with same fuse

 

[don_resqcapt19]

Most if not all non fused Square D safety switches (not certain about other brands) have higher ratings if there is certain fuse types ahead of them. Basically they end up with same rating as the same series switch in a fused disconnect unit with same fuse

Have not seen a fuse on a motor branch circuit in a long time. Most cases the starter has an instantaneous only trip motor circuit protector.