Motor Protection

[Brandon Loyd]

Trying to size a breaker for a 240V, 20hp, 50amp motor. 403.52 (I think) says 300 or 800% depending on breaker type for short circuit protection. Ugly's says I need 100 amp breaker with #4 wire. 100amp is 200% of FLA, #4 is only good for 95amps. I am confused.

 

[augie47]

430 Part II for your conductors Code 20HP 240 is 54 amps x 1.25 = 67.5 amps #4 THHN
430 Part IV for your SCGF protection. Inverse Time C/B 250% = 135 amp
breaker
#4 wire with 100 amp or 125 amp breaker would not be uncommon.
(The wire is considered to be protected from overload by your overload device, the breaker is just for short circuit =/ground fault protection and must be sized large enough to handle the inrush.)

 

[kwired]

I suggest you see what the breaker manufacturer recommends as many of them will suggest a smaller breaker than NEC recommends. I have in front of me a motor data calculator for Square D and for your motor it recommends a 90 amp breaker, which is only 180% of your 50 amp motor. NEC will allow up to 250% inverse time breaker which is a 125 amp breaker. It can be increased only if it will not allow starting of the motor.

 

[Brandon Loyd]

Motor breaker size

Motor breaker size

Are most breakers inverse time breakers or instantaneous? How can I tell? I thought inverse time had adjustable dials. The situation I was describing is an existing condition. 100 amp breaker in and out, #4 wire, 20 hp motor with 50 FLA. Based on the code, isn't the breaker undersized for this motor. Or the lower the better as long as the motor can start up?

 

[Brandon Loyd]

Are most breakers inverse time or instantaneous? How can I tell? I thought inverse time had adjustable dials. The situation I was describing is an existing condition. 100 amp breaker, #4 in and out, 20 hp motor with 50 FLA. Based on the code, isn't the breaker undersized for this motor. Or the lower the better as long as the motor can start up?

 

[augie47]

Most of the ones I see are Inverse. Instantaneous are used when listed as part of a factory assembly.
The 250% is max (with exceptions)....
as long as the 100 does not nuisance trip..it should be fine.

 

[Brandon Loyd]

Wire protection

Wire protection

Most of the ones I see are Inverse. Instantaneous are used when listed as part of a factory assembly.
The 250% is max (with exceptions)....
as long as the 100 does not nuisance trip..it should be fine.

Can the wires legally protected by the motor overload heaters since the breaker is oversized foe inrush?

 

[Brandon Loyd]

Motor wire protection

Motor wire protection

In an MCC combo starter where the breaker is oversized for startup, can the overload heaters then legally protect the wire from overheating since the breaker is to big?

 

[raider1]

In an MCC combo starter where the breaker is oversized for startup, can the overload heaters then legally protect the wire from overheating since the breaker is to big?

Yes, the overload device will protect the conductors from overload and the breaker is only providing short circuit and ground fault protection for the branch circuit conductors and the motor.

Chris

 

[Brandon Loyd]

I have 150amp fuses feeding dist. Blocks, then #4 feeding 100amp breaker which feeds a motor. Could the motor have internal overloads and could those legally protect the conductors?

 

[raider1]

I have 150amp fuses feeding dist. Blocks, then #4 feeding 100amp breaker which feeds a motor. Could the motor have internal overloads and could those legally protect the conductors?

The motor must have overload protection in accordance with Part III of Article 430. This can be done by a separate overload device or by an intergral overload in the motor.

Chris