Elevator feed

[mustang1]

Have a 30hp motor 208v 3ph with fla of 90amps. Engineer has 3-#1 and 1-#6 ground with a 100amp breaker. Is this correct??

 

[DetroitEE]

Have a 30hp motor 208v 3ph with fla of 90amps. Engineer has 3-#1 and 1-#6 ground with a 100amp breaker. Is this correct??

There's nothing wrong with it, it's code compliant; the conductors may have been upsized for voltage drop. Although I would probably have specified a larger breaker to avoid tripping from in-rush current.

 

[steve66]

Although I would probably have specified a larger breaker to avoid tripping from in-rush current.

Yes, I agree, and I think the minimum size for a breaker feeding a single motor is 125% of the motor rated current. That would mean a 125A breaker minimum.

On top of that, Elevator manufacturers usually point to the NEC 430.52 exceptions, and encourage using the largest circuit breaker size allowed. I assume this is because the motor can be expected to do a lot of starting, and also because you don't want to trap people in the elevator due to an undersized circuit breaker.

 

[david luchini]

and I think the minimum size for a breaker feeding a single motor is 125% of the motor rated current. That would mean a 125A breaker minimum.

I don't believe that there is any minimum required breaker size for a single motor based on the motor flc. However, the motor OCPD must be capable of carrying the starting current of the motor. If the 100A c/b can carry the starting current, then it is compliant.

 

[DetroitEE]

I don't believe that there is any minimum required breaker size for a single motor based on the motor flc. However, the motor OCPD must be capable of carrying the starting current of the motor. If the 100A c/b can carry the starting current, then it is compliant.

Correct; the 125% rule is for the conductor ampacity for continuous duty motors, not the OCPD. Also, an elevator is not considered a contiunous duty motor per Article 620.

That being said, it would be rare that you would want to protect conductors at a lower OCPD rating than the conductor ampacity because of the previously stated in-rush issue.

 

[steve66]

I don't believe that there is any minimum required breaker size for a single motor based on the motor flc. However, the motor OCPD must be capable of carrying the starting current of the motor. If the 100A c/b can carry the starting current, then it is compliant.

I'm so used to using 125% on motors, I'm not really sure if its a code requirement, or just standard practice.

But it looks to me that 220.18 would apply, since the branch circuit rating would be equal to the circuit breaker rating.

Steve

 

[david luchini]

That being said, it would be rare that you would want to protect conductors at a lower OCPD rating than the conductor ampacity because of the previously stated in-rush issue.

I don't see that the conductor ampacity has anything to do with the OCPD rating. For instance, if you used a 110A c/b with #2 wire for this motor, and the 110A c/b carried the starting current; if you increased the wire size to 1/0 for voltage drop, would you increase the c/b to 150A or leave it at 110A?

 

[Ragin Cajun]

Elevator cut sheets I see have the OCP size listed.

100A is way too small. Typical 30HP motor would use 175A OCB.

To confuse maters, most elevators I see use soft start.

RC