Motor Overload Protection

[cbishop7]

NEC section 430 and industry standards are different...Been looking into this and can't seem to understand it fully.

The motor calculations part 1 and 2 in the Mike Holt's forum stipulates sizing overload protection at the motor at 125% (per NEC 430.32) and the short-circuit/ground-fault per NEC 430.52 (175%, 250%, etc).

My old "cheat sheets" and what seems to be industry standards for sizing of protection is to match the circuit breaker size and disconnect & fuse sizes the same values at the higher %.

For example, a 30HP (40FLA) motor at 480, 3PH according to my sheet would have a 70A/3P circuit breaker in the panel with a 100A disconnect and 70A fuses located at the motor with #6 wire.

But isn't that a violation of NEC 430.32? Wouldn't I need 50A fuses?

Am I missing something here? My other thought is if the motor has a controller with the proper overload protection but then why have a CB and fused disconnects as well?

Any help would be much appreciated.

 

[augie47]

Your "sheet" is probably listing the branch circuit short circuit/ground fault
protection as selected by Section IV of 430.
That is the figure shown on most of the manufacturer slide rules/data sheets I have seen.
The 125% is for branch circuit overload which, in most cases, is handled by a starter overload means.

 

[Dennis Alwon]

Okay so you have a 30 hp motor 3 phase at 480V which we know by T. 430.250 the full load current is 40 amps. Now for the branch circuit we would need 40 amps times 125% which is 50 amps. 310.16 tells us at 75C we could use a #8 ad we can use an OCPD much higher. Let's say that we are allow 175% for the OCP. That gives us 70 amps.

Your concern is a #8 with a protection of 70 amps. Correct?

The wire is still protect thru the ground fault and short circuit of the OCPD. The wire also will be protected for overload by the overload protection of the motor or the overload protection you install.

That is my take.

 

[cbishop7]

Would make sense...then what purpose does the fused disconnect serve as the circuit breaker would be providing the short circuit/ground fault protection on the circuit correct? Couldn't you just go with a non-fused disconnect to serve as the disconnecting means at the motor (for servicing)?

Thanks again.

 

[raider1]

Would make sense...then what purpose does the fused disconnect serve as the circuit breaker would be providing the short circuit/ground fault protection on the circuit correct? Couldn't you just go with a non-fused disconnect to serve as the disconnecting means at the motor (for servicing)?

Thanks again.

Yes, a non-fused disconnect can be used as the disconnecting means required by 430.102(B).

Chris

 

[augie47]

Since you are in rngineering, I would like to mention that in reference to your statement "then what purpose does the fused disconnect serve". In regard to motors with starters ahead of the disconnect, possibly little if any advantage, but, from field experience in the case of feeder breakers feeding disconnects for equipment such as HVAC equipment, way too often the end use equipment has a requirement for protection less than the feeder breaker and the ability to meet this requirement is enhanced by a fusible disconnect.
I can't recall the number of times I have seen a feeder with a 50 or 60 amp breaker for a RTU that ends up with a MaxOCP tag of 45 amps.
The ability to quickly install 45 amp fuses to meet that requirement versus changing the breaker is often advantageous.

 

[cbishop7]

Awesome, all makes sense now. Appreciate the help all!

 

[mayanees]

... on motor disconnects..

... on motor disconnects..

A word of advice on motor disconnects...

I was always a fan of an unfused disconnect, but have learned that's only okay when the environment that the disconnect is in has less than 10kA fault current unless the upstream OCPD is listed with the disconnect.

Unfused disconnects are rated at 10 KA withstand, and unless the upstream OCPD is listed and tested to operate with the disconnect, then a fused disconnect is necessary. This is to say that if you stay within the same manufacturer of the starter and the disconnect you're likely safe. But otherwise, if the fault current at the motor is 10kA or higher, use a fused disconnect with fuses rated for the application.

John M

 

[augie47]

good point... often overlooked

 

[jim dungar]

But otherwise, if the fault current at the motor is 10kA or higher, use a fused disconnect with fuses rated for the application.

And, as long as we are on mis-application trivia, a fusible switch is only rated at 10kA unless fuse rejection clips have been installed.

 

[jwjrw]

Since you are in rngineering, I would like to mention that in reference to your statement "then what purpose does the fused disconnect serve". In regard to motors with starters ahead of the disconnect, possibly little if any advantage, but, from field experience in the case of feeder breakers feeding disconnects for equipment such as HVAC equipment, way too often the end use equipment has a requirement for protection less than the feeder breaker and the ability to meet this requirement is enhanced by a fusible disconnect.
I can't recall the number of times I have seen a feeder with a 50 or 60 amp breaker for a RTU that ends up with a MaxOCP tag of 45 amps.
The ability to quickly install 45 amp fuses to meet that requirement versus changing the breaker is often advantageous.

Especially if you have bolt in breakers!