Derating question

Status
Not open for further replies.
Greentagger

Greentagger

Senior Member
Location
Texas
Occupation
Master Electrician, Electrical Inspector
Installation consists of 4-3 phase circuits feeding motors. 12 ccc installed in one conduit. #12’s on 20 A breakers . They end up on motor starters with overload protection. FLC’ s on each motor is approximately 6A apiece. According to Table 310.15(B)(3)(a), 12 ccc would have to be derated 50% , which wouldn’t correspond with a 20A breaker. Could the overload protection device for these motors be assumed that they would instead meet this requirement, since it would be set below the 15A derated value of conductors? Thanks for any help.
 
Motor conductors are sized based on the motor current not the size of the OCPD. After derating they need to large enough to satisfy the size requirements in Article 430 typically 125% of the motor current rating.
 
I'd say the conductors are compliant. And they could have even used #14 conductors. We would need to know the exact info for the motors to see if the 20 amp OCPD is correct.
 
You need to size the conductors from 430.250 not the motor name plate. The motor nameplate is used to size the starter overloads. With THHN assuming a dry location #14s or #12s looks like either one is ok. Breakers would need to be 15a most likely.

As @texie said need the motor data to confirm
 
Eddie702 said:
Breakers would need to be 15a most likely.
Click to expand...
If the table value for the motor is 6 amps or less and using an inverse time breaker then 15 amp would be max. The O.P. just gave us a ballpark amp number of "FLC’ s on each motor is approximately 6A apiece". Funny how the little details can effect the answer in a question like this.
I get the sense that the OP may be thinking the 20 amp breaker is to large for #12 given the derating required and not understanding that the ampacity of the conductors has no bearing on the breaker size in this application.
 
Last edited:
I do apologize. It was a very general and ballpark question. I was really trying to ask if derating values of motor leads would depend on breaker size (motor branch circuit and ground fault protection) value (which would be larger than (motor and branch circuit overload protection ) from the nameplate . Trying to determine whether the smaller value of protection (nameplate overload value) could be used to validate the conductors comply with derating. Clear as mud now? Thanks for your patience. Seems as though the smaller value of protection would be compliant?
texie said:
If the table value for the motor is 6 amps or less and using an inverse time breaker then 15 amp would be max. The O.P. just gave us a ballpark amp number of "FLC’ s on each motor is approximately 6A apiece". Funny how the little details can effect the answer in a question like this.
I get the sense that the OP may be thinking the 20 amp breaker is to large for #12 given the derating required and not understanding that the ampacity of the conductors has no bearing on the breaker size in this application.
Click to expand...
 
Greentagger said:
I do apologize. It was a very general and ballpark question. I was really trying to ask if derating values of motor leads would depend on breaker size (motor branch circuit and ground fault protection) value (which would be larger than (motor and branch circuit overload protection ) from the nameplate . Trying to determine whether the smaller value of protection (nameplate overload value) could be used to validate the conductors comply with derating. Clear as mud now? Thanks for your patience. Seems as though the smaller value of protection would be compliant?
Click to expand...
You don't use motor nameplate values or overload protection values for conductor sizing and the size of the OCPD relative to the conductor ampacity is irrelevant. You need to know the NEC Table value for the motor and pick a conductor that has a final adjusted ampacity of 125% of that value.
The max. inverse time breaker size is 250% of the Table value and has nothing to do with the conductor ampacity in this application. A motor circuit most often has an OCPD much larger than the ampacity of the conductor would imply.
In your case if the motor Table value is 6 amps, then the minimum conductor ampacity would be 7 amps (6 X 1.25).
The maximum inverse time breaker would be 250% of 6 amps which means a max breaker would be 15 amp (6 X 250%). A 20 amp breaker would be to to large no matter what size the conductors are.
In your case we can pretty much guess that the conductors are large enough based on the math above. But the max inverse time breaker can't be determined without knowing the Table value for the motor in question.
 
Last edited:
Hopefully this question is not too much of a tangent. It helps me to remember these things when I understand the rationale for them (which sometimes is tough with NEC guidance).

@texie it looks like for a motor circuit we could end up with a cable that is not protected by the OCPD. Is that normal for a typical installation, or is the feeder upsized to correspond with OCPD size in practice?

Maybe if I look at the breaker TCC compared to the motor characteristic curve along with the cable damage curve it would be clear why the requirements are written as they are.
 
EmagSamurai said:
@texie it looks like for a motor circuit we could end up with a cable that is not protected by the OCPD. Is that normal for a typical installation, or is the feeder upsized to correspond with OCPD size in practice?
Click to expand...

The OCPD will provide Short-Circuit and Ground Fault protection to the circuit conductors. The motor overload device will provide the overload protection to the circuit conductors. The two devices together protect the motor circuit conductors.
 
Status
Not open for further replies.
Top