wire size

[stev11]

hi

I have a motor and it is FLA=20A and a compressor with RLA=15,Then the selected OCPD and wire are 50A,12AWG
and for comp, 25A,14AWG

But, how can I know if the wire size is protected by the OCPD or not??

Thank you.

 

[augie47]

A couple of side notes first. NEC 430.6 shows that you would normally use the table values for motors, not the FLA, when selecting the MOCP and MCA. For compressors those values are normally determined by the manufacturer.

As far as your question, if the motor and compressor have internal or separate overload devices, they prevent the conductor from being overloaded.
If the motor/compressor do not have overload then normally it would need to be provided.
The MOCP protection you show are actually short-circuit/ground-fault values.

 

[Dennis Alwon]

I agree with Gus. The overcurrent protective device is only there for ground fault and short circuits. The overloads in the motors will protect the conductors from getting to much current.

 

[stev11]

A couple of side notes first. NEC 430.6 shows that you would normally use the table values for motors, not the FLA, when selecting the MOCP and MCA. For compressors those values are normally determined by the manufacturer.

As far as your question, if the motor and compressor have internal or separate overload devices, they prevent the conductor from being overloaded.
If the motor/compressor do not have overload then normally it would need to be provided.
The MOCP protection you show are actually short-circuit/ground-fault values.

ok, but the motor is 3phase motor,4 or 6 poles and the name plate with horse power and rated current.
now, is this acceptable to use FLA or rated current to calculate the wire size and OCPD???

Thanx

 

[ActionDave]

ok, but the motor is 3phase motor,4 or 6 poles and the name plate with horse power and rated current.
now, is this acceptable to use FLA or rated current to calculate the wire size and OCPD???

Thanx

If you are working under NEC you would use table 430.250 and the horse power rating of the motor for OCPD. FLA is only used for thermal protection.

 

[fifty60]

I am sure the NEC addresses this but I do not have the Code in front of me. What if you are wanting to use a duel element or inverse time circuit breaker for both short circuit/ground fault and overload protection? Do we go to the tables at the end of 430 or do we use the nameplate? I am talking about when a motor does not have thermal protection, and you want to provide overload/short circuit protection with a single device.

 

[ActionDave]

I am sure the NEC addresses this but I do not have the Code in front of me. What if you are wanting to use a duel element or inverse time circuit breaker for both short circuit/ground fault and overload protection? Do we go to the tables at the end of 430 or do we use the nameplate? I am talking about when a motor does not have thermal protection, and you want to provide overload/short circuit protection with a single device.

What size motors are you wanting to protect? Unless they are fractional horse power I don't think you can do this. If they aren't I don't think one would want to. I think it would be difficult to protect the motor properly and have it start reliably.

 

[rbalex]

If you are working under NEC you would use table 430.250 and the horse power rating of the motor for OCPD. FLA is only used for thermal protection.

While this statement is essentially correct, one must start with Section 430.6 which describes where the Table(s) must be used or how the nameplate Amperes are used.