Minimum Conductor Size for Motor

[sryan]

I am installing a new 10 HP motor, 480 V 14FLA. I will be using a 30A breaker in the MCC bucket. I am wanting to use the existing 12 gauge from an old motor. Is #12 allowed for the 30A breaker. I estimate with a 3% voltage drop, I can run up to 350 ft. Is this allowed?

 

[petersonra]

I am installing a new 10 HP motor, 480 V 14FLA. I will be using a 30A breaker in the MCC bucket. I am wanting to use the existing 12 gauge from an old motor. Is #12 allowed for the 30A breaker. I estimate with a 3% voltage drop, I can run up to 350 ft. Is this allowed?

the minimum conductor ampacity is 125% of the FLC taken from the chart in the code.

the maximum cb rating is 250% of this value.

 

[Dennis Alwon]

#12 will be compliant

 

[infinity]

T430.250 10HP@480 volts=14 amps
14*125%=17.5 amps. You could use #14 conductors with the 30 amp OCPD.

 

[Jraef]

Is there a motor starter involved, or are you just feeding this motor with an MCC feeder circuit breaker alone? Makes a big difference.

 

[Julius Right]

Let's say the OCPD it is a separate protection for the motor and its job is overcurrent protection. Then , according to NEC 2014 Table 430.72(B) Maximum Rating of Overcurrent Protective Device in Amperes for 12 awg [see note 1. Value specified in 310.15 as applicable.] it has to be 20 A [for 60oC ] or 25 [for 75oC] insulation rating.[Table 310.15(B)(16) ].
However, art.240.4 Protection of Conductors. (D)small conductors. pos. (5) 12 AWG Copper. 20 amperes. So, in my opinion, 20 A MCB rated is required.:weeping:

 

[petersonra]

Let's say the OCPD it is a separate protection for the motor and its job is overcurrent protection. Then , according to NEC 2014 Table 430.72(B) Maximum Rating of Overcurrent Protective Device in Amperes for 12 awg [see note 1. Value specified in 310.15 as applicable.] it has to be 20 A [for 60oC ] or 25 [for 75oC] insulation rating.[Table 310.15(B)(16) ].
However, art.240.4 Protection of Conductors. (D)small conductors. pos. (5) 12 AWG Copper. 20 amperes. So, in my opinion, 20 A MCB rated is required.:weeping:

In most cases in a motor circuit, the CB is there only for short circuit protection.

 

[ActionDave]

However, art.240.4 Protection of Conductors. (D)small conductors. pos. (5) 12 AWG Copper. 20 amperes. So, in my opinion, 20 A MCB rated is required.:weeping:

240.4(D) tells you to look at 240.4(G) and that tells you you don't have to use 240.4(D) for motors.

 

[Julius Right]

240.4(D) tells you to look at 240.4(G) and that tells you you don't have to use 240.4(D) for motors.

I agree with you.(G) states I have to go to art.430. However, as you can see, I started from this article and I closed the loop here. I see I choose the wrong article.:slaphead: This one could be better:
art.110.14 Electrical Connections.(1) Equipment Provisions.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
In this case 25 A will be required, I think.

 

[infinity]

I agree with you.(G) states I have to go to art.430. However, as you can see, I started from this article and I closed the loop here. I see I choose the wrong article.:slaphead: This one could be better:
art.110.14 Electrical Connections.(1) Equipment Provisions.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
In this case 25 A will be required, I think.

25 amp what, OCPD? Conductor?

 

[Smart $]

I agree with you.(G) states I have to go to art.430. However, as you can see, I started from this article and I closed the loop here. I see I choose the wrong article.:slaphead: This one could be better:
art.110.14 Electrical Connections.(1) Equipment Provisions.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
In this case 25 A will be required, I think.

Nope. 17.5A as infinity calculated above is the required minimum conductor ampacity @ 75°C. As long as there is separate overload protection, the OCPD [inverse-time breaker] rating can go up 250% of FLC of 14A... or 35A (more if need be under no-start condition) .

PS: Table 430.72(B) is for motor control circuits.

 

[david luchini]

I agree with you.(G) states I have to go to art.430. However, as you can see, I started from this article and I closed the loop here. I see I choose the wrong article.:slaphead: This one could be better:
art.110.14 Electrical Connections.(1) Equipment Provisions.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
In this case 25 A will be required, I think.

A conductor with an ampacity of 17.5 is required (#14 would be sufficient)

An OCPD of not greater than 35A is required.

Edit: Smart$ always types faster.

 

[Julius Right]

25 amp what, OCPD? Conductor?

This is for conductor, of course. However, if I well interpret the 430.72(B) Maximum Rating of Overcurrent Protective Device in Amperes- through Note1- it seems to me it has to be for OCPD also.

 

[ActionDave]

This is for conductor, of course. However, if I well interpret the 430.72(B) Maximum Rating of Overcurrent Protective Device in Amperes- through Note1- it seems to me it has to be for OCPD also.

That is in part VI of art. 430 which applies to motor control circuits.

 

[Julius Right]

25 amp what, OCPD? Conductor?

This is for conductor, of course. However, if I well interpret the 430.72(B) Maximum Rating of Overcurrent Protective Device in Amperes- through Note1- it seems to me it has to be for OCPD also.

 

[Dennis Alwon]

430.72 is for motor control circuits. The overcurrent protective device can be 250% higher than the fla of the motor for a polyphase motor. The motor itself usually has overload protection so the breaker is only there for short circuit and ground fault protection

 

[david luchini]

This is for conductor, of course.

As has already been pointed out, the conductor only needs to have an ampacity of 17.5, not 25, per 430.22.

 

[Julius Right]

Thanks for your remarks. I did not check the actual current and I take the 12# as fact. I see now 14# copper is what it is required indeed. I agree with 250% full load current as OCPD rating. However, in my opinion, an overload protecting device of 1.25 [or 1.40] FLC has to be provided.

 

[kwired]

Thanks for your remarks. I did not check the actual current and I take the 12# as fact. I see now 14# copper is what it is required indeed. I agree with 250% full load current as OCPD rating. However, in my opinion, an overload protecting device of 1.25 [or 1.40] FLC has to be provided.

Yes overload protection is required. Though it sort of appears to be for the motor only it does inherently protect the conductors from overload as well.

 

[Dennis Alwon]

Thanks for your remarks. I did not check the actual current and I take the 12# as fact. I see now 14# copper is what it is required indeed. I agree with 250% full load current as OCPD rating. However, in my opinion, an overload protecting device of 1.25 [or 1.40] FLC has to be provided.

I believe the 140% is only if the required percentages below don't work well with a motor

430.32 Continuous-Duty Motors.

(A) More Than 1 Horsepower. Each motor used in a
continuous duty application and rated more than 1 hp
shall be protected against overload by one of the means
in 430.32(A)(1) through (A)(4).


(1) Separate Overload Device. A separate overload device
that is responsive to motor current. This device shall be selected
to trip or shall be rated at no more than the following
percent of the motor nameplate full-load current rating:


Motors with a marked service factor
1.15 or greater 125%


Motors with a marked temperature rise 40°C or less 125%
All other motors 115%

430.32(C) Selection of Overload Device. Where the sensing element
or setting or sizing of the overload device selected in
accordance with 430.32(A)(1) and 430.32(B)(1) is not sufficient
to start the motor or to carry the load, higher size
sensing elements or incremental settings or sizing shall be
permitted to be used, provided the trip current of the overload
device does not exceed the following percentage of
motor nameplate full-load current rating:
Motors with marked service factor 1.15 or greater 140%
Motors with a marked temperature rise
40°C or less 140%
All other motors 130%