# Motor Calculation Check



## B-ri (Dec 11, 2013)

Contactor:


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## Dennis Alwon (May 9, 2009)

That should work fine however depending on distance you may want a larger conductor


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## dronai (Apr 11, 2011)

Why not a 20A ?


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## IMM_Doctor (Mar 24, 2009)

I may be rusty also, but me thinks 14awg is light.

430.22 Min circuit FLA x 1.25. ..... 14.2 x 1.25= 17.75
I would use 12awg copper.

430.52 Inverse time CB FLA x2.50. ..... 14.2 x 2.50= 35a


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## tates1882 (Sep 3, 2010)

IMM_Doctor said:


> I may be rusty also, but me thinks 14awg is light.
> 
> 430.22 Min circuit FLA x 1.25. ..... 14.2 x 1.25= 17.75
> I would use 12awg copper.
> ...



125% is for a continuous load. (3 or more hours without cycling off). I doubt the ops unit runs for more than 3 hours. 


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## RIVETER (Sep 26, 2009)

B-ri said:


> I'm a little rusty on motors and would greatly appreciate if someone could confirm if my calculations are right:
> 
> 3hp 208v-230v 3 phase compressor motor
> Rla= 14.2
> ...


How do you figure #14 is large enough for that amount of current?


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## tates1882 (Sep 3, 2010)

RIVETER said:


> How do you figure #14 is large enough for that amount of current?



14 is good for 20 amps although you normally can't use an ocpd larger then 15 amps, in this case is one of the few times you can use 14 for 20 


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## tates1882 (Sep 3, 2010)

B-ri said:


> I'm a little rusty on motors and would greatly appreciate if someone could confirm if my calculations are right:
> 
> 3hp 208v-230v 3 phase compressor motor
> Rla= 14.2
> ...



Where'd you come up with 14.2 amps? I get 10.6 for 3hp at 208v 3phase. 


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## RIVETER (Sep 26, 2009)

tates1882 said:


> 14 is good for 20 amps although you normally can't use an ocpd larger then 15 amps, in this case is one of the few times you can use 14 for 20
> 
> 
> Sent from my iPhone using Tapatalk


I can't possibly know it all...just ask my wife, but are you talking for a continuous load?


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## IMM_Doctor (Mar 24, 2009)

tates1882 said:


> 125% is for a continuous load. (3 or more hours without cycling off). I doubt the ops unit runs for more than 3 hours.
> 
> 
> Sent from my iPhone using Tapatalk


OP said "Compressor". No mention of reduced duty cycle. I assume continuous.

If application is truly intermittent or reduced duty cycle, you can chip away at the 125%. See table 430.22(e).


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## tates1882 (Sep 3, 2010)

RIVETER said:


> I can't possibly know it all...just ask my wife, but are you talking for a continuous load?



Code wise yes. 14 would be legal to carry 20 amps. 

Would I install 14 where it was carrying more then 15 amps, no. 


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## B-ri (Dec 11, 2013)

tates1882 said:


> Where'd you come up with 14.2 amps? I get 10.6 for 3hp at 208v 3phase.


RLA=14.2 is on the nameplate



This compressor is for a walk in cooler.


The way I got my answer was table 430.250 3ph motor FLC (230v) = 9.6

9.6 x 1.25 = 12A, which gives me 14 guage

9.6 x 2.5 = 24A, which gives me the 25a 3 pole breaker


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## RIVETER (Sep 26, 2009)

tates1882 said:


> Code wise yes. 14 would be legal to carry 20 amps.
> 
> Would I install 14 where it was carrying more then 15 amps, no.
> 
> ...


Good to know but I was thinking #14 cu=15amps...then 80% of that is somewhat less depending on the insulation.


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## IMM_Doctor (Mar 24, 2009)

B-ri said:


> RLA=14.2 is on the nameplate
> 
> 
> 
> ...


You and I used the same multiples for minimum wire and max OCPD.
You stated 14.2 RLA, I used that as FLA. You plugged in 3HP at 230v in to table 430.250.

Are we both right? What is the difference between RLA and FLA?

But at the end of the day, I typically leak my math upwards to be sure that the circuit is sufficient. (on the aide of caution).


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## B-ri (Dec 11, 2013)

IMM_Doctor said:


> You and I used the same multiples for minimum wire and max OCPD.
> You stated 14.2 RLA, I used that as FLA. You plugged in 3HP at 230v in to table 430.250.
> 
> Are we both right? What is the difference between RLA and FLA?
> ...



I plan on upsizing my wire, my main concern is installing the proper OCPD.

I was taught that RLA is the same as FLA, but when sizing motors you use the FLC (430.250)


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## erics37 (May 7, 2009)

B-ri said:


> This compressor is for a walk in cooler.


Is it a hermetic compressor, all sealed in a casing and operating within the refrigerant?

If so, use Article 440 which will tell you to use the nameplate current values.

If it's not hermetic and is a normal motor with an external shaft running a compressor directly or with belts or whatever else, then use Article 430. That means you have to use the values in Table 430.250 for determining conductor size, overcurrent, etc. The nameplate current value is only for overload sizing if separate overload device. Yours is thermally protected so not really applicable.


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## B-ri (Dec 11, 2013)

erics37 said:


> Is it a hermetic compressor, all sealed in a casing and operating within the refrigerant?
> 
> If so, use Article 440 which will tell you to use the nameplate current values.
> 
> If it's not hermetic and is a normal motor with an external shaft running a compressor directly or with belts or whatever else, then use Article 430. That means you have to use the values in Table 430.250 for determining conductor size, overcurrent, etc. The nameplate current value is only for overload sizing if separate overload device. Yours is thermally protected so not really applicable.



"Semi-Hermetic"


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## Ridge (Mar 25, 2011)

Your compressor would be considered "hermetic" as it is cooled by the refrigeration process. Article 440 would be where to look. I have worked on walk in coolers and many compressors in a heavy industrial setting and there is the potential under the right circumstances for the compressor to freeze up(ice up) and continue to run, depending on what the cooler is being used for, in peak hours in may run for 8 or more hours with the door opening constantly.


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