# Motor Calculation



## Black Dog (Oct 16, 2011)

dronai said:


> I have to submit a drawing for permits for a couple of lift motors. Is this right?
> 
> Nameplate:
> 2 HP, 1 Phase ,208V, 17A, AC,
> ...


Looks good to me....


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## 51360 (Jun 9, 2014)

dronai said:


> I have to submit a drawing for permits for a couple of lift motors. Is this right?
> 
> Nameplate:
> 2 HP, 1 Phase ,208V, 17A, AC,
> ...


In the CEC, we can reduce the branch circuit conductors due to duty. Using the NEC, you probably can as well, ( looks like it ).

But I wouldn't in this case! 

Borgi


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## JRaef (Mar 23, 2009)

dronai said:


> ...
> OL's- NEC 430.32 (c) 130%
> ...


You may have made a couple of classic errors, made by a LOT of people, on this issue.

First, the way 430.32 (A) (1) actually reads is ;


> 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: ...


 (emphasis added). 

What happens in the real world however, is that the manufacturer's of the OL relays have, in 99% of the cases, ALREADY put that percentage into the calculations of their OL heater selections or settings. So when you select heaters or settings, YOU MUST FOLLOW THE INSTRUCTIONS FROM THE MFR on selecting the correct unit, not the NEC rules in their raw form. For example, if you use a Sq. D bimetal OL relay, the heater selection chart is ALREADY set for 125% of the amps that you use for selection, and their instructions tell you to *USE THE MOTOR NAMEPLATE FLA*. So if you apply the 125% to the nameplate FLA value BEFORE you select them, then you are really selecting at 156.25% of the actual nameplate FLA (125% of 125%). In addition, on a motor that has a 1.0 SF, or a motor that is not markered with a SF, which, by rule, must be considered to be 1.0SF, you actually must use the FLA x .90 to select the OL heater element, because of the fact that the 1.25 is already factored in.

Second, you have preemptively gone to 430.32 (C) to go even higher (130%), without FIRST knowing that the motor will not start with the percentages shown in (A). I have seen AHJs bounce people on that issue, asking for them to show documentation on the fact that it failed to start. Many AHJs are not that detailed any longer, but still, doing so without it being necessary puts the motor at risk.

These two mistakes can be traced back to a very large portion of motor failures due to overloading / overheating, because a LOT of electricians don't read the instructions on the OL relays, which tell you exactly how to select the proper heaters or settings.


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## dronai (Apr 11, 2011)

Thanks Jreaf. I think the starter may be included in the lift package, but I put that in case it is not.

Question, According to the Duty, I can use #12 because of the 14.46A instead of 17A ? (85%)


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## JRaef (Mar 23, 2009)

dronai said:


> Thanks Jreaf. I think the starter may be included in the lift package, but I put that in case it is not.
> 
> Question, According to the Duty, I can use #12 because of the 14.46A instead of 17A ? (85%)


 By my interpretation of 430.22 (E), I think so. I'm not an AHJ however, and I've been known to be slapped in the hand by them... Might be worth asking in advance, but really, how much extra would it cost to run #10?


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## Dennis Alwon (May 9, 2009)

You don't need #10 if the conductors are rated at the 75C column since #12 can be used at 25 amps 75C


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## dronai (Apr 11, 2011)

JRaef said:


> By my interpretation of 430.22 (E), I think so. I'm not an AHJ however, and I've been known to be slapped in the hand by them... Might be worth asking in advance, but really, how much extra would it cost to run #10?


 Not much, just practicing this part of the code. 



Dennis Alwon said:


> You don't need #10 if the conductors are rated at the 75C column since #12 can be used at 25 amps 75C


 Thanks Dennis


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