NEC HP Ratings

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noochstyle

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Does anyone know the formula that NEC 2005 uses to come up with the FLA from the HP? I know its a function of HP,Voltage, and Phase. Is there anything else they take into account? What PF do they use or efficiency rating?

Thank you,
John
 
I understand that, I am just looking for a good way to estimate the motor HP like NEC does they say a .5HP has a FLA of 1.1 @ 460-3PH? How did they come up with 1.1A? Is it an average based on data, or do they have a rule of thumb with a formula? If they have a formula I would like to use it so I can make quicker estimates etc.
 
I understand how to do it I am trying to make a little algorithm for myself when I do large scale one lines I want to automatically know the CB/Fuse size and I don't want to use all the charts I would love to know the FLA (as the NEC states it ) Basically if they have a rule of thumb for converting HP to Amps; that would be what I am looking for. I am getting the impression that they use an average based on some data set to calculate there FLA from HP.

Thank you,
John
 
I believe the NEC did not use any "normal" formula when it created its FLA vs. HP tables. The NEC tables are intended to be much higher than any motor you can find in the real world. This artificially high FLA is why the NEC table values are not to be used to size motor running protection.
 
As I have stated in other posts, the simplest best rule of thumb is take the HP, and call it KVA. Calc your current as normal.

i.e. 0.5Hp = 0.5KVA, or 500 VA. If this is 480V, 3 ph, then it would be .6 amps.

Yes, that is less then the beloved NEC tables say, but frankly I have never been able to figure out how they dervided at those nunmbers. If you work backwards you can use a 0.8 pf and the eff would come out at something like 53%, which is unreasonable.

Especially for estimating, IMO, use the KVA method. I don't recall it failing me yet. Besides, the mechanical guys will change the motor sizes 10 times before they show up on site.
 
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