What is the formula of FLA for Motors (for the following Chart)

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Engr44

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NY
Hi everyone,)
i want to know that what is the FLA (Full load ampere) formula for the following FLA currents for motors chart and calculators.

Link 1: http://www.electricaltechnology.org/2012/04/motor-hp-input-voltages-full-load.html
Link:2: http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm
Link 2: http://www.shipcopumps.com/tools/mathwizard/index.php?category=extras&id=motor-amperage
Link 2: Written by Sir Mike Holt (http://ecmweb.com/nec/motor-calculations)

i mean, they used the same current rating for their chart and calculators... and say that taken out from NEC tables... but, is there a specific electrical formula available for that charts??

because, by using the general amperage or FLA current formulas, current rating is different than the above charts and calculators...

Thanks
 

rbalex

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Mission Viejo, CA
Occupation
Professional Electrical Engineer
...is there a specific electrical formula available for that charts??
Welcome to the forums.

The NEC Tables (charts) T430.247 to T430.250 are based on the FLA of the worst motor any NEMA manufacturer makes. This is so motors of the same horsepower and frame size are basically interchangeable and, if a motor is replaced, the only motor circuit component that may need to be revised is the motor overload protection.
 

Jraef

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Occupation
Electrical Engineer
The information in those tables, as rbalex said, is empirical data, selected to be the worst case scenario from the highest volume producers.

There is a formula though: A = P (HP x 746 or kW x 1000) / V x PF x Eff x 1.732

The problem is not the lack of a formula, the problem is that it contains two variables (PF and Eff) that can only be defined empirically, by testing. That's why technically, Goldigger is right as well. You cannot assign an ABSOLUTE formula in which no variables must be used to determine the motor FLA.
 
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