Motor kW vs. NEC Chart

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Sparky2791

Sparky2791

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Hello –

I never quite understood the whole 1 HP is equal to .746kW formula.

Especially if you compare it to the NEC motor tables.

Take for example a 10HP motor @ 208 3phase

The whole .746kW per HP suggests this motor a 7.46kW load.

The NEC table (which according to 430.6 is what I am to use to determine FLA to size OCP among other things) lists this motor at 32.2FLA So using a pf of .8 it works out that the motor is 9.3kW.

I have a project with many motors and the process engineer lists all the motors in the building as HP and as kW. When I checked the math they are using the .746/HP for each motor. I feel better converting every motor to kVA based on NEC chart to get actual power for all the motors and sizing the transformer feeding the building . This way the apparent power will be less based on the motors power factors.

Appreciate the thoughts anyone can offer on this subject.
 
Most countries use kW. In fact U.S., Liberia and Myanmar are the only remaining Imperial Units.
Having said that, I was quite conversant HP and other units with old motors still in service to this day.
Note that kVA is not power.
 
The horsepower rating of the motor is its delivered (output) power.
The KW rating of the motor is basically its consumed (input) power.
The difference includes the motor power factor as well as the motor efficiency.

Just because you can covert between units of measure, doesn't mean you should do so without considering the outcome.
 
OK so lets say my total KW is 461. Selecting transformer size would be 461/.8 = 577 KVA Next standard sizes transformer is a 750KVA.
 
jim dungar said:
The horsepower rating of the motor is its delivered (output) power.
The KW rating of the motor is basically its consumed (input) power.
The difference includes the motor power factor as well as the motor efficiency.

Just because you can covert between units of measure, doesn't mean you should do so without considering the outcome.
Click to expand...
Power factor doesn't enter into it.
 
Also, KVA is not equal to KW with motors. Power factor will increase the current to more than what the amps calculate to using KW.

It is physics that defines one HP as 746 KW.
 
LibertyEngineering said:
Does not explain why 746W per HP clearly to me, sorry. I need to take more classes to understand it better
Click to expand...

James Watt measured the performance of draft horses, and determined an average draft horse is capable of sustaining 550 ft-lbf/second of output power, as a way to express the performance of a steam engine in a way that is familiar to people accustomed to using horses for mechanical work. This is equivalent to 746 kg-m^2/sec^3, when you translate to the SI system. The kg-m^2/sec^3 unit of power we call a Watt, is named in honor of him. 746 W to HP is simply a unit translation.

In the context of motors, HP commonly refers to mechanical output power, and kW refers to electrical input power. This is why they don't correspond to exactly 746W/HP, because it is a different power value that each unit indicates. The motor has its own inefficiencies, which is why kW in will be greater than kW out.
 
Carultch said:
James Watt measured the performance of draft horses, and determined an average draft horse is capable of sustaining 550 ft-lbf/second of output power, as a way to express the performance of a steam engine in a way that is familiar to people accustomed to using horses for mechanical work. This is equivalent to 746 kg-m^2/sec^3, when you translate to the SI system. The kg-m^2/sec^3 unit of power we call a Watt, is named in honor of him. 746 W to HP is simply a unit translation.

In the context of motors, HP commonly refers to mechanical output power, and kW refers to electrical input power. This is why they don't correspond to exactly 746W/HP, because it is a different power value that each unit indicates. The motor has its own inefficiencies, which is why kW in will be greater than kW out.
Click to expand...

In the context of motors, KW is the mechanical output power.
 
dkidd said:
In the context of motors, KW is the mechanical output power.
Click to expand...

Either unit could be used to measure either value of power. A motor built for the rest of the world, will have both values would be in kW. You might also have dual units on the nameplate for the output power. If a value in both units is present, and the values are not consistent, kW is electrical input, and HP is mechanical output. kW is the unit that directly relates to Volts and Amps, after accounting for k meaning thousand and the power factor. Horsepower is a unit built from factors such as force, radius, and rotation rate, and therefore corresponds to mechanical power. It is rare (if ever) that HP would be the unit for a non-mechanical form of power.

I prefer seeing both values in kW, not only because I don't instinctively have an idea of the size of a horsepower without translating the unit, but also because it gives you more insight on what is happening inside the motor. In the limit of perfection, a motor could be 100%, but in practice of course, there will be some energy lost to heat. Seeing 10kW in, and 9.5 kW out, tells you that it is 95% efficient. Given the same motor ratings as 10 kW in and 12.7 HP out, the motor efficiency is not immediately obvious.
 
jim dungar said:
The horsepower rating of the motor is its delivered (output) power.
The KW rating of the motor is basically its consumed (input) power. [I, GeorgeB, disagree, delivered also]
The difference includes the motor power factor as well as the motor efficiency.

Just because you can covert between units of measure, doesn't mean you should do so without considering the outcome.
Click to expand...
A motor nameplate give the delivered power; a 7.5 kW is for all practical purposes identical to a 10 HP motor. The electrical load in kVA will vary some with motor design, but will usually be higher for efficiency (say 6%-8% at this size) and perhaps 25% higher considering power factor of about 0.8.

So, 10 kVA via electrical metering would be typical for a 10 HP or 7.5 kW motor.
 
The OP asked why their calculations did not produce similar results even though both used KW.
The NEC tables contains information related to the input (consumed) power of the motor. All calculations using these data which result in a KW value are not directly comparable to the output rating of the motor in HP, converted to KW, without taking into consideration the losses of the motor.

I often take the simple approach of saying input KVA is equal to the output HP, as George showed.
 
Carultch said:
Either unit could be used to measure either value of power. A motor built for the rest of the world, will have both values would be in kW. You might also have dual units on the nameplate for the output power. If a value in both units is present, and the values are not consistent, kW is electrical input, and HP is mechanical output. kW is the unit that directly relates to Volts and Amps, after accounting for k meaning thousand and the power factor. Horsepower is a unit built from factors such as force, radius, and rotation rate, and therefore corresponds to mechanical power. It is rare (if ever) that HP would be the unit for a non-mechanical form of power.

I prefer seeing both values in kW, not only because I don't instinctively have an idea of the size of a horsepower without translating the unit, but also because it gives you more insight on what is happening inside the motor. In the limit of perfection, a motor could be 100%, but in practice of course, there will be some energy lost to heat. Seeing 10kW in, and 9.5 kW out, tells you that it is 95% efficient. Given the same motor ratings as 10 kW in and 12.7 HP out, the motor efficiency is not immediately obvious.
Click to expand...
IEC motors, which are the most prevalent, are rated in kW output. They are not generally expressed in input kW.
 
In NORTH AMERICA, as it relates to motors:
"HP" is an expression of MECHANICAL power production, as in Torque in lb.-ft. x RPM / 5250
"kW" is an expression of ELECTRICAL power consumption, as in kVA x PF x Efficiency (x 1.732 for 3 phase)

In other countries, they use the term "kW" for BOTH the mechanical output and the electrical consumption, so you have to add delineators,
"Delivered kW" is the mechanical power delivered to the shaft (machine), so Torque in N-m x RPM / 9550
"Absorbed kW" is the amount of energy consumed by the motor, so again, kVA x PF x Eff. (x 1.732 for 3 phase)

The 746W/HP is a generalized term relating to only the MECHANICAL definitions, but we use it as a shortcut for electrical calcs too, it's just not absolute because Eff and PF vary.
 
Hopefully OP has figured out after reading some the replies that motor input power is always going to be higher than output power because of inefficiencies in the motor, then there might be even more current because of power factor.

The NEC tables basically list the worst possible case of typical rated amps a motor of a certain HP rating will draw at full load (basically worst case efficiency and power factor motor you would likely find is the current value in the charts). Most motors you will encounter will have lesser nameplate rating because they have better efficiency and/or power factor. I think they mostly do this so that if you had say an application that needs 10 HP motor - you will have sufficient conductor already in place should that motor need replaced and it would happen to have higher nameplate amp rating than the original even though it is still 10 HP output rating.

HP and kW are just two different units of power similar to inches and centimeters being two different units of distance. You can convert from one to the other rather easily and .746 is the conversion factor for HP to kW
 
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