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I'm working on a hot bar w/2 4kw heating elements @208 3phase. By my calculation, 4k x 2= 8kw / 208 = 38amps. Does this sound right? It came from the factory w/a 30 amp breaker.

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Three phase amps, resistive loads only, amps = Watts divided by volts divided by the square root of 3.

8000/208/1.73 = 22.2 Amps.

Getting to divide by the square root of 3 is the advantage of 3-phase power!

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Are these 3Ø heaters or single Ø?

It makes a big difference.

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Motors are inductive loads. This is where this mechanical engineer bails out!

Any EE's out there???

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Three Phase Watts on a motor is equal to:

Ave Volts X Ave Amps X %Efficiency X Power Factor X 1.73

The %Eff listed on the motor will be close enough.
The power factor listed on the motor will also be close enough.
The 1.73 is the square root of 3

You will have to measure the voltages across each leg and use the average.

You will have to measure the amperage on each leg and use the average.

Remember, the wattage that you obtain will only be the motor's wattage under the conditions when you took the readings. For example, if you take the readings while the motor is not fully loaded, you will get a lower reading than when the motor is fully loaded. A motor operating a blower in an HVAC system is a good example. If the CFM changes, so will the power consumption (wattage)

By the way, if you divide the motor's calculated wattage by 746, you will get the motor's actual operating horsepower as opposed to the motor's rated horsepower listed on the name plate. This actual HP is called "Brake Horsepower" and is the actual amount of work the motor is doing.

Norm

7. Originally posted by NormChris

Three Phase Watts on a motor is equal to:

Ave Volts X Ave Amps X %Efficiency X Power Factor X 1.73

The %Eff listed on the motor will be close enough.
The power factor listed on the motor will also be close enough.
The 1.73 is the square root of 3

You will have to measure the voltages across each leg and use the average.

You will have to measure the amperage on each leg and use the average.

Remember, the wattage that you obtain will only be the motor's wattage under the conditions when you took the readings. For example, if you take the readings while the motor is not fully loaded, you will get a lower reading than when the motor is fully loaded. A motor operating a blower in an HVAC system is a good example. If the CFM changes, so will the power consumption (wattage)

By the way, if you divide the motor's calculated wattage by 746, you will get the motor's actual operating horsepower as opposed to the motor's rated horsepower listed on the name plate. This actual HP is called "Brake Horsepower" and is the actual amount of work the motor is doing.

Norm
Great Post Norm,
Bernie

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Thankfully I only need the resistive load, but I'm , going to print this for future reference.

Thanks again.

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