# Thread: Formula for converting HP to AMPs or Visa Versa

1. ## Formula for converting HP to AMPs or Visa Versa

Does anyone know this formula?

Thanks

2. Power = current * voltage

First, you'll need to convert HP to Watts: 1 HP = 745.7 Watts

Second, you'll need some voltage. I assume it'll be like 120 V or 240 V, right?

3. 746 Watts = 1 Hp

Hp X 746 Watts
---------------- = AMPS
Voltage

10 Hp Motor At 220 Volts

Is

10 Hp X 746 Watts
---------------------- = 33.90 Amps
220 Volts

Amps X Volts = Watts

Ken

4. Ken again!!

When I was a kid, the voltage was 110 volts and 220 volts. Then a bit later it was 115 volts and 230 volts. And now it is 120 volts and 240 volts.

I just looked at my voltage meter at the house and it reads 123+ volts!!!!! So it could be 246 volts now.

So use the correct numbers in the formulas. Pretty sure the 746 watts per horse is the same.

My son has a lots of old electronic test equipment, radios, tape recorders etc that he has to use a varrible transformer to lower the voltage to operate this equipment. It may be the same when using some of the old equipment motors. So if you do, better check and see if they don't run to hot.

KEN

5. Volts x amps = watts and 746 watts = 1 horse power. Working backwords however from horsepower to watts you need to take into account motor efficiency. HP*746/efficiency = electrical power input. Most home use motors are in the upper 80 or low 90% range. High efficiency industrial motors are more like 97 or 98% efficient.

P.S. this is all for single phase power. 3 phase has an additional factor of 1.414 in it somewhere that I can't remember right now.

Dean

6. For 3-phase, figure 2.5 AMPs current draw per HP.

7. Originally Posted by Ken Frantz
746 Watts = 1 Hp

Hp X 746 Watts
---------------- = AMPS
Voltage

10 Hp Motor At 220 Volts

Is

10 Hp X 746 Watts
---------------------- = 33.90 Amps
220 Volts

Amps X Volts = Watts

Ken
Correct for pure power, without allowing for the power factor and friction losses, etc. Actual draw, fully loaded, would probably be somewhere in the 42-45 amp range.

8. Member
Join Date
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"Rule Of Thumb For Small Motors" ( And we all know what happens to thumbs that stick out there!)
1HP=746 watts.
HP=IxVxPFxEFF/746,where PF( Power Factor)=.86 and EFF ( Motor Efficiency)=.81. Using this formula current can be calculated as follows for a 1HP single phase motor.
I=HPx746/(VxPFxEFF) or,
I=1X746/ (120 x.86x.81) or,
I=8.9 amps.
Current for the same Motor connected to 240 would be.
I=1x746/ (240x.86x.81) or,
I=4.46 Amps.
I saw an efficiency(EFF) value of 90% or more posted. This seems a little high to me for a typical low cost motor found on woodworking machines, if this value is indeed true the formulas can still be calculated using this value as a substitute for the EFF value I used.
A good site for this type of info is Rick Christopher's website at http://waterfront-woods.home.att.net/.
Hopefully I didn't mess these numbers up again, I got a little screwed up on them last week.Pretty embarassing when you consider that I do this electrical/ electronic stuff for a living.

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