How do you determine amperage?

Factual Questions
1

How do you determine amperage of an electric tool? I believe that there is a formula that involves the wattage and voltage in order to get the amperage. Can anybody tell me what it is?

ben

2

P = EI

Watts = Volts * Amps

3

Thanks.

ben

4

The best way is to look on the tool nameplate where the current will be given.

For alternating currents it is always necessary to take into account the fact that the voltage and current will most likely not be in phase. The horsepower rating of the tool motor will tell you what the in-phase component of the current it. For example a 1/4 HP motor at 120 v. has an in-phase current of about 1.5 amp. (746/[4*120]) Most small motors will have a large quadrature component, probably greater than the in-phase component so let’s guess 2 amp. This would make the total current in the example 2.5 amp. (sqrt[1.5[sup]2[/sup] + 2[sup]2[/sup]])

This just happens to be the current on the nameplate of my Craftsman 1/2" hand drill.

If it is a DC machine like a Makita, then don’t worry about the current. Just buy the correct battery for the tool and that’s all that’s needed.

5

Oh, yeah. AC.

Here’s some Ohm’s law for AC current:

P = EI cos x

x = phase angle in degrees.