Electrical Testing

# Kill That Watt

Kilowatt calculations typically are used to determine the amounts of power used by inductive loads such as motors.

In our last issue, we discussed watt calculations and how you can make practical use of them. So for kilowatts, is it the same thing except the units are 1,000 times larger?

For the purpose of calculating kilowatts, mostly the answer is yes. To calculate watts, multiply volts times amps. To get kilowatts, divide that number by 1,000; note that extra steps may be involved because we use kilowatts differently from the way we use watts.

Generally, we use watts to determine the amount of power used by a particular small load or by a combination of small loads in a branch circuit. For this reason, we do not include power factor in the calculation.

We typically use kilowatts to determine the amounts of power used by inductive loads such as motors. These are much larger loads than the lamps (light bulbs) for which we may be using watts. And because they are inductive, we include power factor in the calculation; we multiply voltage times current times power factor.

There’s yet another wrinkle with kilowatts. The types of loads for which we calculate kilowatts are often (unlike lights) two-phase or three-phase loads. This means we also have to introduce a phase multiplier.

For two-phase loads, the multiplier is two. So your kilowatt number is:

(E x I x PF x 2) ÷ 1,000

For three-phase loads, the multiplier isn’t three. In fact, it’s a bit less than 2. It’s the square root of three. Industry practice is to go out two decimal places, making that multiplier 1.73. The formula looks like this:

(E x I x PF x 1.73) ÷ 1,000

These two formulas produce almost the same result. But for three-phase, it’s a little smaller kilowatt value.

If you leave power factor out of the formula, you have another very useful number that’s expressed as kilovolt-amperes, or kVA. Generally, we use this number when sizing power sources. While lamps are sized in watts and motors are sized in kilowatts, transformers are sized in kVA.

Of course, you can manipulate these formulas algebraically to meet your needs. For example, you know the kW rating of a motor and you know the voltage. You can use the same formula to determine how much current it will draw.

But make sure you are working with the correct formula. As you move from the lowest power level (e.g., a 20W indicating lamp on an operator’s control panel) through the midrange (e.g., a 37kW motor) to the source (e.g., a 100kVA transformer), you need to change the engineering units (and formulas to match them).