Ecmweb 7390 Kilowatts
Ecmweb 7390 Kilowatts
Ecmweb 7390 Kilowatts
Ecmweb 7390 Kilowatts
Ecmweb 7390 Kilowatts

Kill That Watt

May 14, 2015
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.

Viktorus/iStock/Thinkstock

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).

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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