The watts calculation is very simple. It’s just voltage times current, or E x I. But how is this useful?

Many loads today are specified in watts. These include not just lamps or luminaires, but also motors. Let’s look at a lighting example.

A plant engineer wants to replace the existing 400W metal-halide lamps because they’re nearing the end of their expected life. They haven’t changed color yet, but the concern is that when they do, they will present an explosion hazard. The original specification didn’t allow for this issue; there’s not any sort of coating or housing that eliminates this hazard.

Upgrading to a better lamp would solve this problem and not require a fixture change. But interrupting production is costly, and that upgrade won’t solve the problem of scheduled relamping. Nor will it solve the restrike problem, something that became apparent during a recent power loss.

These and other issues have resulted in a decision to replace the whole system with an LED equivalent. The plant engineer noticed that the new lamps would consume only 150W. He wants to consolidate some branch circuits and install additional lamps in areas where the present layout has coverage issues because of partitions and large machinery.

You look at the drawings and see that there now are 12 lamps on each branch circuit. You see the breakers are all 20A. A dozen 400W lamps is a total load of 4,800W. This is a 277V system. Dividing watts by volts, you see the total load is just over 17A.

But lighting is a continuous load. That means the breakers should be loaded to no more than 80% to comply with the NEC. The maximum load is 16A, not the 17A that now exists. There are too many lamps on each circuit; someone didn’t do their watts calculations or didn’t multiply the continuous load by 125%.

How many LED replacements can you put on the same 20A circuit while keeping the load to no greater than 80% of breaker rating? First, multiply 277V by 16A. That gives you the maximum watts, which is 4,432W. Now divide that number by 150, and you arrive at 29 plus some digits. You can install 29 LED units on each circuit; this assumes the wattage rating includes the power supply, also.

Armed with this information, which you determined with some basic calculations, the plant engineer can now make those layout decisions and have the electricians consolidate branch circuits as is practicable.

You also can use watts calculations to determine the expected resistance (or impedance) of the loads on a circuit and then compare this with measurements of actual conditions to determine whether there are load problems that need further investigation. There are all kinds of uses for this simple calculation and its algebraic variations.