What is a Load?

What is a Load?

Throughout this course, we have discussed electrical terms like watts, amps, and volts. In Lesson 4, you learned about Ohm’s laws. Now, we’re going to learn about power and how we can use that to calculate loads.

The “Power Triangle” operates just the same as the Ohm’s Law triangle. If you need to find Power, Current (I), or Voltage, just cover the missing one. Using that, we can find that Power is the Current multiplied by the Voltage.

So what does that mean? It means….Watt! The watt is the basic unit of power, and we can use watt ratings to calculate the needs of a room.  Let’s say we want to do a remodel for a bedroom. We need to talk to the client to see what he or she may want to put in the room. There could be a TV, lamps, and perhaps a window air conditioner. We can either look at the power consumption (sometimes listed on a label for each device) or we can estimate the power consumption of the devices in the room (each 60-watt lamp with equivalent LED lights uses 9 watts, a 32-inch TV uses 60 watts of power, etc). As an electrician, you can recognize that the air conditioner can use a lot of power on its own. Medium-sized window units can use anywhere from 1000 to 1500 watts. Since we are doing this remodel with the future in mind, it is always best to calculate toward the upper end of these estimates.

It is recommended that a circuit should have no more than 80% of its maximum load. This allows for future additions to the room (perhaps a video game station) without maxing out the circuit(s) available. If, in your design, you are reaching that 80% limit, you can either increase the number of circuits or increase the capacity of a circuit.

Adding another circuit is one option. A 15-amp circuit at 120 volts has a maximum power load of 1800 watts. But 80% of 1800 is 1440 watts. Looking back at the above bedroom remodel, the window air conditioner may need to be on its own circuit, and the rest of the room on another. 

Another option is adding a more robust circuit and breaker. A 20-amp circuit at 120 volts has a maximum load of 2400 watts. 80% of that is 1920 watts. That would allow the TV, AC, lamps, and other devices to run on the same circuit. One thing to consider is that you will need to increase the size of the wire to run a 20-amp circuit. The NEC states that a copper wire in a 20-amp circuit needs to be at least 12-gauge. The extra thickness of wire may increase your cost per foot calculation when running wire.