How To Calculate Safe Electrical Load Capacities

Using Ohm's Law to Determine Safe Load Capacity

Overloaded electrical socket in home office
Cultura RM/Rob Prideaux/Collection Mix: Subjects/Getty Images

We all have a mountain of electrical appliances around the house and many, if not all, of them, have some sort of motor running them. These may include furnaces, dishwashers, sump pump, garbage disposal, and microwaves. Each of these motorized gadgets needs a dedicated circuit just for their own use. You see, they should not be on a shared circuit with anything else. Doing this would overload the circuit and could cause the circuit breaker to trip or the fuse to blow.

So how is one to know what sized circuit to put each of these items on? You see, the circuits are protected by either circuit breakers or fuses that limit the amount of amperage allowed to flow through that circuit. They watch over the circuit's power draw like a watchdog. But still, how do we determine the right size for these circuit breakers and fuses? This is one time that guessing is not an option. You'll need to know the actual draw from each of the appliances and I'm going to tell you how to find out the appliance's needs are.

Motors have a nameplate rating that is listed on the side of the motor. It lists the type, serial number, voltage, whether it is AC or DC, the RPM's, and the amperage rating. If you know the voltage and amperage rating, you can determine the wattage or total capacity needed for the safe operation of the motor. Think of a simple hair dryer that is 1,500 watts running on a 120-volt circuit. That would be wattage divided by voltage equals amperage. So 1500/120=12.5 amps. As you can see, the load is pretty heavy on a 20-amp circuit and at the allowable maximum for a 15-amp circuit. You certainly wouldn't want to add any other appliance or, as you can see, it would overload the circuit.

By using Ohm's Law, we can determine what the wattage of the motor is and determine what size breaker or fuse is needed to protect it. One example is above, but I'm going to show you more of the math so that it all makes sense to you.

In order to do this calculation, you simply take the amperage (AMPS) times the voltage (VOLTS) to give you the power (WATTAGE). But we're not done yet. A 15-amp circuit that is running on 120 volts has a total capacity of 1,800 watts. To determine the safe capacity, you need to multiply the 1,800 watts times 80% to give you 1,440 watts. The rating of your motor should not exceed this rating. so let's say your motor is 120 volts and 13 amps. 120V X 13A = 1,560 watts. Now consider a 20-amp circuit gives you 20A X 120V = 2,400 watts. 2,400 watts X 80% = 1,920 watts of safe capacity, more than enough for this installation. You can see that the 20-amp circuit will fit this installation well.

Again, the golden rule of thumb is only one appliance on a circuit. My theory is you can never have enough circuits or outlets in your home. This undoubtedly came from one of my first rental houses. The home only had 120 volts coming in, yes that's right, it didn't even have a 240-volt service! There were only four circuits in the whole house and one was the lighting. To make it worse, there were only two outlets in each room. I knew from the minute I walked in that there were going to be big changes to this house. Needless to say, I rewired the home and installed a 100-amp service with 240 volts. 

Play it smart by reading the nameplate information, doing a little math, and sizing the circuit protection properly, you can safely operate the motors on any circuit.