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How To Calculate Safe Electrical Load Capacities

How the use of Ohm's Law helps you deterimine the safe capacity.

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We all have a mountain of electrical appliances around the house and may, if not all, of them have some sort of motor running them. These may include furnaces, dishwashers, sump pump, garbage disposal, and microwaves. These motorized gadgets need a dedicated circuit just for their own use. You see, they should not be on a shared circuit with anything else.

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?

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.

By using Ohm's Law, we can determine what the wattage of the motor is and determine what sized breaker or fuse is needed to protect it.

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 multipy 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.

By reading the nameplate information, doing a little math, and sizing the ciruit protection properly, you can safely operate the motor.

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