How to calculate electrical circuit loads
Every electrical circuit has a load limit which is the maximum load it can handle safely without overheating. Circuit breakers in a main power panel are designed to help prevent a circuit from overheating by limiting the amount of current that is allowed to flow through that circuit.
Calculating the load on a circuit is a good indicator of whether a new circuit needs to be added or if some appliances need to be moved to other circuits.
Locate the circuit for which you want to calculate the circuit load. Take the breaker size and multiply it by the rated voltage. For example, a 20 amp breaker that operates at 120 volts has a maximum load of 2400 watts. The National Electric Code recommends that a circuit not be loaded more than 80 per cent of its maximum capacity.
Calculate the recommended maximum. Multiply the breaker maximum capacity by 80 per cent. This equals 2400 watts times 80 per cent or 1920 watts.
Locate all of the appliances and devices that are connected to this circuit. Check each device for how many watts of power it uses. Write down the wattage of each device. If the wattage is not given, multiply the voltage of the device by the amount of current it uses to get the power in watts.
Add the wattage of all of these devices together to get the total load on the circuit. Take the total load and divide it by the maximum recommended load to get a percentage. For example, if the total loads add up to 800 watts and this is a 20 amp circuit, then the load usage is 800 watts divided by 1920 watts which equals 0.416 or 42 per cent. This means that this circuit is operating at 42 per cent of its recommended maximum load.
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