How to Calculate Electrical Load Capacity for Safe Usage

Electrical appliances run by motors are a commonality in homes, like furnaces, dishwashers, air conditioners, and garbage disposals, to name a few. According to the electrical code, each of these motorized devices needs a dedicated circuit just for their own use. In order to figure out what the correct circuit size is for each device, you can calculate the maximum power demand that will be placed on a circuit, then choose a circuit size that accommodates that demand, plus a margin for safety.

Avoid any circuit breaker trips with this guide on calculating safe electrical load capacities and converting amps and volts to watts.

Before You Begin

To note, permanent heating appliances have a fairly heavy electrical load, and most require their own dedicated circuits. Allowing these appliances to share a circuit with other devices can easily overload the circuit, so avoid having anything too powerful on a shared circuit.

Older homes that have not had their wiring updated often have such appliances installed on circuits shared with other devices, and in these situations, it is quite common for circuit breakers to trip or fuses to blow.

First, Determine Circuit Capacity

Figuring the electrical requirements or demand of an appliance begins with an understanding of a simple relationship between amps, watts, and volts—the three key means of measuring electricity.

A relationship principle known as Ohm's Law states that amperage (A) x volts (V) = watts (W). Using this simple relationship principle, you can calculate the available wattage of any given circuit size:

• 15-amp 120-volt circuit: 15 amps x 120 volts = 1,800 watts
• 20-amp 120-volt circuit: 20 amps x 120-volts = 2,400 watts
• 25-amp 120-volt circuit: 25 amps x 120 volts = 3,000 watts
• 20-amp 240-volt circuit: 20 amps x 240 volts = 4,800 watts
• 25-amp 240-volt circuit: 25 amps x 240 volts = 6,000 watts
• 30-amp 240-volt circuit: 30 amps x 240 volts = 7,200 watts
• 40-amp 240-volt circuit: 40 amps x 240 volts = 9,600 watts
• 50-amp 240-volt circuit: 50 amps x 240 volts = 12,000 watts
• 60-amp 240-volt circuit: 60 amps x 240 volts = 14,400 watts

The simple A x V = W formula can be restated in a number of ways, such as W ÷ V = A, or W ÷ A = V.

How to Calculate Circuit Load Demand

Choosing the correct size for a dedicated appliance circuit is simple, and it ensures that the appliance's electrical demand is well within the capacity of the circuit. The load can be measured in either amp or watts, and it is fairly easy to calculate based on the information printed on the appliance motor specification label.

• 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, most importantly, 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 that motor.
• Heating appliances generally have their wattage ratings printed on the faceplate.

A Sample Circuit Calculation

Let's use bathroom appliances as an example. A hairdryer rated at 1,500 watts running on a 120-volt bathroom branch circuit. Using the W ÷ V = A variation of Ohm's law, you can calculate:

1,500 watts ÷ 120 volts = 12.5 amps

Therefore, your hair dryer running a maximum heat can draw 12.5 amps of power. If you consider that a vent fan and bathroom light fixture might also be operating at the same time, you can see that a 15-amp bathroom circuit with a total capacity of 1,800 watts might be hard-pressed to handle such a load.

Let's imagine that our sample bathroom has a vent fan that draws 120 watts of power, a light fixture that has three 60-watt bulbs (180 watts total), and an electrical outlet where that 1,500-watt hairdryer might be plugged in. All of these could easily be drawing power at the same time. The likely maximum load on that circuit could reach 1,800 watts, putting it right at the maximum that a 15-amp circuit (providing 1,800 watts) could handle. If you put a single 100-watt lightbulb in the bathroom light fixture, you create a situation where a tripped circuit breaker is likely.

In our sample bathroom, a 20-amp circuit providing 2,400 watts of power can quite easily handle 1,800 watts of demand, with a 25 percent safety margin. This is the reason why most electrical codes call for a 20-amp branch circuit to serve a bathroom. Kitchens are another location where 120-volt branch circuits serving outlets are virtually always 20-amp circuits. In modern homes, it is normally only general lighting circuits that are still wired as 15-amp circuits.

Information on Dedicated Appliance Circuits

Exactly the same principle is used to calculate the demand on a circuit serving a single appliance, such as a microwave oven, garbage disposal, or air conditioner. Here are some of the appliances that may require dedicated electrical circuits (check with local building codes for exact requirements):

• Microwave
• Electric oven
• Garbage disposal
• Dishwasher
• Washing machine
• Trash compactor
• Refrigerator
• Room air conditioner
• Furnace
• Electric water heaters
• Electric ranges
• Electric clothes dryer
• Central air conditioner

A large microwave oven with a built-in vent fan and light fixture can easily demand 1,200 to 1,500 watts of power, and an electrician wiring a dedicated circuit for this appliance would likely install a 20-amp circuit that provides 2,400 watts of available power. On the other hand, a large 1 hp garbage disposer drawing 7 amps (840 watts), can easily be served by a dedicated 15-amp circuit with 1,800 watts of available power.

The same method of calculation can be used for any dedicated appliance circuit serving a single appliance. For example, a 240-volt electric water heater rated for 5,500 watts can be calculated in this way: A = 5,500 ÷ 240, or A = 22.9. But because the circuit requires a 20 percent safety margin, the circuit needs to provide at least 27.48 amps (120 percent of 22.9 = 27.48 amps). An electrician would install a 30-amp 240-volt circuit to serve such a water heater.

Most electricians will slightly oversize the dedicated circuit size to allow for future changes. For example, if you have a fairly small 800-watt microwave oven, the electrician will normally install a 20-amp circuit even though a 15-amp circuit can easily handle this appliance. This is done so that the circuit will be able to handle future appliances that may be larger than the ones you have now.