How To Calculate Safe Electrical Load Capacities

Using Ohm's Law

Overloaded electrical socket in home office
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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, air conditioners, sump pumps, garbage disposals, and microwaves. According to the electrical code, each of these motorized gadgets needs a dedicated circuit just for their own use. Permanent heating appliances also 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, since by nature they have a fairly heavy power draw, especially when they first start up. 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.

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

So how is one to know what circuit size is required by each appliance? If you undersize a circuit feeding large central air conditioner, for example, you may find yourself with a situation in which your air conditioner circuit trips whenever it is running at maximum power. Calculating the correct size for a dedicated appliance circuit involves calculating the maximum power demand that will be placed on a circuit, then choosing a circuit size that accommodates that demand, plus a margin for safety.

Circuit Capacity

Figuring the electrical of an appliance begins with a 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 a correct size for a dedicated appliance circuit involve fairly simple arithmetic to make sure that 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

For example, think of a simple hair dryer rated at 1,500 watts running on a 120-volt bathrrom branch circuit. Using the W ÷ V = A variation of Ohm's law, you can calculate that 1,500 watts ÷ 120 volts = 12.5 amps. Your hair dryer running a maximum heat can draw 12.5 amps of power. But 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 van that draw 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 hair dryer 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. But if you put a single 100-watt lightbulb in the bathroom light fixture, you create a situation where a tripped circuit breaker is likely.

Electrician's usually calculate circuit load with a 20 percent safety margin, making sure that the the maximum appliance and fixture load on the circuit is no more than 80 percent of the available amperage and wattage provided by the circuit. In our sample bathroom, a 20-amp circuit providing 2,400 watts of power can quite easily handle 1,800 watts of demand, with 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 locations 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.

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. 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.