How to Calculate Safe Electrical Load
Capacities
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 startup.
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):
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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:
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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 bathroom
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 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 1,500watt 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
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 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.
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.