The energy usage of most of our home appliances is measured or displayed by their wattage. The rating for all the circuit breakers in the breaker panel is in amps. Establishing how many watts a 15-amp breaker can handle can be a little confusing, so we have broken it down to explain the relationship better.
A 15-amp circuit breaker can handle a maximum of 1800 watts. Though, the permissible maximum for a 15-amp circuit breaker is only 1440 watts. The National Electrical Code (NEC) puts a cap on the maximum watts you can put through a breaker at 80% of its total capacity.
Many people may think that comparing watts and amps is like comparing apples to oranges, but the two units of measure are closely related. We have detailed the relationship between these two units of measurement so you can quickly work out how many watts a 15-amp breaker can handle and some other practical calculations.
How Many Watts Can You Put Through a 15-Amp Breaker?
The number of watts you can put on a circuit controlled by a 15-amp breaker has two limiting factors.
- The first factor is the amount of current the circuit breaker can handle before tripping and breaking the connection.
- The second factor is the limitation imposed by the National Electrical Code or NEC on how many watts are allowed to be on the circuit.
Let’s look at the relationship between the units and then figure out how many watts in total a 15-amp breaker can theoretically support.
The unit watts indicate an appliance’s power and play a role in how much current, or amps, the appliance will draw from the circuit. Watts are, therefore, a function of the current in amps, combined with a third factor, the number of volts in the circuit.
The explanation of the relationship is a simple formula.
Current (amps) = Power (watts) / Voltage (volts)
You can calculate the missing value with any mathematical equation if you know some of the values. The voltage in most homes is known; it will either be 120 volts or 240 volts. We need to make our unknown unit, watts, the focus of the equation, which we can do by rearranging the values as shown below.
Power (watts) = Current (amps) * Voltage (volts)
Plugging our known values into the equation, we can calculate the maximum watts that the 15-amp circuit breaker will handle.
watts = 15 amps * 120 volts
Total watts = 1800 watts
This value, 1800 watts, is the total wattage that the circuit breaker will handle before tripping. While this is the maximum, the second limiting factor is the allowable wattage that the NEC prescribes as the maximum wattage on a 15-amp circuit.
If you constantly overload a circuit beyond maximum wattage capacity, a sudden spike in current draw created when an appliance starts up will cause the circuit breaker to trip. Running at full load all the time will also cause the wiring in the circuit to heat up, which is a potential safety risk.
If the wires in the circuit become too hot, the insulation around the wires can melt, creating a dangerous short circuit, which can cause a fire or electrocute someone.
For this reason, the NEC limits the maximum watts on a circuit to be no more than 80% of the maximum watts the breaker can handle.
We can work out our final figure of the permissible watts on a 15-amp circuit using this restriction.
1800 watts * 0.8 = 1440 watts
You should distinguish between the watts that a 15-amp circuit breaker can handle and the maximum permissible watts on the same breaker.
In our case, the maximum watts the 15-amp breaker can handle is 1800 watts, but the permissible maximum according to the NEC is 1440 watts. If you need more watts, you’ll need to upgrade your breaker, assuming your wiring is large enough to support the upgrade.
How Many 100-Watt Bulbs Can Be On a 15-Amp Circuit?
Figure out how many 100-watt light bulbs can be on a 15-amp circuit is an excellent way to illustrate the calculations we have discussed.
We have seen that the maximum number of watts that we can put through a 15-amp circuit breaker is 1440 watts. We can now calculate how many 100-watt light bulbs we can put on the circuit before exceeding this maximum wattage.
1440 watts / 100 watts = 14.4 light bulbs
Our result shows that you can connect a maximum of 14 100-watt light bulbs to a 15-amp circuit breaker.
Another way to solve the equation is to convert the wattage of the light bulbs to amps. The maximum amperage the circuit breaker can handle is 15 amps, but the prescribed maximum by the NEC is 80% of this capacity, which is 12 amps.
We can find out the number of amps each light bulb draws by using our previous equation.
current (amps) = power (watts) / voltage (volts)
amps = 100 watts / 120 volts
= 0.833 amps
This result shows us that each 100-watt light bulb will draw 0.833 amps. We can now use this calculated value in the maximum load we can put through the 15-amp breaker, which is 12-amps.
12 amps / 0.833 amps = 14.4 light bulbs
As you can see, this gives us the same answer that we achieved when we used the formula where watts were the focus of the equation.
The calculations used to establish the load that you can place on a 15-amp circuit are not complicated mathematics. The main factor to remember is the relationship each unit has in the equation.
The maximums that the NEC restricts on these circuits are not the only factors that need consideration when planning your wiring circuits. The National Electrical Code also limits the number of outlets that can be placed on the circuit.
The rule-of-thumb number is one electrical outlet for every 1.5 amps of the circuit breaker’s prescribed 80% maximum load. In our case of a 15-amp circuit breaker where the maximum is 12 amps, we have a limitation of 8 outlets.
Remember that if you are in any doubt about the calculations or the local restrictions in your area, consulting a qualified electrician is always the safest option to choose over a DIY method.