What are the max watts on a 15 amp circuit or 20 amp circuit before the breaker will trip?
Let’s do the math.
Watts = Volts * Amps
Most homes in the United States have outlet voltages around 120V.
So on a 15 amp circuit, the maximum watts are 15A*120V = 1800 watts.
80% Rated Circuit Breakers
Many 15A breakers nowadays can handle 1800 watts (100% load) continuously like this one from Siemens:
However, some circuit breakers are rated at 80% of their current rating for continuous loads.
The NEC (National Electric Code) defines a continuous load as “a load where the maximum current is expected to continue for three hours or more.”
Section 384-16(c) of the NEC also states that a standard OCPD (overcurrent protection device) can be loaded to only 80% of its rating for continuous loads.
In this case, the max watts on a 15 amp circuit with a standard breaker are 0.8*1800 = 1440 watts.
20 Amp Circuit Breaker
The math is similar for a 20 amp circuit breaker.
Here the maximum watts are 20A*120V = 2400 watts for a 100% continuous current rated circuit breaker.
On a standard 20A circuit breaker rated for 80%, the maximum watts are 0.8*20A*120V = 1920 watts.
For more information on how the NEC describes circuit breaker loading, check out this article.
So, the answer is that for continuous loads of three hours or more it depends on the circuit breaker you have. For instantaneous loads though, a 15A circuit breaker can handle 1800 watts and a 20A circuit can handle 2400 watts.