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If I turn on a light bulb while running a 15 amp saw, the breaker is likely to not trip despite pulling more than 15 amps (eg. 15.54 amps is being pulled with a 60 watt bulb).

One of three things is happening is my guess, either:

a) The 15 amp motors and claimed current used is overstated and I'm not really using more than 15 amps or

b) Breakers actually trip beyond their 15 amps rating. Perhaps something like 17 amps of current will actually make it trip?

c) The 15 amp motor w/ light bulb actually WILL trip the breaker, but it's just going to take "a while" for the bimetal in the breaker to heat up enough to trip.

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    Put a Kill-a-Watt on it, you'll see what it's actually drawing. 15A is an idealized rating based on good cutting through perfect wood. It'll be lower if you use a soft hand, and higher if you push it harshly into a knot, and higher when you start the motor ("inrush current"). Breakers' purpose is to shut down overloaded wiring before it overheats, and wire has thermal mass, so it takes awhile to overheat on a slight overload. Breakers permit this. – Harper Nov 25 '16 at 15:33
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The motor rating on the saw is the FLA (full load amperage). With the saw running without a load (cutting the material is the load) the saw will not be pulling the full load amperage. Many small motors are rated at peak draw (closed to locked rotor current) the motor never draws this amount until the motor is almost stalled. The "phony FLA" is used on air compressors, vacuum cleaners, table saws and many hand tools to get the buyer to pick that manufacturer If the actual current draw is measured you might find the saw is only drawing 1/2 the current listed on the name plate. A true 120v 1 HP motor FLA is listed as 16 amps This size motor would trip a 15 amp breaker (on start up even tripping a 20 amp breaker) because the starting current is usually 2-4 times the FLA for the time required to get to speed. I would doubt that your breaker is defective but the only way to tell with certainty is to measure the actual current on the branch circuit.

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