New construction wired by professional electrician, Montana USA. Dedicated circuit for a 14" chop saw with 10 ga. wire and a 20 amp breaker. Saw was used previously on a 15 amp circuit with no problems. The saw is rated to draw 15 amps. The saw is a 14" Milwaukee abrasive wheel cut off saw with cord and plug, it is plugged into a 20 amp duplex GFI outlet. It trips the breaker immediately upon energizing. The electrician says he can not install a 25 amp breaker as it would be out of code. Any suggestions? In the comments section of this question mention was made to slow blow circuit breakers, I have been unable to find any information on these. Does someone know where info. can be found? I tried Square-D's site.
10 gauge wire can have a 30 amp breaker under the current National Electric Code. (and past ones too). Most places use the NEC, but some make changes.
I do not have my book right here, but if needed I can get you the exact section in the code for that data.
I am 'sure' he carries a current copy of the NEC with him, for reference.
Aloysius is also completely correct, a 15 amp rated motor will draw a much higher momentary start-up current.
The current will also increase if you stall the saw.
When you pull the trigger of the saw, the motor winding looks like nearly a dead short (maybe 0.6 ohms) for a few AC line cycles. Until the motor spins up and/or heats up, the current is primarily limited only by series resistance in the building wiring and the motor windings. During motor start, the type and length of wire makes a huge difference.
You didn't describe in detail the older circuit, but since it had a 15A breaker, they probably used 14AWG wire. Compared with 10AWG of your new circuit, 14AWG has 2.6 times higher resistance per unit length. And resistance goes up linearly with distance. If your old outlet was 10 times as far away from the breaker, then it had ten times the 2.6X increase, or 26 times more resistance in the wire. The additional resistance would have reduced the inrush.
Here is a real world calculation of inrush current for the 2 scenarios.
10AWG at 15ft has 0.03 ohms
14AWG at 150ft has 0.78 ohms
1HP motor winding has 0.6 ohms (assumption)
I = V / R
10AWG example: 120 / (0.03 + 0.6) = 190 A inrush
14AWG example: 120 / (0.78 + 0.6) = 87 A inrush
So you can see how you might be pulling 2X more inrush current (190A vs 87A) simply by plugging the saw into an outlet with heavier and shorter building wiring. A little research on circuit breaker trip points revealed that SquareD QO breakers will trip in one line cycle (16.6 ms) or faster with as little as 120A (6X rating). But there is a lot of variation such that the exact same part number breaker from a different box could require 220A (11X rating) to trip that fast.
In the world of circuit breakers, they use terms like "high magnetic" or "high inrush" instead of "slow blow". Siemens, SquareD, and GE make these versions of their standard 15A and 20A breakers. They usually have "HM" or "HID" at the end of the part number. The QO-HM version is guaranteed to require at least 220A (11X rated current) to trip within 1 line cycle.
In summary you probably got a breaker that was at the very low end of the trip point threshold. Couple that with lower resistance wiring and the inrush is just too much. I bet you could replace the 20A breaker with another of the same, from a different store, and it would work.