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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.

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    Does it trip when you plug it in, or when you pull the trigger? How old is the saw? Does the saw work when plugged into another receptacle, what about another receptacle on another circuit?
    – Tester101
    Mar 15 '15 at 18:44
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    Is it a GFCI breaker?
    – friedo
    Mar 15 '15 at 20:15
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    Motors often have higher than rated startup loads. (As an example, my 15A compressor draws almost 30A for just a moment.) Ask your electrician about slow trip breakers. Mar 15 '15 at 20:41
  • Best option IF possible (likely not, but worth checking) would be to switch the saw to 230V operation. Otherwise using a breaker that has a slower trip would be the norm.
    – Ecnerwal
    Mar 15 '15 at 20:47
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    It seems that all the comments miss a critical point: Does the saw still work properly when plugged into the old 15 amp protected circuit and/or any other circuit in the house/garage/shop? If so, it sounds to me like it's an issue with the new circuit.
    – FreeMan
    Mar 16 '15 at 13:53
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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.

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    Throwing a bigger breaker at the problem is likely a band aid, not a solution.
    – Tester101
    Mar 16 '15 at 11:32
  • Understood and accepted. The questions I attempted to answer were the issue of the 25 amp breaker being out of code, and a possible reason for a saw to trip a 20 amp breaker. I do not know of a source for a thermal only, or at least variable magnetic trip breaker for a load center built to the newest building codes. So on that question, I provided no information. If you will look at his follow-up top level question, and read the information posted there, you will see that I identified a breaker by part #, a question about the series of his load center, and a complete reference point in the NEC
    – Some Guy
    Mar 16 '15 at 14:08
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    @SomeGuy, the problem is not with a 30A breaker and #10 wire. The problem is with a 15 or 20A receptacle and a 30A breaker. ..... Also, he states it previously worked fine on a 15A circuit. Obviously this is not an issue with startup current. Jan 8 '17 at 14:00
  • You need to look at the parts of the Code which say which receptacles can be on which breakers. The fastest way to find this is exact phrase search "15 or 20". That refers to which breakers may feed the common NEMA 5-15 recep. Sep 21 '19 at 20:48
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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.

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    Thank You good info. Our electrician changed it out (finally) with a 30 A. breaker and it works well now. Only trips on a heavy load. Mostly when cutting material that is larger than the saw was designed for.
    – Scott G.
    Apr 9 at 2:45
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Thank You good info. Our electrician changed it out (finally) with a 30 A. breaker and it works well now. Only trips on a heavy load. Mostly when cutting material that is larger than the saw was designed for. –

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