Air compressor circuit breaker size?
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    Default Air compressor circuit breaker size?

    While trying to justify a 40 amp circuit breaker for my 5hp, 3 phase, 240 V air compressor I came across a chart of unknown origin that called for 2x the FLAs of the motor, which is 13.9 according to the motor data plate. The manual for the IR air compressor calls for 3x the FLAs. The 40 amp breaker would only be slightly less than 3x rule. Would I be safe with the 40 amp breaker? I tried out the compressor on a 30 amp breaker and it ran fine.

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    Breaker size on a "general branch circuit", that supplies several machines, is the size to protect the wire gauge used. it applies to most any circuit, with a few exceptions.

    Most motors are protected at 125% of FLA, per the NEC. This complies with the derating to 80% for continuous loads.

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    Quote Originally Posted by beeser View Post
    While trying to justify a 40 amp circuit breaker for my 5hp, 3 phase, 240 V air compressor I came across a chart of unknown origin that called for 2x the FLAs of the motor, which is 13.9 according to the motor data plate. The manual for the IR air compressor calls for 3x the FLAs. The 40 amp breaker would only be slightly less than 3x rule. Would I be safe with the 40 amp breaker? I tried out the compressor on a 30 amp breaker and it ran fine.
    .
    most motors pull 400% amps on startup easily confirmed using a clamp on amp meter. if 14 amp motor pulls 56 amps on startup depending if circuit breaker is fast reacting or slow reacting in time. normally a the circuit is protected with a large enough circuit breaker AND a motor starter is used with its own amp protection device often its a heater strip or adjustable circuit breaker. for example it might be 10.0 - 15.0 dial on overload and you use screwdriver to adjust to what you want or need. overload can be set for 13.5 amps so not limited to 15. amp breakers overloads tend to be slow reacting but much closer to motor normal amps. magnetic contactor motor starter also in event of power failure relays are spring loaded to shutoff and it requires pressing the starter on button again. as a circuit breaker that cools off and auto turns back on can be dangerous in certain situations.

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    Default Compressor circuit sizing.

    Beesr,

    Selecting a circuit makeup, and protecting a motor are separate functions, normally.
    The NEC requires that you use a code table for the motor HP size, to determine the the motor amps to be used to calculate from. In your case 15.2A for a 5HP 230V 3P.

    The NEC table allow for a worse case scenario, for different motors of the same HP.

    For breakers (thermal/magnetic), min size would be 125%, max size is normally 175%.

    15.2A X 1.25 = 19A, so the min size is 20A, next std size.

    Max size 175% = 26.6A, so a 30A can be used.

    Those numbers are for the supply circuit.

    For the motor, you need to use the actual FLA from the motor nameplate.

    A 5Hp unit will normally run fine for years on a 25 or 30A circuit.

    The wire needs to be protected by the breaker you select.

    #10 AWG for 25 & 30 A
    Your 40A would require #8 AWG, and is normally not needed.

    SAF

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    The size wire run to the compressor from my 3 phase panel is 10 gauge, which is the recommended size in the IR manual. What was throwing me off is the following in the manual under the heading of Fuses.

    "The momentary starting current of an electric motor is greater than its full-load current; therefore, use a fuse with a capacity three times greater than the full-load motor current. For example: If the full-load current of a 5 hp. compressor is 12.9 amperes, 40 ampere fuses should be used...."

    I don't know if this makes a difference but I installed an unloader to the compressor. It didn't have one originally. It seems to me the momentary starting current would be reduced as a result. Based on the comments above, especially from SAF (thanks again for the input) and the addition of the unloader I plan to use one of the existing 30 amp circuits. It currently serves a metal lathe that has a dual speed, 5 hp motor. I would appreciate any comments or cautions if heading in the wrong direction.

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    There are strict rules for sizing of OCPDs in dedicated motor circuits, laid out in the NEC table 430.52. If the circuit is not just for one motor, then it is a “Feeder” and the motor rules don’t apply any longer. For feeders, your breaker size is dictated by your conductor size so in your case because you ran #10, you are limited to 30A.

    Per NEC table 430.52 for dedicated motor circuits though;

    300% of the motor FLA only applies to NON Tine-Delay fuses. But don’t even consider that option, waste of time and money because they WILL blow.

    Time Delay fuses are limited to 175% of FLA, maximum.

    Inverses-Time circuit breakers, aka “Thermal Magnetic” breakers and what we generally think of as CBs are limited to 250% max.

    Instantaneous Trip breakers, aka “MCPs”, have different rules but are ONLY allowed to be used as components in FACTORY built combination motor starter assemblies. You cannot use them on your own, so pay no attention to that column.

    Maximum Fuse or OCPD Device Calculator NEC Table 430.52


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