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Thread: 30 amp breaker with Romex 12-2

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    optimizer is offline New Member
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    30 amp breaker with Romex 12-2

    I have two well pumps hooked up to my Square D 20 Amp Double Pole Circuit Breaker via a ROMEX® 12-2 NM-B with Ground Indoor Electrical Wire.

    The problem is that once in a while the breaker will trip. I was told by the well guy that the breaker may need to be increased to 30 amps, but the wire is 12-2. Do I need to run 10-2 wire to go with the 30 amp breaker or will the 12-2 wire work?

    I don't want to melt the wire and burn the house down.

    Thanks

  2. #2
    HayZee518's Avatar
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    12-2 will only safely pass 20 amps. 10-2 is rated for 30 amps.

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    LazyPup's Avatar
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    Hayzee,,

    correct me if i am wrong, but in the post he states he has two pumps on this circuit.

    If the circuit is capable of supplying enough current to operate two pumps simultaneously it would appear to me that the ampacity of the circuit would be too high to adequately provide overcurrent protection when only one pump is running.

    I would prefer to either run two dedictated circuits from the main panel or run one circuit from the main panel with a small subpanel at the pump location with breakers properly sized to the amp load of the individual pumps.

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    optimizer is offline New Member
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    We had two dedicated breakers at the main panel when the pumps were running individually, but I hired a water filtration company to treat my hard/low ph water.

    To make a long story short we added a larger pressure tank and have the pumps set so that both pumps turn on every time water is needed, so the water from both wells would mix and it can be treated properly.

    The filtration guys decided that one breaker at the main panel was appropriate, but they did tell me that an 30 amp breaker might be necessary if it tripped the first time the new configuration was turned on. It did not trip until a few days later.

    The 12-2 wire was already in place and I think the filtration guy, who also seemed to be a savy electrician, was OK to just upgrade from a 20 to a 30 amp breaker without re-wiring to 10-2.

    Is just using a 30 amp double pole breaker with 12-2 acceptable or is that dangerous?

    PS: I have absolutely no electrician skills at all, so go easy on me.

    Thanks.

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    HayZee518's Avatar
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    Its your house - you want to switch to a double 30 - on a 12/2 circuit your perrogative - when the house burns down don't say we didn't warn you!

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    Something is not making a whole lot of sense to me here.

    Typically a residential well pump is a 1HP pump.

    This is being supplied by a double pole breaker, therefore it is a 220v circuit which is also typical of residential well pumps.

    746 watts = 1 Horse power electrical

    Watts = voltage x amperage,,,therefore we can divide the wattage by the voltage to determine the amperage...

    746 watts / 220v = 3.39amps

    Allowing that motors are not 100% efficient, the actual motor rating is probably in the order of 220v 5a

    When a motor starts it requires a momentary surge of power to overcome inertia and start the armature rotating. That is technically defined as the "Locked Rotor Amperage" and is typically 3 times the normal run amps of the motor, therefore the instantaneous start current would be in the order of 15amps, howerver the locked rotor current is normally an instantaneous draw that only lasts a few milli-seconds and a breaker that is properly sized to the motor will tolerate the short duration of start current.

    If they have both motors on a common circuit the combined run current should then be approximately 10amps, however, the instantaneous start current would be 15amps x 2 or 30amps, which explains why they told you to install a 30amp breaker.

    The problem with that reasoning is that if you provide 30amps to a motor that only requires 5amps to run, you would have flames coming out of the motor before the circuit breaker would trip out.

    In order to provide any safety the control circuit would need to have a sequencer switch to insure one pump was up to RPM before the second pump could start to prevent a simultaneous draw of start current.

    In my personal opinion a safer method would be to install a subpanel with a separate breaker for each motor properly sized to the run amp draw of the motor.

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    HayZee518's Avatar
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    I agree with lazypup completely. A lot of pump houses use what is called an alternator that allows two pumps to operate but not at the same time. In an electrical connection, one pump becomes the lead pump with theother kicking in shortly after the first one. Then when they shut off the second pump becomes the lead pump and so on.

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