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What size fuse to use?

Discussion in 'Electronic Basics' started by Butter, Mar 6, 2008.

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  1. Butter

    Butter Guest

    I've got a solenoid with 8 ohms of resistence. Its AC and don't know
    where to go to find out how I'd determine what size fuse I'd need. And
    where would I go for single phase motors too. I haven't got any books
    that tell how to determine what size to use.
  2. Charles

    Charles Guest

    At 120 volts that would be 15 Amps, but that ignores inductive reactance.
    AC solenoids have more reactance than resistance. Perhaps you can contact
    the manufacturer of the solenoid to determine normal current and then fuse
    the circuit at 1.6 times that value. I'd guess 1 amp or so.

    Time to invest in some books and some basic test equipment?
  3. bud--

    bud-- Guest

    Single phase motors - depends on the motor. Comments for US practice:

    Small motors are typically 'impedance protected' - the impedance of the
    motor limits the current.

    Larger motors, maybe 1/3HP up to a few HP, typically have built in
    thermal protectors that switch the motor off when the motor gets hot.

    As motors get larger they are protected by 'motor starters' which have
    protection that is closely sized to the motor current rating and
    designed for motor protection.

    In the last 2 cases a fuse or circuit breaker generally only protects
    the supply wiring from short circuits and can be a lot larger than you
    might expect.

    In any case, motors can draw 6x their normal running current when they
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