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. Thanks Rosco
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?
. 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 start.
-------------- A problem with the 8 ohms for an AC selenoid is that the solenoid has inductance which also limits the current. And the inductance is higher if a core is pulled into the solenoid. What is the solenoid current in its normal condition (usually core pulled in)?