NEC generally calls for 175% of Full Load Ampere rating for dual element time delay fuses for motors. Fast acting fuses, inverse time or instantaneous breakers are all rated differently for the short circuit protection.
However, this only covers short circuit protection, and not an overload condition. Thermal or electronic 'overloads' located after the control circuit directly before the wires to the motor is the generally accepted practice for protecting against an overloaded motor. Fishing line or seaweed maybe binding the shaft and preventing it from spinning freely, for instance. Or prop stuck in the mud. This is in addition to the fuses. Depending on the motor's service factor and temperature rise specs, the overloads usually shut off at no more than 115% or 125% of the FLA.
Overloads have a much tighter current limit range, but they're somewhat slower acting to allow for inrush at startup. (fuses and breakers often vary widely in the actual trip current) They're meant to recreate the motor's windings' temperature to trip the control circuit. Also, they're resettable.
See Article 430 of the NEC for more info.
On Thursday, May 27, 2021, 12:39:03 PM CDT, Dan Pfeiffer <dan@pfeiffer.net> wrote:
For those running 48V 8-20kW systems, what are you doing for fuse protection on your battery bank? Brands and sizes? My 12kW motor has a constant capacity of 250 amps. The peak capacity is 600. I don't know what I might actually see for short duration peaks but I was thinking a 300 amp fuse might blow in normal operating if I had a momentary peak on the motor draw. What are others using?
Dan Pfeiffer
Dan Pfeiffer
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