I'm adding fuses and a relay for a 10 HP motor on 480v. I figure that it draws about 15-20 amps under load but that does that account for the start up in rush current? Should I be using a 50-60 breaker/fuses? If I'm using fuses, I assume that I would need to divide the amps by phases. ie 60 amps total and 3 phases 3, 20 amp fuses?
Try 15A fuse per phase first,that giving a better protection to motor. If motor is always running in full load condition,touch the fuse case to monitor the fuse temperature, If they are quite hot then replacing them with 20A per phase to make them last longer.
Jan 26, 2018
To amplify Rouse's statements, if you are wiring according the National Electrical Code (NEC), then for a standard design B induction motor, you must provide running overload protection at no more than 115% of the nameplate full-load amps for motors with a service factor less than 1.15, or no more than 125% for service factor 1.15. Normally this requirement is met by the overload relay build into a starter. You also must provide protection against short circuits. To that end, you may use fuses or breakers, rated up to 175% or 250%, respectively, to protect the circuit that supplies power to the starter. The wiring from those fuses or breaker to the starter need only be sized at 125% of the nominal (NEC) current rating for that motor. There is another option. You can use dual-element, time-delay (DETD) fuses to provide both running overload protection and short-circuit protection. You may then use an appropriately-rated contactor or other control device to start and stop the motor. The DETD fuses are sized based on the 115% requirement (or 125% requirement). They are a little more expensive than single-element fuses, but they might allow you to eliminate other expensive electrical control components. The above fuse and breaker values are maximum values. You may use lower-value ones to provide better protection where your motor doesn't run at full load and/or doesn't have a high inrush current. Regarding the DETD fuses, if your available short circuit current is sufficiently low, you may use time-delay fuses instead of dual-element time delay fuses. To make this determination, you have to know what the available fault current is, and what the ampere interrupting rating of the fuse is. If you don't know it (and it isn't straight-forward to figure out) then you should stick with DETD fuses to cover your bases.
Jan 26, 2018
You are about correct for a three-phase motor. I'm reading 14 amps, but this depends on the type of motor and varies. You always figure amps per phase in circuits. Breaker ampere trip ratings are also listed per phase. Motor inrush can be four to seven times full load amp rating for motors (depending on the type of motor). This value is often referred to as locked rotor amps. If you size a breaker or fuse as you would another type of circuit, such as a lighting circuit, you will continually trip a breaker or clear a fuse. If you are in the U.S. and need to provide this per NEC, then the requirement is, you can select a circuit breaker up to 250% of the full load amps. If the motor is protected by fuses, you are allowed a fuse rating up to 175% of the full load motor rating. Assuming 14 amps full load, the apparent breaker rating would be 35 amps. You can select a 40 amp breaker since it is the next commonly available breaker rating above 35 amps. Feeder circuit conductors for the motor need to be sized 125% of the full load rating per the NEC. 14 x 125% 17.5; so #12 AWG (which is rated up to 20 amps) circuit conductors are minimum.
Jan 26, 2018