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For calculations of motors the code book states that a 1.5 HP motor draws 10 amps. For correct sizing the motor's nameplate amperage should be used.

Using 10 amps, 30 amp non time delay fuse, 15 amp for time delay fuse, or 25 amp for breaker. A #10 copper conductor with an insulation factor of 90 degrees C should be used on this amperage.

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Answer for USA, Canada and countries running a 60 Hz supply service.

The motor's amperage has to be found to answer this question.

The formula to use is I = HP x 746/1.73 x E x %eff x pf.

A standard motor's efficiency between 5 to 100 HP is .84 to .91.

A standard motor's power factor between 10 to 100 HP is .86 to .92.

Amps = 746 x 1.5/1.73 x 480 x .84 x .86 = 1119/200 = 5.6 amps

The breaker has to be sizes to 250% of the motors full load amps. 5.6 x 250% = 14 amps.

A three pole 15 amp breaker connected to three conductor 600 volt rated cable will be needed.

As for overload protection without knowing the brand of contactor it is hard to say what the proper number of the heaters will be.

To obtain the proper overload protection base the overload heaters on the 5.6 amps or the motors full load amps that will be on the motor's nameplate.

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

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12y ago
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225 amp depending on many factors.

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11y ago
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Q: What size breaker for 150 hp 480 volt motor?
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What size of wire and breaker do you need for a 10 KW electric furnace?

To answer this question, wire size is rated in the amount of amperage that it can legally carry. The formula to find amperage when the HP is known is I = HP x 746/1.73 x E x %eff x pf. A standard motor's efficiency between 5 to 100 HP is .84 to .91. A standard motor's power factor between 10 to 100 HP is .86 to .92. Amps = 10 x 746 = 7460 = 7460/1.73 x 220 x .84 x .86 = 7460/275 = 27.1 amps. The electrical code states that a motor conductor has to be rated at 125% of the motors full load amperage. 27.1 x 125% = 33.8 A #8 copper conductor with an insulation factor of 60, 75 or 90 degrees C is rated at 40, 50 and 55 amps respectively.


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