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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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βˆ™ 11y ago
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βˆ™ 11y ago

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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βˆ™ 11y ago

225 amp depending on many factors.

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Q: What size breaker for 150 hp 480 volt motor?
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What size breaker for a 20 horse power motor with 3 phase 208 volts?

For a 20 horsepower motor with 3-phase 208 volts, you would typically use a 70 amp circuit breaker to protect the motor and wiring. This calculation is based on the full load amp (FLA) rating of the motor, which for a 20 HP motor at 208 volts typically falls around 70 amps. It's important to consult the motor's technical specifications and local electrical codes to ensure proper breaker sizing.


How do you find out how big your main breaker box is?

You can determine the size of your main breaker box by looking at the number of circuits it can handle. A typical residential main breaker box is usually 100 amps, 150 amps, or 200 amps. You can also check the label on your breaker box or consult with an electrician for assistance.


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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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Wire sizing is based on the amperage needed by the load. The equation used to find amperage when the HP is shown is; A = HP x 746/1.73 x V x %eff x pf. Amps = 25 x 746 = 18650/1.73 x 240 x .87 x .87 = 18650/314 = 60 amps. 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. Motor feeders have to be rated at 125% of the motors full load amperage. 60 x 125% = 75 amps. A #4 copper conductor with an insulation rating of 75 or 90 degrees C are both rated at 85 amps. The breaker has to be 250% of the motors full load amperage 60 x 250% = 150 amps The breaker will be a three pole 150 amp breaker connected to three #4 copper conductors to operate a 25 HP 240 volt three phase motor.


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What size breaker for a 20 horse power motor with 3 phase 208 volts?

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