Find the minimum size branch circuit and feeder conductors (using THHN) for each

Question

Find the minimum size branch circuit and feeder conductors (using THHN) for each of the motors as well as the feeder, breaker/fuse sizes for each motor and the feeder, thermal overload sizes. The feeder overcurrent protection is a fused disconnect (time delay fuse) that supplies a circuit breaker panel with mains. The overcurrent protection for each motor is an inverse time circuit breaker.

M1: 25HP, 230V, 3-phase, 65A, 50 degree temp rise

M2: 15HP, 230V, 3-phase, 38A, SF = 1.3

M3: 10HP, 230V, 3-phase, 25A, temp rise = 40 degrees

pleas inclode explnation.

Explanation / Answer

The National Electrical Code requirements for conductor sizing and overcurrent protection have always been confusing and complex. Factors that must be considered include:

NEC 240.4 requires the branch circuit, feeder, and service conductors to be protected against overcurrent in accordance with their ampacities as specified in 310.15. Section 240.4(A) through (G) contains rules that modify the general requirement and permit conductors to be protected in a manner other than their ampacities from 310.15, including:

With so many different Code rules that modify the general requirements, it does become overwhelming to select a circuit conductor and overcurrent protection device. However the following steps and examples should help you understand the basic rules of conductor sizing and protection.

Step 1 - Size the overcurrent device in accordance with 210.20(A) and 215.3. These two NEC rules require the overcurrent device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load.

Step 2 - Select the conductor to comply with 210.19(A), 215.2, and 230.42(A). Sections 210.19(A), 215.2 and 230.42(A) require the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load.

In addition, 110.14(C) requires consideration of the temperature rating of the equipment terminals when sizing conductors. Circuit conductors must be sized according to the 60

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