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Motor Load Calculations

Monday, May 9th, 2011

NEC 430.24 describes the requirements for sizing a feeder serving multiple motors. The feeder must be large enough to handle 125% of the largest motor load, and 100% of all of the other motor loads.

Design Master Electrical has two main types of motor loads: Single Motor and Multiple Motors. The software uses these load types to determine the largest motor when calculating a feeder size. It always looks at all of the connected motors. For example, when determining the largest motor for a panel, we look at all of the motors connected to the panel, plus all of the motors connected to panels fed from the panel.

The largest “Single Motor” load is used as the largest motor.

If there are multiple “Single Motor” loads, the largest is used as the largest motor.

If there are no “Single Motor” loads, the largest “Multiple Motors” load is used as the largest motor.

For a piece of equipment with a single motor, use the Single Motor load type.

If the equipment has multiple motors, you have two choices:

  • If there are four or fewer motors and no other load types, you can specify each motor as a Single Motor.
  • If there are more than four motors and you need additional load types, specify the largest motor as Single Motor and the sum of the other motors as Multiple Motors. Even if the sum of the other motors is larger than the single largest motor, the largest motor can be determined.

If no motors are designated as a Single Motor, the software does not have enough information to determine the largest motor load. In this case, the largest Multiple Motors load will be used . The largest motor will be smaller than that, but how much smaller cannot be determined. Sizing that group of multiple motors at 125% might result in a feeder that is too large, but is preferable to a feeder that is too small.

Motor Subtypes

There are multiple subtypes of motor loads that you can use, depending upon the type of motor.

Single Motor and Multiple Motors: Use these two values when none of the other motor options apply.

Cooling, Single Motor, and Cooling, Multiple Motors, and Heating and Cooling Motor: All cooling loads are assumed to be motors. The Single Motor and Multiple Motor types work as described above. The General Loads and Dwelling Unit Loads work the same way.

Appliance Motor: Use this value when you have a dwelling unit appliance load that is also a motor. A garage door opener is one example. The motor is treated as a Single Motor.

Fixed Motor Loads

There are two types of fixed motor loads: Largest Motor and Other Motors.

Use the Largest Motor field for the load of the largest motor on the circuit. It will be treated as a Single Motor load. If there is a single motor on the circuit, use this field.

Use the Other Motors field for the sum load of all of the other motors on the circuit. It will be treated as a Multiple Motors load.

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