Skip to main content

Motor Load Calculations

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, it looks at all of the motors connected to the panel itself, plus all of the motors connected to downstream panels.

The hierarchy for determining the largest motor is as follows:

  1. If there are one or more Single Motor loads, the largest of those is used as the largest motor.
  2. If there are no Single Motor loads, the largest Multiple Motors load is used as the largest motor.

When to Use Single Motor or Multiple Motors​

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 separate Single Motor load on the equipment.
  • If there are more than four motors and you need to specify more load types, specify the largest motor as a Single Motor load and the sum of the other motors as a Multiple Motors load.
    This way, even if the sum of the other motors is larger than the single largest motor, the largest motor can be determined.

Motor Subtypes​

There are multiple subtypes of motor loads that you can use, depending upon the type of motor. Use the Single Motor and Multiple Motors load types when none of the types below apply.

  • Cooling, Single Motor, Cooling, Multiple Motors, and Heating and Cooling Motor: All cooling loads are assumed to be motors. The Single Motor and Multiple Motors 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​

When defining fixed loads, there are two types of motor loads:

  • Largest Motor: Use this 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.

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