Largest Motor for One-Family Dwellings

Sections 220.50(A) and (B) refer to the following four code sections for determining motor loads:

430.24, Several Motors or a Motor(s) and Other Loads

430.25, Multimotor and Combination-Load Equipment

430.26, Feeder Demand Factor Article 440-Part IV, Hermetic Refrigerant Motor Compressors

Section 430.24 requires conductors supplying several motors to have an ampacity based on the full load table currents of all motors supplied plus 25% of the highest-rated motor. Sections 430.25 and 430.26 apply to special motor applications that are not generally found in dwelling-unit applications. Part IV of Article 440 covers the circuit conductors for HVAC equipment. It does not cover the blower motors found in air-handling units of split air conditioning units, which are covered by Article 430.

For example, what is the largest motor load for a dwelling with a 120 V blower rated at 660 VA, a 120 attic fan rated at 1400 VA, and a 120 V vent motor rated at 400 VA?

Largest motor = 1400

Largest motor load = 1400 × 25%

Largest motor load = 350 VA

Test your Knowledge: Determining the Largest Motor Load for One-Family Dwellings

1. What is the demand load if the largest motor is a 1600 VA pool pump?

Answer: 1600 x 25% = 400 VA – EXCELLENT!

Largest Motor for Multifamily Dwellings

Despite the fact that each unit in a multifamily dwelling may have the same motor load, 220.50 requires by its reference to 430.24 that only 25% of the largest motor in the entire multifamily dwelling be added to the standard calculation for multifamily dwellings. Once the largest motor in the multifamily dwelling is found, add 25% of the largest motor’s load to the service demand. If there are multiple largest motors of the same size, only use one of the motors.

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