You've correctly identified a potential area of confusion with the 2020 NEC regarding dwelling unit service calculations and the application of the 25% increase for the largest motor. Let's break down the issue and clarify the application of both 220.60 and the traditional motor sizing rules.

Understanding the Two Provisions 220.60 (Noncoincident Loads)

This section addresses situations where two or more large loads are unlikely to operate simultaneously. The classic example is heating and air conditioning in a residence. If the air conditioning load is smaller than the heating load, and it contains the largest motor, then 220.60 allows you to add 25% of the full-load current of the air conditioner to the smaller of the two (heating or air conditioning) to determine the total load. If heating is smaller than AC, then you would NOT need to increase the AC load as it is already the larger of the two.

Traditional Motor Sizing (Implied in many examples and calculations)

This is the more general rule of thumb to add 25% to the largest motor's full-load current. It's based on the understanding that motors draw a significantly higher inrush current when starting, which can stress the service conductors. This rule is implied and enforced through sizing the conductors to handle the motor starting current per Article 430.

The Overlap and Potential for Double-Counting

The core of your question is: If you've already increased the A/C load by 25% per 220.60, do you need to again increase the largest motor (which is often the A/C) by 25% later in the service calculation?The Answer: Typically, No - YOU ARE CORRECT.

Here's why:1. Purpose of the Increases

Both the 220.60 increase and the traditional motor sizing increase aim to account for the inrush current of the largest motor.
2.

Avoid Double Counting

Once you've accounted for the motor's inrush current through 220.60, there is no need to do it again.
3.

220.60 is Specifically Designed to Address the Largest Motor in Noncoincident Loads

The intent of 220.60 is that by applying the 25% increase to the smaller of the noncoincident loads (when the A/C is the larger motor load), you are already accounting for the inrush current of the largest motor within the context of the whole service calculation.

Why Annex D1(b) Might Seem Confusing

The Annex D examples in the NEC are for illustrative purposes. They are examples, not mandates. They may not always reflect the most efficient or up-to-date application of the code. The fact that Annex D1(b) still shows a separate 25% increase for the largest motor, even with the 220.60 provision in place, is a valid point of concern and may reflect a need for those examples to be updated.

Here's a more explicit breakdown of the process


Calculate General Lighting Load

Per 220.12.
2.

Calculate Small Appliance and Laundry Loads

Per 220.14.
3.

Calculate Fixed Appliance Loads

Per 220.174.

Determine Heating and Air Conditioning Loads

Calculate separately.5.

Apply 220.60 (if applicable)

Determine which is the smaller of the heating and A/C loads. If the A/C contains the largest motor, add 25% of the A/C full-load current to the smaller load (heating). If heating is smaller, then do not add 25% to the AC load. Use the resulting values (after the 25% addition) for heating and A/C in the overall service calculation.6.

Calculate other loads (e.g., ranges, dryers, etc.).7. Calculate total service load, no further adjustments for the motorIn summary

If you correctly apply 220.60 (increasing the smaller of the noncoincident loads by 25% if the A/C contains the largest motor), you should not then add another 25% to the largest motor later in the service calculation. You've already accounted for the motor inrush current. The Annex D example has not caught up to the code yet, and could mislead you.

Important Considerations



Local Amendments

Always check for local amendments to the NEC, as some jurisdictions may have specific requirements.

Engineering Judgment

If there are unusual motor characteristics or other circumstances that warrant it, an engineer may choose to add a further safety margin, but this would be a departure from the standard code application.

Conductor Sizing

Remember that you still need to size the individual branch circuit conductors for the A/C motor according to Article 430, which will involve considering the motor's nameplate information and relevant overcurrent protection rules.*

Manufacturer's Instructions: Always follow manufacturer's instructions for appliance and equipment connections.By correctly interpreting and applying 220.60, you can avoid double-counting the motor inrush current and size the service conductors appropriately for dwelling units. Consult with a qualified electrical engineer or inspector if you have any doubts or site-specific concerns. Flag for review