Yes, you can use Article 220.87 and Article 220.54 in conjunction, but you need to be careful about how you apply them and understand the implications. Here's a breakdown

1. Purpose of Each Article:
Article 220.87: Determining Existing Loads (Optional Method)

This article allows you to use actual recorded maximum demand data (typically from the past year) to determine the existing load on a service. This is often more accurate than relying on nameplate ratings and standard demand factors, especially if you believe the building's actual usage is lower than what a calculation using standard methods would suggest.



Article 220.54: Electric Clothes Dryers — Dwelling Unit(s)

This article provides demand factors to apply to the total number of electric clothes dryers in dwelling units when calculating the service or feeder load. The demand factors are based on the number of dryers.

2. The Process

Here's the general approach you'd take:1.

Determine the Existing Load (Article 220.87)

Obtain at least one year of billing demand data. Calculate the maximum demand excluding space heating and cooling, as per 220.87(1). This is crucial. You now have the maximum demand of the existing load, not including heating or cooling. Let's call this "Existing Demand (Excluding HVAC)".2.

Calculate the Available Capacity

Multiply the service ampacity (400 amps) by the voltage and then by 1.73 if you are using three phase to find the total amount of power available. (400 240 = 96,000 Watts for single phase) Subtract the "Existing Demand (Excluding HVAC)" from the total power available to determine the amount of power available for new loads.3.

Calculate the Dryer Load (Article 220.54)

Assume each dryer is rated at 5kW (5000 watts) unless you have specific nameplate information. This is the NEC default. Use Table 220.54 to determine the demand factor for the total number of dryers after you add the new ones. For example, if you eventually have 20 dryers, the demand factor will be 35%. If you eventually have 50 dryers, the demand factor is 26%. This number will be the amount of power that the dryers will demand.4.

Determine the Number of Additional Dryers

Figure out how many dryers you can add without exceeding the remaining capacity. This may require some trial and error.

3. Important Considerations and Cautions



Accuracy of Demand Data

The accuracy of your load calculation is entirely dependent on the quality and completeness of the billing demand data. Make sure it's reliable.

Future Expansion

Consider potential future load growth beyond just the dryers. Will residents be adding more appliances, electronics, or EV chargers in the near future? It's best to leave some headroom.

Code Official's Approval

Using Article 220.87 is optional, and the local electrical inspector/AHJ (Authority Having Jurisdiction) has the final say. Be prepared to justify your calculations and approach. They might require additional documentation or have specific local interpretations.

Feeder Capacity

You're focusing on the 400-amp service fuse. Make sure the feeder conductors between the service equipment and the apartment panels are also adequately sized for the additional load.

Panel Capacity

Don't forget to check the capacity of the individual apartment panels. You need to make sure that the panels in the apartments that will be getting dryers have enough space and available capacity for the 240V dryer circuit.

Example Scenario

Let's say: 400 amp service at 240 volts. Article 220.87 yields a maximum "Existing Demand (Excluding HVAC)" of 60,000 watts Available Power: (400 amps 240 volts) - 60,000 watts = 36,000 watts available. Assume you currently have 10 dryers.Now, let's see how many more dryers we can add.

Trial 1

Adding 5 more dryers (total of 15). Table 220.54 demand factor for 15 dryers: 41% Total Dryer Load: 15 dryers 5000 watts/dryer 0.41 = 30,750 watts. Since 30,750 watts is less than 36,000 watts, adding 5 more dryers is potentially okay.

Trial 2

Adding 10 more dryers(total of 20) Table 220.54 demand factor for 20 dryers: 35% Total Dryer Load: 20 dryers 5000 watts/dryer 0.35 = 35,000 watts. Since 35,000 watts is less than 36,000 watts, adding 10 more dryers is potentially okay.

Trial 3

Adding 12 more dryers (total of 22). Table 220.54 demand factor for 22 dryers: 34% Total Dryer Load: 22 dryers 5000 watts/dryer 0.34 = 37,400 watts. Since 37,400 watts is more than 36,000 watts, adding 12 more dryers will overload the service.

Important Note: This is a simplified example. You must perform a thorough load calculation, considering all existing loads, potential future growth, and all applicable code requirements. Consulting with a qualified electrical engineer or contractor is highly recommended* for this type of project. They can ensure compliance with all applicable codes and standards and provide accurate calculations for your specific situation. Flag for review