Okay, let's break down the optional calculation for your situation, focusing on the heating and cooling components, and address the nuances of using the optional calculation with supplemental electric heat.

Understanding 220.82(C)(3): The Key is "Central Electric Space Heating"

The crucial phrase is "central electric space heating system." This is where the interpretation comes in. The 65% demand factor applies only to a central electric space heating system. It doesn't automatically apply to all electric heating. What is a "Central Electric Space Heating System?" The intent of this clause is to reduce the calculated load where the heating is supplied by a central resistance heating system designed to heat the entire building. Think baseboard heaters throughout the entire dwelling, or a ducted electric furnace. Your Situation

You have a mix: infloor electric heat, controlled in zones by thermostats and supplemental mini-splits that will also provide heat. The infloor heat is distributed and not a single central system as much as a distributed system.

How to Apply 220.82(C)(3) to Your Specific Case

Here's how I recommend approaching this, considering code intent and practical application:1. Evaluate the "Central" Nature of the Infloor Heat



Individual Thermostats

Because the infloor heat is split into multiple zones with individual thermostats, it's less like a single central system. This suggests you cannot apply the 65% reduction to all the infloor heat.

Central Controller

The 9900W of infloor heat controlled by a "central controller" is more central system-like. However, the presence of multiple thermostats throughout the zones still implies a degree of individual zone control.2.

Apply the Appropriate Demand Factors



Mini-Splits (Cooling and Heating)

Use 100% of the nameplate rating of both mini-splits. This is because they are your primary cooling, and also contributing to heating. Mini-Split 1: 5500W Mini-Split 2: 3360W

Infloor Heat (Individual Thermostats)

Use 100% of the nameplate rating of all the infloor heaters on their own thermostats. 2 x 1700W = 3400W 1 x 2600W = 2600W

Infloor Heat (Central Controller)

This is the trickiest part. I would recommend 100% due to it being supplemental and the other infloor heats are also being included at 100%. 9900W = 9900W3.

Compare Heating and Cooling Loads



Total Heating Load

5500W + 3360W + 3400W + 2600W + 9900W = 24760W

Cooling Load (Mini-Splits)

This is nameplate only since these units can provide heat, not a smaller MCA load from article 440. Mini-Split 1: 5500W Mini-Split 2: 3360W Total: 8860W

Incorporate only the LARGER of the total heating load and cooling load into the 220.82 calculation.4. Complete the 220.82 Optional Calculation

Follow all the steps in 220.82, using the heating/cooling load calculated above.

Why This Approach? Conservative

This approach is reasonably conservative. You're not taking the 65% reduction on the zone controlled infloor heat, which reflects the fact that those zones likely will be used independently and potentially simultaneously.

Code Intent

It aligns with the code's intent, which is to give a reduction only for true central electric space heating systems designed to heat the entire house at once.

Real-World Usage

It considers the likely usage pattern of your customer. If they're zoning the heating with thermostats, it's more likely different areas will be heated simultaneously.

Standard Calculation (Important Note)

Yes, you are correct. Even in the standard calculation, if you have both heating and cooling, you omit the smaller of the two loads. Since the heating load is likely larger (given all the infloor heat), you would omit the cooling load. All the loads are at 100% though, no 65% reduction here.

Additional Considerations Local Inspector

The final authority on interpretation is the local electrical inspector. Discuss your proposed approach with them before doing the work to avoid issues. They may have local interpretations or preferences.

Future Expansion

Consider potential future electrical needs. Are there plans for electric vehicle charging, hot tubs, or other significant loads? It's better to have extra capacity than to be close to the limit.

Voltage Drop

After determining the load, ensure your service conductors and branch circuits are adequately sized to minimize voltage drop, especially with the infloor heating.

In summary: You cannot use the 65% demand factor for all the heating, only if you consider the central controller one a central unit. The most conservative approach, and likely the most accurate given the zoning and separate thermostats, is to include 100% of the mini-split loads and the infloor heat loads. Verify with your local inspector before proceeding. Flag for review