Okay, let's clarify the application of the optional calculation method with tankless water heaters under the NEC 2020. Your statement has the right idea, but let's refine it for accuracy.

Understanding the Optional Calculation Method (NEC 220.82 - 220.85)

The optional method allows you to calculate the service or feeder size for a dwelling unit in a simplified way. It's an alternative to the standard calculation outlined in Article 220, Parts I through III.

Tankless Water Heaters and the Optional Method

Tankless water heaters, due to their potentially very high instantaneous demand, require special consideration in the optional calculation method.

Here's the breakdown of how to handle a tankless water heater within the optional calculation, specifically referring to NEC 2020


Determine the Connected Load

Find the nameplate rating (in volt-amperes or watts) of the tankless water heater. This is the full potential load if it were to run at 100% duty cycle, which it typically doesn't.2.

Application of Demand Factors (NEC 220.84(B))



Tankless Water Heaters (with Controlled Temperature Limiting Devices)

NEC 220.84(B)(1) allows you to take the larger of the following two values: The connected load of the tankless water heater. The load calculated by applying 100% of the first tankless water heater and 25% of the nameplate rating of all other tankless water heaters.

Tankless Water Heaters (without Controlled Temperature Limiting Devices)

NEC 220.84(B)(2) requires you to apply 100% to the load.
3.

Inclusion in the Overall Calculation

Add the adjusted demand load of the tankless water heater to all other loads calculated as follows (per NEC 220.82):

(a) General Loads

Includes things like general lighting and receptacles. Take the first 10 kVA at 100%.

(b) Remainder of General Loads

Anything exceeding the initial 10 kVA is taken at 40%.

This is where your 40% comes in, but it applies to the sum of all other loads after you include the appropriately adjusted tankless water heater load. (c) Electric Ranges, Wall-Mounted Ovens, Counter-Mounted Cooking Units

Use Table 220.55.

(d) Electric Clothes Dryers

The larger of nameplate rating or 5 kVA.

(e) Air Conditioning and Heat Pump Equipment

100% of the nameplate rating of the central AC/heat pump compressor plus 100% of the supplemental heating (unless both the AC and supplemental heating won't operate simultaneously. Then, take the larger).

(f) Electric Space Heating

65% of the nameplate(s) rating of the non-coincident heating load of air conditioner and heat pumps.4.

Determine the Service Size

Calculate the total demand load (in kVA) by adding the results from the above steps. Then, divide by the system voltage (e.g., 240V for a typical residential service) to determine the required amperage.

Example (Simplified)

Let's say you have the following: Tankless Water Heater: 50A at 240V = 12,000 VA (12 kVA) General Lighting/Receptacles: 6,000 VA (6 kVA) Electric Range: 8 kVA (from Table 220.55) Air Conditioner: 3,000 VA (3 kVA) Electric Dryer: 5,000 VA (5 kVA)

Calculation


Tankless Water Heater

Since we only have one tankless water heater, we use its full connected load of 12 kVA. Assuming the load has a controlled limiting device.2.

General Loads (excluding tankless)

6 kVA3.

First 10 kVA of General Loads

We have 6kVA of general and 12 kVA from the tankless for 18 kVA. 10 kVA \ 100% = 10 kVA 8 kVA \ 40% = 3.2 kVA Air Conditioner 3 kVA \ 100% = 3 kVA Electric Range = 8 kVA Electric Dryer = 5 kVATotal: 10+ 3.2+3+8+5= 29.2 kVAService Size Calculation: 29.2/240 V1000= 121.67 A

Key Points



Read NEC 220.82-220.84 carefully. These sections contain the specific rules for the optional method. Pay close attention to the details and examples. Tankless Water Heater Control Devices

Note the different treatment of tankless heaters with or without controlled-temperature limiting devices.

Total Load

Make sure you are applying the 40% reduction after including the properly adjusted tankless water heater load into the total load subject to the reduction.

Local Codes

Always check with your local electrical inspector, as they may have specific interpretations or amendments to the NEC.

Important Disclaimer: Electrical calculations can be complex and have significant safety implications. I highly recommend consulting with a qualified and licensed electrician for your specific installation. This explanation is for educational purposes only and should not be considered a substitute for professional advice. Flag for review