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A tankless water heater is a very small but very powerful and extremely efficient electric water heating system. The tankless water heater, whose technology has earned 5 U.S. Patents and many more foreign patents, is much more than just a water heater.

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National Electric Code Rules - Load Calculations

Optional Calculations for Computing Feeder and Service Loads 220-30. 
Optional Calculation – New Dwelling Unit

Feeder and Service Load. For a dwelling unit having the total connected load served by a single 3-wire, 120/240-volt or 208Y/120-volt set of service-entrance or feeder conductors with an ampacity of 100 or greater, it shall be permissible to compute the feeder and service loads in accordance with this section instead of the method specified in Part B of this article. The calculated load shall be the result of adding the loads from (b) and (c). Feeder and service-entrance conductors whose demand load is determined by this optional calculation shall be permitted to have the neutral load determined by Section 220-22.

General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:

  1. 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit specified in Section 220-16
  2. 3 volt-amperes per square foot (0.093 m2) for general lighting and general-use receptacles
  3. The nameplate rating of all appliances that are fastened in place, permanently connected, or located to be on a specific circuit, ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and water heaters
  4. The nameplate ampere or kVA rating of all motors and of all low-power-factor loads

Heating and Air-Conditioning Load. Include the largest of the following six selections (load in kVA).

  1. 100 percent of the nameplate rating(s) of the air conditioning and cooling.
  2. 100 percent of the nameplate ratings of the heat pump compressors and supplemental heating unless the controller prevents the compressor and supplemental heating from operating at the same time.
  3. 100 percent of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value. Systems qualifying under this selection shall not be calculated under any other selection in (c).
  4. 65 percent of the nameplate rating(s) of the central electric space heating, including integral supplemental heating in heat pumps where the controller prevents the compressor and supplemental heating from operating at the same time.
  5. 65 percent of the nameplate rating(s) of electric space heating if less than four separately controlled units.
  6. 40 percent of the nameplate rating(s) of electric space heating if four or more separately controlled units.

Optional Calculation for Additional Loads in Existing Dwelling Unit

For an existing dwelling unit presently being served by an existing 120/240-volt or 208Y/120-volt, 3-wire service, it shall be permissible to compute load calculations as follows:

Load (kVa) Percent of Load 
First 8 kVA of load at 100 
Remainder of load at 40

Load calculations shall include lighting at 3 volt-amperes/ft2 (0.093 m2); 1500 volt-amperes for each 2-wire, small-appliance branch circuit and each laundry branch circuit as specified in Section 220-16; range or wall-mounted oven and counter-mounted cooking unit; and other appliances that are permanently connected or fastened in place, at nameplate rating.

If air-conditioning equipment or electric space-heating equipment is to be installed, the following formula shall be applied to determine if the existing service is of sufficient size.

Air-conditioning equipment*.................. 100%
Central electric space heating*............. 100%
Less than four seperately controlled space-heating units*.............................. 100%
First 8 kVA of all other loads.................. 100%
Remainder of all other loads................. 40%

*Use larger connected load of air conditioning and space heating, but not both.

Other loads shall include the following:

  1. 1500 volt-amperes for each 20-ampere appliance circuit
  2. Lighting and portable appliances at 3 volt-amperes/ft2 (0.093 m2)
  3. Household range or wall-mounted oven and counter-mounted cooking unit
  4. All other appliances fastened in place, including four or more separately controlled space-heating units, at nameplate rating