Tankless Water Heaters - Introduction to Electrical Guide

Our tankless water heater is a flow-through electric tankless water heater that generally requires more power (kW) to operate than an electric storage tank heater. The trade-off of using more power to heat only as it is needed proves to be a better alternative to using less power while heating a storage tank heater; whether you need it or not. The energy savings associated with the flow-through (or on-demand) technology of the tankless water heater versus storage tank heating is discussed in the Cost Comparison section of this manual.

Like most electric storage tank water heaters, the tankless heater requires 240 volts (AC) (or 208 VAC) to operate. Several heaters require multiple double pole circuits and breakers (see the chart below for 240 volt electrical ratings and requirements). Ratings for 208 volt commercial service and 208 volt ratings can be found in the Product Description & Specification section of this manual.

Model	Max. Power Rating (kW)	Voltage Rating (VAC)	Max. Current Rating (Amps)	Number of Circuits (2 wire)	Number of Circuit Breakers (Double Pole)	Circuit Breaker Size (Amps)
RA-9	9	240	37.5	1	1	40
RA-11	11	240	46	1	1	50
RA-14	14	240	58	2	2	30
RA-18	18	240	75	2	2	40
RA-22	22	240	91	2	2	50
RA-28	28	240	116	4	4	30

The heater is considered a non-continuous heating appliance according to the definitions in the National Electric Code, sections NEC 410 and 411. An appliance load that is not continuous for 3 hours or more is considered non-continuous. The heater, when used for standard domestic hot water applications, is considered a non-continuous heating appliance. Due to the diversity of water heating in a home, the load (amps) contribution of the tankless water heater to the overall service load of the home or building can be calculated using the optional methods of National Electrical Code, sections NEC 220-30 or 220-31. The methods and rules for calculating these loads can be found in the following section of this manual under B. National Electric Code Rules — Load Calculations.

For new dwellings, the service load should be calculated using NEC 220-30. For existing dwellings, the service load should be calculated using NEC 220-31. By both calculation methods, the tankless water heater load is generally added to the service load at 40% of it’s maximum nameplate rating. For instance, the maximum current (amp) rating of the Model RA-28 is 116 amps and 40% of this rating is about 47 amps. The 47 amps is typically the load added to the overall service load of the dwelling when using the optional calculation methods as described in NEC 220-30 and 220-31, not the maximum current rating, 116 amps. As a result, Model RA-28 will fit in most homes up to 3000 square feet that have a 200 amp whole-house electrical service. Also, it is important to refer to the Product Specifications for flow rate and temperature rise ratings of each model before selecting the model(s) for a home or building.

During operation, the tankless water heater is designed to use only the power necessary to heat the water for various combinations of temperature rise and flow rate. Also, it is designed to distribute the power evenly to it’s heating elements. This is called the "Power Sharing" technology.

The on-board microprocessor control determines the temperature rise and flow rate through temperature sensors mounted on the heating chamber. The control then staggers the application of power to the heating elements using voltage modulation. The result is a smooth and efficient use of power to heat the water. This advanced control technology is extremely important in eliminating light flicker and fluctuations within the home or building. Also, the tankless water heater will use only about 40 to 60% of it’s power rating for most domestic water heating applications in a home, such as a standard shower, bath or kitchen sink. The tankless water heater may use more power when heating water for multiple tasks or for higher flow faucets, such as bath tub and washing machine faucets.

Electrical disconnect devices do not contain circuit breakers and are not required by the National Electrical Code (NEC) for residential appliances such as the tankless water heater or any appliance rated less than 300 volts. However, disconnects may be required by the NEC for motor loads and for appliances with multiple circuits in commercial applications. The tankless water heater does not contain any motors, but some models require multiple circuits and circuit breakers. A disconnect may be required for the heater in commercial applications.

Electrical sub-panels, containing circuit breakers, may be used with appliances such as the tankless water heater in residential and commercial applications. Particularly for the models requiring multiple circuits, which are models RA-14, RA-18, RA-22 and RA-28 (see previous Chart — Electrical Ratings in this section).

In new residential construction, their is generally enough breaker spaces in the main electrical panel to accommodate multiple circuit breakers for the tankless water heater. However, in existing homes, the main electrical panel may be nearly full with circuit breakers serving existing load. In these cases, a single large breaker, rated for the entire load of the tankless water heater, can be installed at the main panel. From the main panel, a single circuit or sub-feed is installed to a sub-panel where the appropriate number of circuit breakers can be installed for the tankless water heater. Refer to the "Electrical Wiring & Breaker Guides" in this section for options that can be used to serve various tankless water heater models requiring multiple circuits.

As a non-continuous heating appliance, the branch circuit wires and breakers must be sized to 100% of the maximum ampere rating of the appliance. It is recommended that the wire and breakers of the branch circuits and sub-feeds be rated for at least 75 degrees C. This is particularly important to avoid over heating of the wires at the connections to the breakers. Over heating at the breaker connections may cause nuisance or premature breaker trips. Refer to D. National Electrical Code Rules — Branch Circuit Protection in this section of the Product Manual for further detail and explanation.

Tankless water heaters are manufactured with common 240 volt (AC) heating elements designed for optimum operation on a standard residential 240 volt (AC) electric service. Also, the tankless water heater will operate at 208 VAC, a common commercial voltage, with standard 240 VAC heating elements. However, when operating the heater at 208 VAC, the power rating and the heat output rating is significantly reduced. Tankless water heater models can be special ordered with 208 VAC heating elements to help maximize the power and heating output. Refer to the Product Specifications Table for details on the various voltage ratings.