As electrical power becomes more expensive, people focus on better methods of conserving it and reducing monthly electric utility bills. When analyzing the power usage for a home or a business, many are questioning the feasibility of installing storage-type water heaters where the water is maintained at a specific temperature. The usage of timers, which would turn the storage water heaters on and off, is an inexpensive method of limiting the amount of wasted power. However, family or business schedules may not make this option convenient or practical since storage tank water heaters take time to heat the water for use. Passive solar water heaters may be an efficient way to heat swimming pool water or as a method to supplement the storage tank water heater, but would not be feasible as the only source of hot water for general use within a building. Tankless water heaters are becoming a very popular method for delivering the specific amount of hot water to be used without wasting power.

Tankless water heaters—also called instantaneous or demand water heaters—are available for natural gas, propane and electric systems in many sizes. When there is a hot water demand from the dishwasher, clothes washer, shower, hot water tap or other hot water draw, cold water enters into the tankless unit and is heated by the electric or gas heating elements. These tankless water heaters can be sized appropriately for each hot water load with the heaters located at each point of hot water use or can be sized large enough to handle multiple hot water loads simultaneously. Where individual heaters are installed at each hot water point of use, electrical power or gas must be supplied to the point of use, rather than to one central location for whole-house or whole-building hot water.

A natural gas or propane tankless water heater requires an electrical control circuit to supply power for a control board that provides sensing of the difference between the incoming water temperature and the desired water temperature. The control board determines the amount of heat necessary to obtain the desired temperature of the hot water, which initiates an electronic ignition sequence causing the water to be heated to the desired temperature and then be delivered to the point of use. Most natural gas or propane units require a 15- or 20-ampere (A), 120-volt (V) circuit for power to the control board and electronic ignition device. The manufacturer of the tankless unit normally determines the size of the circuit and provides, as part of the installation, instructions and the nameplate of the tankless water heater.

Since the control circuit is an individual load, the ampere rating or setting of the overcurrent device determines the circuit rating in accordance with 210.3 and 422.10(A) in the National Electrical Code as a noncontinuous load. The size of the circuit conductor is based on the load and the size of the overcurrent device protecting the circuit.

Electrical circuits that supply electrical tankless water heaters are sized based on the wattage of the individual units with voltage rated at 120V, 208V, 240V and 277V, depending on the voltage available at the site and at the location of the tankless water heater. Based on 422.11(E) and having a resistance-heating element rated at less than 48A, single nonmotor-operated appliance branch circuits can have branch-circuit overcurrent protection that does not exceed the marking on the appliance. If the appliance is not marked with an overcurrent protection device size and the appliance is rated at 13.3A or less, the overcurrent device cannot exceed 20A. If the appliance is rated over 13.3A and is not marked with an overcurrent device size, the overcurrent device cannot exceed 150 percent of the rated appliance current. Where this 150 percent value does not correspond to a standard size overcurrent device based on 240.6(A), the next standard size can be installed.

Where the tankless water heater’s electrical heating elements exceed a 48A rating, 422.11(F)(3) permits resistance-type immersion electric heating elements to be contained within an American Society of Mechanical Engineers-rated and stamped vessel or listed instantaneous tankless water heater. The heating elements for the tankless water heater are permitted to be subdivided into circuits not exceeding 120A with the branch circuits protected at not more than 150A. Whole-house or whole-building tankless or instantaneous water heaters can be sized from 12 kilowatts (kW) and 50A at 240V to 36kW and 150A at 240V or higher. Ensure any existing or new electrical service is large enough to handle these instantaneous tankless water heaters since increasing the size of the electrical service can be expensive.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and mark.c.ode@us.ul.com.