ELECTRICAL SERVICE REQUIREMENTS & SAMPLE LOAD CALCULATIONS

ELECTRICAL SERVICE REQUIREMENTS  & SAMPLE LOAD CALCULATIONS

A. Introduction to Seisco Electrical Requirements

The Seisco is a flow-through electric 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 with the

Seisco, 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 Seisco versus storage tank heating is discussed in the

Cost Comparison section of this manual.

Like most electric storage tank water heaters, the Seisco heater requires 240 volts (AC) (or

208 VAC) to operate. Several Seisco heaters require multiple double pole circuits and breakers

Electrical Service:

The Seisco 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 Seisco 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 Seisco 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.  

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 Seisco 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 Seisco 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, the Seisco

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 Seisco Product Specifications for flow rate

and temperature rise ratings of each model before selecting the Seisco(s) for a home or building.

Power/Voltage Modulation:

During operation, the Seisco 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 and is patented by Seisco.

The on-board microprocessor control of the Seisco determines the temperature rise and flow

rate through it’s 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

Seisco 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 Seisco heater may

use more power when heating water for multiple tasks or for higher flow faucets, such as bath

tub and washing machine faucets.

Disconnects and Sub-panels:

Electrical disconnect devices do not contain circuit breakers and are not required by the

National Electrical Code (NEC) for residential appliances such as the Seisco 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 Seisco water

heater does not contain any motors, but some models require multiple circuits and circuit

breakers. A disconnect may be required for the Seisco in commercial applications.

Electrical sub-panels, containing circuit breakers, may be used with appliances such as the

Seisco 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-

In new residential construction, their is generally enough breaker spaces in the main electrical

panel to accommodate multiple circuit breakers for the Seisco 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 Seisco 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 Seisco heater. Refer to

the “Electrical Wiring & Breaker Guides” in this section for options that can be used to serve

various Seisco Models requiring multiple circuits.

Branch Circuits and Breakers:

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

National Electrical Code Rules – Branch Circuit Protection in this section of the Seisco

Product Manual for further detail and explanation.

Voltage Rating:

Seisco 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 Seisco 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. Seisco 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.

SEISCO Product Manual — Electrical Requirements

National Electrical Code Rules – Load Calculations

 (a) 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 .