Residential electric water heaters are
listed under UL Standard 174, Household Electric Storage Tank Water
Heaters. Electrical contractors many times are also in the plumbing,
heating, and appliance business.
They need to know more than how to do
the electrical hookup. The following text discusses electrical as
well as other data about electric water heaters that will prove
useful.
All homes require a supply of hot
water. To meet this need, one or more automatic water heaters are
generally installed as close as practical to the areas having the
greatest need for hot water.
Water piping carries the heated water
from the water heater to the various plumbing fixtures and to
appliances such as dishwashers and clothes washers.
The wattage ratings of electric water
heaters can vary greatly, depending on the size of the heater in
gallons, the speed of recovery desired, local electric utility
regulations, and codes. Typical wattage ratings are 1500, 2000, 2500,
3000, 3800, 4500, and 5500 watts.
A resistance heating element might be
dual rated. For example, the element might be marked 5500 watts at
250 volts, and 3800 watts at 208 volts.
Most residential-type water heaters are
connected to 240 volts except for the smaller 2-, 4-, and 6-gallon
point of use sizes generally rated 1500 watts at 120 volts.
Commercial electric water heaters can be rated single-phase or
3-phase, 208, 240, 277, or 480 volts.
UL requires that the power (wattage)
input must not exceed 105% of the water heater’s nameplate rating.
All testing is done with a supply voltage equal to the heating
element’s rated voltage.
Most heating elements may burn out
prematurely if operated at voltages 5% higher than for which they
are rated.
To help reduce the premature burnout of
heating elements, some manufacturers will supply 250-volt heating
elements, yet will mark the nameplate 240 volts, with its
corresponding wattage at 240 volts.
This allows a safety factor if slightly
higher than normal voltages are experienced.
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