Electric Work: How to figure Volts=Amps-Watts for residential water heater

Tuesday, April 28, 2015

How to figure Volts=Amps-Watts for residential water heater

Off peak with timer EH40-80

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Most heating circuits will require at least a 30-amp, 240-volt breaker.

The size of cable is typically 10-gauge, ...

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1. Find label on side of water heater
Take photo of label for records.

Water heaters with 2 elements are rated 208-240Volt
Homes are usually wired 240Volt

Condo buildings etc can be wired for 208Volt
Water heater work with any voltage

Resources:
Water heater manufacturers
Find age of water heater using serial number on label
Connect water heater to generator
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Water heater label

Look for Volts and Watts on label Always test volts first,
and then look for wattsLabel shows Phase:

Phase 1 or single-phase
is household or condo electric service
(Phase 3 shows on some commercial water heaters)

Voltage:
Label shows 208-240 Volts since these are typical voltages

Each element is 5500 watts:
Label shows Two 5500 Watt elements @240V
Each element is rated 4125 watts when connected to 208V
instead of 240V

Total connected 5500 watts
means both elements are never ON at same time.

Note about 208Volt:
5500 Watt rating drops to 4125 watts when tank wired to 208V.

Why? Water heater elements are like light bulbs...
When power drops, then light bulbs dim, and produces less wattage output.

Same with elements:
when voltage drops, the element is less hot and
wattage output is lower

Resources: Tank wiring can be changed:
How to wire thermostats and elements
Off peak water heater wiring
Convert 3-phase non-balanced water heater to single-phase
How electric water heaters work
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Different water heater label/ 4500 watts @ 240volts

Label shows 4500 watt elements @240Volt
Total connected 4500 watts, which means both elements are not ON at same time.
Homes, houses, and many ordinary apartment buildings have 240Volts
While many apartment and condo buildings have 208Volt service

Note: 4500 watt element @ 240V changes to 3375 watt element @ 208 volts

Watt rating drops when voltage drops
Always test voltage first before figuring watts
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Element rated for 4500 watts @ 240Volt

Reducing voltage reduces wattage
Resistive loads like water heater elements, watt consumption drops when voltage drops.
For example light bulb gets dim in a brown-out when voltage drops across large area of homes.

Example formula shows that wattage for 120Volts
is 25% of wattage for 240Volts

Save water heater costs during peak electric pricing by converting water heater to 120V.

Not recommended for ordinary household usage.
Only for peak hours.

Lower wattage heats more slowly.
4500 watt @ 240 volt heats 21 gallons per hour
1125 watts @ 120 volt heats 5 gallons per hour.

Tank stays hot.
Electric consumption spreads across more hours.

It still takes same amount of energy to heat water, so cost per heated gallon stays same.

Resource: How to convert water heater to 120Volts
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Formulas:
Volt x Amp = Watt

Amp = Watt ÷ Volt

More Electrical formulas

Water heater label shows Volts and Watts
Volts x Amps = Watts --Same as-- Amps = Watts ÷ Volts
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Calculate amp size of breaker using formula:
5500 Watt element ÷ 240 Volts = 22.91 Amps or 23 Amps

Other examples
4500 Watt element ÷ 240 Volts = 18.75 Amps
3500 Watt element ÷ 240 Volts = 14.5 Amps
1500 Watt element ÷ 240 Volts = 6.25 Amps

40 Watt light bulb ÷ 120 Volts = 0.33 Amps
If resistive load,

such as light bulb or water heater element, receives less voltage, then the watt rating is less.
If 40 watt light bulb receives 80 volts, then wattage is reduced::
(80 ÷ 120) squared = 44% ... so 40 watt bulb rating is reduced to

40 x .44 = 17.6 watts

Resource:
Choose correct wire and breaker size
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Volts and watts show on end of element

When replacing elements, use same Wattage shown on tank label
Any generic element will work as replacement
If using smaller wattage, tank will heat slower.

Tank warranty is voided when installing higher wattage element than shows on label
And this can be dangerous if wire size and breaker do not match load. Best to match element rating with tank label. Generic hardware store elements are good choice. Expensive 'lifetime' or 'extended warranty' elements are probably money-wasteful for homeowner.

Resources:
Select wire and breaker size How to replace element
How to test element
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water heater circuit breaker

10 gauge wire and 30 amp breaker
for 4500 watt and 5500 watt elements @ 240Volts

5500 Watt element ÷ 240 Volts = 22.91 Amps or 23 Amps4500 Watt element ÷ 240 Volts = 18.75 Amps

Oversized breaker and wire will run cooler, and breaker box and breaker willl last longer

Resources:
Select wire and breaker size
How to replace circuit breaker
Troubleshoot: water heater is blowing breaker
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12 gauge wire and 20 amp breaker
for 3800 watt and 1500 watt elements @ 240Volts3500 Watt element ÷ 240 Volts = 14.5 Amps

1500 Watt element ÷ 240 Volts = 6.25 AmpsResources:

Select wire and breaker size
How to replace circuit breaker
Troubleshoot: water heater is blowing breaker
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wire and breaker size chart

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Wire and breaker chart

For ordinary residential non-simultaneous water heater
Test voltage
Identify element wattage
Look for total connected watts on label
Use chart to find correct wire and breaker

Resources:
Select wire and breaker size
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What does Watt rating mean?
Think about the difference between 40 Watt and 100 Watt light bulb.
Higher the Watts, the hotter the light bulb, and the higher output of total energy
So 5500 Watt element gets hotter than 3500 Watt element, and 5500 Watt uses more total energy
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Why are elements different Wattage?
Think about a light bulb. A closet can be lit with one 40 Watt bulb. But a large room requires more Wattage.
So very large water heaters use 5500 Watt element since they heat more water.
Most residential tanks have 4500 Watt elements
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If elements burn out frequently, reduce element wattage, or install lower density element.

Resources:
How to select and replace element
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No image available
AO Smith Resources:
Sears Kenmore (AO Smith) sell sheet/ requires 30 amp breaker and 10 gauge wire
How to adjust water heater temperature
9-ways to save with water heater
How to wire thermostats and elements
Off peak water heater wiring
How to replace elements

Smaller wattage on lower element? Probably does not save much.

DualWatt™ Lower element:
Choose between 3800 watt and 5500 watt connection on lower element.
Upper element remains 5500 watt.

Does smaller wattage on lower element save money?
Probably not.
Why? because it takes same number of Kwh to heat gallon of water, no matter which size element. All cold water entering tank must be heated to thermostat set point

Exceptions where DualWatt element might save money:
1) Tanks wired for Off-peak, but then any element will achieve same result
2) States that have peak electric pricing, the lower wattage will reduce consumption a bit by heating water more slowly, thus spreading electric usage over more time.
3) Lower wattage elements generally lasts longer and do not burn out as frequently as higher wattage elements. Or replace element with low density element.
4) Replace upper element with 3800 watt, and set lower element to 3500 watt, and you can use 12 gauge wire and 20 amp breaker without upgrading electric to 30 amp breaker and 10 gauge wire

Turning down temperatyure on lower element is better strategy for saving.
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Thermostats and elements

Rasources:
How to wire thermostats and element s
How to troubleshoot electric water heater

Ready for peak electric prices?
How-to-wire-water-heater-with-switches-and-timers
How to wire off-peak water heater thermostats
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Household water heaters are wired for NON-simultaneous operation.

Both elements are not ON at same time
Resources:
How to wire water heater thermostats
How water heaters work
How to wire off-peak water heater
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240 volt water heater circuit

Basic 240Volt water heater circuit

Example shows water heater on dedicated 240 Volt breaker.
Dedicated means the breaker does not supply power to other circiuts.
Another example: Microwave oven is normally on dedicated 120 Volt breaker.

Resources:
How to replace circuit breaker
How to install subpanel
Troubleshoot electric water heater
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Electric meter on water heater

How to install electric meter on water heater
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color-code household wire

Use 10 gauge wire for water heaters; Orange-colored 10 gauge wire for 30 Amp breakers

30 Amp breaker x 240 Volts = 7200 watt => 80% rule applies:

7200 Watts x 80% = 5760 W safe maximum

80% rules applies to household electrical loads

Other rating:

Yellow-colored 12 gauge wire for 20 Amp breakers

White-colored 14 gauge wire for 15 Amp breakers

18 gauge lamp cord or 1 residential can opener, pencil sharpener, lamp etc

Resource:
Ratings and color code for wire
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What size circuit breaker? 30 Amp

For safety > a circuit breaker is allowed to handle 80% of Amps shown

Water heater safety:
Always use 10 gauge wire and 30 amp breaker

30 Amp breaker x 80% rule = 24 Amps safe maximum

Calculation above shows that 5500 Watt water uses 23 Amps

And 30 Amp circuit breaker can carry 24 Amps

So a 30 Amp circuit breaker can safely handle
the 23 Amps needed for 5500 Watt water heater

Oversized breaker and wire run cooler and save money, but make sure breaker size and wire size matchResource:
Ratings for breaker and wire
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Cost to operate water heater
How much does it cost to operate gas and electric water heaters
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Small water heater

Avg electric price by state
Avg state price chart in pdf

How much does it cost to run Electric water heater:
3 hours is approximate daily run-time for a water heater [depending on usage and temperature of incoming water]
Bath uses 12-15 Gallons hot water <> shower uses 5-9 Gallons hot water > Hot water use chart

5500 Watts x 3 hours = 16,500 watt-hours or 16.5 Kwh per day
For a full 30-day month <=> 30 days x 16.5 Kwh per day = 495 Kwh per month for water heater electricity
Calculate cost <=> 495 Kwh x $.10 per Kwh = $49.50 + tax
As general rule, electric water heaters operate at 99% efficiency with or without water-softener > until sediment reaches bottom element > full efficiency is restored when sediment removed
How to remove sediment Read research pdf
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3 hours a day @ $.10 per Kwh
5500 Watt tank = 495 Kwh per month = $49.50 per month + tax
4500 Watt tank = 405 Kwh per month = $40.50 per month
3500 Watt tank = 315 Kwh per month = $31.50 per month assuming low usage
1500 Watt tank = 135 Kwh per month = $13.50 per month
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Waterheatertimer.org
is a self-help site
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Compare 16 electric and 3 gas water heater timers
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