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How to figure Volts-Amps-Watts for residential water heater
1. Find label on side of water heater
Take photo of label for records.
Water heaters with 2 elements are rated 208-240Volt
Homes in US and Americas are usually wired for 120 / 240Volt
Condo buildings, apartments can be wired for 120 / 208Volt
Commercial buildings have variety of voltages including 120/208/240/277
Ordinary water heater will work with any voltage, up to 480 as long as voltage does not exceed element rating... exception water heater with electronic controls such as heat-pump, tankless, some factory-made solar units etc that must be connected to the rated voltage and no other voltage
Convert-AC-water-heater-to-DC-water-heater.html
Resources:
Connect water heater to generator
Review heat pump
Review Solar water heater
See inside main breaker box
Water heater label
Typical electric water heater label
See larger image
See another image
Look for Volts and Watts on label
Always test volts first, and then look for watts

Label 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

See larger image
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

Water heater lable Other things on label
Serial number/ which also contains code for date of manufacture/ Resource: Find age of water heater
Gallon size of tank
Number of warranty years
Manufacturer and address of manufacturer/ Resource: Water heater manufacturers
Formula for heating water inside tank:
Gallon Per Hour (Electric) = (KW x 3413) divided by (temp. rise x 8.25) or (KW x 414) divided by (temp rise.)
Let's say you have 5500 watt elements.
Let's say incoming water temperature is 65°F and you want to heat tank to 105°, or 40° rise in temperature.

5500 watt or 5.5Kw elements x 3413 = 18771.5
40 degree temperature rise x 8.25 = 330
So in this specific situation, each element would heat 56.88 gallons per hour

If you have 4500 watt elements and incoming water temperature is 55°, and you want to heat to 120°, then the temperature rise is 65°
4.5 KW x 3413 = 15358.5
65 degree temperature rise x 8.25 = 536
So one element would heat 28 gallons per hour

This number is for EACH element, but does NOT consider heat loss from tank, room temperature, or thickness of insulation etc.
Remember, with residential simultaneous tank, only ONE element is ON at one time.
Upper element is ON, or lower element is ON, or BOTH elements are off. At no time are both elements on.
480 volt element
Larger image
Volts and watts are printed on end of each element
480 volt 4000 watt
or
415 volt 3000 watt
When voltage drops, the element wattage also drops

Typically 480 volt is common on 3-phase non-balanced commercial water heaters. The non-balanced 3-phase heater can be converted to single phase but attention must be paid to the element voltage and wattage
Resource: 3-phase non-balanced / and conversion to single phase
How to figure resistive wattage by volts
Ohm's laws says: Volts squared ÷ watts = ohms  
Resources
: See chart   How to test element
Each element has volt and watt rating printed on end of element
If element is rated 4500 watts @ 240 volts.... then volts squared (57600)
÷ 4500 watts = 12.8 ohms
So 12.8 is the ohm rating for the element, no matter what voltage is applied

But what happens when voltage is reduced on the same element?? For example voltage is reduced to 208 volts instead of 240 volts.
Then 208 volts squared (208v x 208v = 43264 volts˛) 
÷ 12.8 ohms = 3380 watts... and the element does not burn as hot

What happens when voltage is reduced to 120 volts on same element?
Then 120 volts squared (120v x 120v = 14400 volts˛)
÷ 12.8 ohm = 1125 watts and heating is reduced to 1/4 the original output

What happens when voltage is reduced on 480 volt, 4000 watt element?
480 Volts squared
(230400) ÷ 4000 watts = 57.6 ohms/ and 57.6 is the ohm rating for element no matter what voltage.
So when voltage is reduced to 415 volts:
415 volts squared
(172225) ÷ 57.6 ohm = 2990 watts (or 3000 watts as shown in photo)

What about increasing voltage instead of lowering voltage?
What happens when 480 volts is connected to element that is rated for 4500 watts and 240 volts? POW ! element burns out
480 volts squared (230400) 
÷ 12.8 ohms = 18,000 watts or 4 times the rating... and the element will burn out immediately

What happens when 240 volts is connected to element that is rated for 2000 watts and 120 volt? POW again
120 Volt squared (14400) ÷ 2000 watts = 7.2 ohm
240 volts squared (43264) ÷ 7.2 ohm element = 6000 watts running through a 2000 watt element causing it to burn out

So buy element that is rated for the voltage  Resource: Elements and element replacement


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
wattage formula

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 element will raise 47 gallons water by 40 degrees in 1 hour.
1125 watts @ 120 volt heats 12 gallons water by 40 degrees in 1 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
Recovery chart
High density element
Low density element
Screw-in elements
High and low density elements
All calculations shown on this page apply to high and low density elements.
Watts ÷ square inches of heating surface = density

High density has less square inches of heating surface, so each square inch of element burns hotter
If both elements have same watts and volts, they will still heat same amount as each other, no matter the density.
All new water heaters come standard with high watt density elements
Purpose of foldback type low density element: they last longer in water that has high mineral content, such as hard water

Resources:
How to select and replace water heater element
How to test if element is working
Upper and lower thermostats
thermostat temperature range
Thermostats are generic: Substitute Aprom with Therm-o-disc or Aprom or 89T etc
Make sure upper thermostat has 3 screws on right side for 2-element tank
Read more about thermostats

Thermostats are rated for any voltage
Thermostats will work with 12-24-36-48 volt AC DC
120-208-240-277-380-415-440 Volt AC up to 480 volt
High voltage DC from direct wire to solar panels will melt thermostats.
Resource for Solar PV water heater:
Convert water heater to high voltage DC
Convert water heater to low voltage DC
How to set up solar array

Thermostats are mechanical and do not depend on voltage to work.
Residential thermostats are rated 90-150 degrees F/ with reset trip out at 170°F
Commercial thermostats are rated 120-180 degree F/ with reset trip out at 200° F
Do NOT use commercial thermostats on residential water heater.
Cautions:
Danger of severe burns with water exceeding 130° F... apply cold water to burns... call for medical help.
Elderly people are not able to feel or respond to hot water ... adjust temperature on water heater to 105-110° F for elderly people... typical hot tub temperature might be 113°F and there is usually a time limit and advisory against anyone with breathing problems to sit in hot tub.
Do not work on water heater thermostats while wearing oxygen. Oxygen is flammable and can be ignited from electric spark. Make sure power is off.
Buy 90-150 degree F for residential tank:
Water heater thermostats at Amazon
Camco thermostats at Amazon
More thermostats
89T short/ dual-element thermostat at Amazon
Example Camco dual-element thermostat at Amazon

Resources:
Select and replace thermostats
120-180° Commercial and 3 phase thermostats
How to wire thermostats
Add another thermostat to gas or electric water heater
Troubleshoot electric water heater

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


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 instead of 120 volts, then wattage is reduced::
(80  ÷  120) squared = 44% ... so 40 watt bulb rating is reduced to 40 watt x .44 =  17.6 watts
Resource:
Choose correct wire and breaker size
Avoid equipment damage from brownout using compressor defender
Water heater element
Larger image
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
water heater circuit breaker
Wire size and breaker must match
Use 10 gauge wire with 30 amp 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 Amps
4500 Watt element ÷ 240 Volts = 18.75 Amps
Using oversized breaker and wire will run cooler, and breaker box and breaker will last longer
Consider 30 amp breaker: safe maximum is 80% rated valve: or 30 amps x .8 = 24 amps
So safe maximum for 30 amp breaker and 10 gauge wire is 24 amps... NOT 30 amps
How many watts? 24 amp safe maximum x 240 volts = 5760 watts (amps x volts = watts)

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

Wire size and breaker must match
Use 12 gauge wire with 20 amp breaker
12 gauge wire and 20 amp breaker
for 3800 watt and 1500 watt elements @ 240Volts


3500 Watt element ÷ 240 Volts = 14.5 Amps
1500 Watt element ÷ 240 Volts = 6.25 Amps
Using oversized breaker and wire will run cooler, and breaker box and breaker will last longer
Consider 20 amp breaker: safe maximum is 80% rated valve: or 20 amps x .8 = 16 amps
So safe maximum for 20 amp breaker and 12 gauge wire is 16 amps... NOT 20 amps
How many watts? 16 amp safe maximum x 240 volts = 3840 watts (amps x volts = watts)
If 20 amp breaker is rated for maximum 3840 watts, then it is too small for 4500 watt water heater
Replace 4500 watt elements with 3500 watt elements/ heating will be slower, but will get the job done

Resources:
Select wire and breaker size
How to replace circuit breaker
Troubleshoot: water heater is blowing breaker
2-pole circuit breaker
Breaker switches must be connected by a bar so both switches operate as one switch
Circuit breaker resources:
Water heater is tripping breaker
How to replace circuit breaker
How to wire gfci
Can AC breaker be used for DC breaker
How to reset circuit breaker
Not enough space for circuit breakers
Circuit breakers
How to install subpanel
Why you need ground wire
Match breaker and wire size
How to wire subpanel
See inside breaker box
How to wire safety switch
How to wire whole house surge protector
Figure volts amps and watts
Figure correct wire and breaker
Main breaker stress test
wire and breaker size chart
Larger image
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 correct wire and breaker size
color-code household wire Use 10 gauge wire for water heaters
Orange-colored 10 gauge wire for 30 Amp breakers
30 Amp breaker x 240Volts = 7200 watts  => 80% rule applies: 7200 Watts x 80% = 5760 Watts safe maximum
          80% rules applies to household electrical loads

Other ratings:
Yellow-colored 12 ga wire for 20 Amp breakers
White-colored 14 ga wire for 15 Amp breakers
18 ga lamp cord or 1 residential can opener, pencil sharpener, floor lamp etc
Resource:
Ratings and color code for wire
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

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 5500-6000 Watt element since they heat more water.
Most residential tanks have 4500 Watt elements
Small 4-20 gallon under-counter water heaters have elements rated for 120 volt and 1500 watt

If elements burn out frequently, reduce element wattage, or install low density element.
Resources:
How to select and replace element
Intermatic IG1240RC3 Avoid damage from surge:
Most ordinary electric water heaters are not damaged by surge.
Exception, water heaters with electronics.
However, powerful lightning strike can damage any water heater, especially modern electronic water heating equipment and other appliances.
Lightning strike will energize nearby wires, causing power surge.
Simple power outage can also cause power spikes when power is restored
Install surge protection at the breaker panel or at the location of equipment
Surge protectors are single-use. Over time, all surge protectors wear out because of constant variations in electric power coming over the grid.

Resources:
Read about Type 1 and Type 2 surge
electric water heater 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

Larger image
Thermostats and elements

Resources:
How to wire thermostats and elements
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
240 volt water heater circuit Basic 240Volt water heater circuit

Example shows water heater on dedicated 240Volt breaker.

Dedicated means the breaker does not supply power to other circuits.
Another example: Microwave oven is normally on dedicated 120Volt breaker

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

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

Use 10 gauge wire for water heaters

Orange-colored 10 gauge wire for 30 Amp breakers
30 Amp breaker x 240Volts = 7200 watts  => 80% rule applies: 7200 Watts x 80% = 5760 Watts safe maximum
          80% rules applies to household electrical loads

Other ratings:
Yellow-colored 12 ga wire for 20 Amp breakers
White-colored 14 ga wire for 15 Amp breakers
18 ga lamp cord or 1 residential can opener, pencil sharpener, floor lamp etc
Resource:
Ratings and color code for wire

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 heater 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 match
Resource:
Ratings for breaker and wire

Cost to operate water heater
How much does it cost to operate gas and electric water heaters


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
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

     3 hours a day @ $.12 per Kwh
    5500 Watt tank = $59.40 per month + tax
    4500 Watt tank = $48.60 per month
    3500 Watt tank = $37.80 per month
    1500 Watt tank = $16.20 per month
     3 hours a day @ $.14 per Kwh
    5500 Watt tank = $69.30 per month + tax
    4500 Watt tank = $56.70 per month
    3500 Watt tank = $44.10 per month
    1500 Watt tank = $18.90 per month
     3 hours a day @ $.16 per Kwh
    5500 Watt tank = $79.20 per month + tax
    4500 Watt tank = $64.80 per month
    3500 Watt tank = $50.40 per month
    1500 Watt tank = $21.60 per month
     3 hours a day @ $.20 per Kwh
    5500 Watt tank = $99.00 per month + tax
    4500 Watt tank = $81.00 per month
    3500 Watt tank = $63.00 per month
    1500 Watt tank = $27.00 per month
Chart  above in pdf

Smaller water heaters are cheaper to run if family can conserve Size chart
Read 9 ways to save with a water heater
Compare 13 electric water heater timers
Electric water heaters operate at 99% efficiency with or without water softener Read
Tempering tank
Tempering tank passively pre-heats cold water
Tempering tank passively pre-heats cold water

Incoming water to household is geothermally heated since pipes are underground

Resources:
Add a tempering tank to preheat very cold incoming water
9 ways to save with water heater
Amp rating for switches

Control water heater with Switches and Timers
The 80% safety rule applies to 15 amp switches and timers, so .80 x 15 Amp = 12 amps maximum
So switch and timer have to be wired to a contactor

Resources:
How to install 15 amp wall timer to control water heater
Control water heater with phone
Control water heater with switches
Ordinary switches can control a water heater
Water heater timers
Compare timers
Water heater Box timers
    Light bulb
15Amp switch  is rated for 1440 Watt
14 - 100Watt light bulbs = 1400Watts
How many light bulbs can 15 Amp light switch safely handle: 14 100watt light bulbs
15 Amp switch x .80 = 12 Amps
12 Amps x 120 Volts = 1440 Watts, therefore *14*  100-Watt light bulbs can be put on a single 15 Amp switch



Ordinary house switch can safely control several light bulbs + a desk lamp + a ceiling fan + etc. up to 1400 Watts
Add up Watts of each item to calculate the total load on the switch.
Same facts hold true for wall plugs which are usually rated 15 Amps and can carry 1400 Watts.

You can see that 1500 Watt microwaves and 1500 Watt space heaters should connect to heavy wires and switches

Resource:
Electricity from power pole to breaker box

Difference between 120 and 240 Volts

All single phase circuits require 2 wires to complete the circuit
    120Volt take 1 hot wire and 1 neutral wire
    240Volt takes 2 hot wires
    No neutral wire is needed to complete 240Volt circuit

240 Volts can deliver energy more efficiently than 120 Volts
240 Volt appliances are more efficient

Resource:
See basic 120 & 240 water heater circuit
AMPS   Highway Information about Amps:
Amps is a measure of the size roadway needed to supply energy to appliance
For example, a 5500 Watt element needs more roadway than a 40 Watt light bulb
Roadway is made up of circuit breaker, wire, and electrical connectors needed to handle the energy

See inside main breaker box
What is Kwh or Kilowatt-hour?
Kwh is the standard unit for billing used by electric company <=> the cost varies from 10˘ - 20˘ per Kwh
1 Kwh = 1000 watt-hours of electricity Average electric price by state
100 Watt light bulb turned on for 10 hours <=> 100 Watts x 10 hours = 1000 watt-hours or 1 Kwh
40 Watt light bulb turned on for 10 hours <=> 40 Watts x 10 hours = 400 watt-hours or .4 Kwh
To calculate cost, multiply Kwh x price <=>
For example .4 Kwh x  $.10 per Kwh = .04 or just 4˘ for each 10 hours of 60 Watt light
appliance watts
small refrigerator
Small Refrigerator: $60 per year
Big refrigerator: $300 per year
Small 786 watt Refrigerator rated 6.5 Amp/ full load      
115 Volt

1.6 Kwh per day = 600 Kwh per year.
@$.10 per Kwh = $60 per year
@$.12 per Kwh = $72 per year

Buy:
Refrigerators at Amazon
Watt meters at Amazon
Big screen TVs at Amazon

Kill a watt meter
Use Kill A Watt meter to measure 120Volt electric usage
Buy:
Kill a watt meter at Amazon
Electric testers at Amazon
Resource:
Kill a watt instruction .pdf
Big screen TV
Big Screen TV: $70-140 per year
Satellite receiver: $18 per year

TV: 300-500 Watts + 3 watt standby
$.10 per Kwh
6 hours TV per day = $70-140 per year
Look at appliance label > each appliance has Watt rating
For example double-oven has a Watt rating of 7.0 KW - that means 7000 Watts
The top oven uses 1/2 total wattage or 3500 watts or 3.5 KW >> the bottom oven uses 1/2 or 3.5 KW
For every hour the top oven is 'on' uses up to 3.5 Kwh of power
When both top and bottom ovens are 'on' for one hour, the appliance uses up to 7.0 Kwh

When buying appliance:
Look at wattage printed on label. Take photo of label with phone. Compare wattage with old appliance. It requires same energy to heat water and heat homes and cook food as before. A giant appliance does not lower bills. Energy Star rating does NOT mean new appliance will pay for itself. With Energy Star, expect $2-6 per year on same size appliance. Appliances give off warm air that works against AC in summer, but helps heater in winter. Reducing consumption and buying smaller wattage appliance is best way to lower energy bill.

Put your TV and satellite box and computer on a power strip/ turn off power strip when not in use
Resource:
Look at power strips that have timers.
Put bath fan, garage light and whole house fan on a countdown timer.

Read more about standby power loss Read1 Read2
Remember, when everybody conserves electricity, the price remains low and fewer resources are used
Additional Links
Troubleshoot Gas valves with manuals Icon gas valve

How to replace elements

Replace thermostats
How to wire tankless electric

Install point of use water heater
Titan tankless
Anode rods anode rods
Recirculation system Point of use water heater
Type 1 and type 2 surge protection surge protection
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Resource:
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Control water heater w/ Z-wave

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