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How to Toggle Electric Water Heater Between 120V and 240V?

How to Switch Water Heater Circuit Between 240V and 120V using Switches?

In electric water heater wiring series, we will be showing how to change the water heater input voltage from 120V to 240V and vise versa using 3-way switch, DPDT switches and T106 timer.

This method is used to reduce the electricity bill. In other words, you can run the electric water heater on 240V AC when prices are lower or same price rate for peak hours and normal operation or when faster recovery of hot water is needed.

You can switch back and run the electric water heater on 120V during the peak hours to reduce the electric bill or less hot water is needed.

Keep in mind that the same power rating water heater can be operate on both 120V or 240V AC

As we know that the same water heater rated for 240V can be operated on 120V. Keep in mind that you can’t operate a 120V water heater on 240V AC. If so, the heater element may damage or burn out which may leads to fire. See the following guidelines.

  • ❌ You can’t connect a 120V rated water heater to 240V or 277V
  • ❌ You can’t connect a 240V rated water heater to 277V or 480V
  • ✔️ You can connect a 240V water heater to 208 or 120V AC supply. (Slow heating)

For the last one, it heating operation will slower 1/4 and suitable option for off peak operation.

When we operate a 240V AC electric water heater on 120V AC supply voltage, it will reduce the heat dissipation amount in water heater tank. In other words, a 4500 Watts water heater will heat slower at 120V AC instead of 240V AC because the overall wattage drops when voltage reduced. This is the same case when we supply 120V to 240V rated light bulb, it will glow dimmer as compared to the rated supply of 240V.

Conversion formula for 240V AC to 120V AC

In case of 4500 Watts water heater elements,

( 120V / 240V )2

0.5 x 0.5 = 0.25

This way, a 4500 Watts water heater will consume

4500 Watts when operated on 240V and

4500 W x 0.25 = 1125 W

1125 Watts when operated on 120V.

Conversion formula for 240V AC to 120V AC

Let see below the different methods showing conversion from 120V and 240V AC circuits for water heater.

Toggle water heater between 120V and 240V using 3-Way Switch

To switch the circuit between 120V and 240V for water heater by using a 30 amperes 3-way switch, just connect a line (L1) from main distribution board via 30A circuit breaker to the water heater. Connect the second line (L2) from CB and neutral from DB to the upper terminal and lower terminal of three way switch respectively. The common terminal on left should be connected to the water heater. Don’t forget to connect the 3-way switch and water heater to the ground/earth wire.

Working of the Circuit:

  • Up Position of 3-Way Switch = L1 and L2 are connected to the Water heater = 240V AC
  • Down Position of 3-Way Switch = L1 and Neutral are connected to the water heater = 120V AC.

How to Toggle water heater between 120V and 240V using 3-Way Switch

Toggle water heater between 120V and 240V using Double Pole Double Throw Switch

The same operation can be done via two pole DPDT (double pole double through switch) as above. To do that, just connect the L1 directly to the water heater through 30A CB. L2 from CB and Neutral from main distribution board should be connected to the A2 and B2 terminal on the right side of 2 pole DPDT switch. The L2 terminal of DPDT switch should be connected directly to the water heater. Do the proper grounding to the double pole double throw switch and water heater as shown below.

Working of the Circuit:

  • Up Position of DPDT Switch = L1 and Neutral are connected (through B2 and L2 terminals) to the water heater = 120V AC
  • Down Position of DPDT Switch = L1 and L2 are connected (through A2 and L2 terminals) to the water heater = 240V AC.

How to Toggle water heater between 120V and 240V using double pole double throw switch

Toggle water heater between 120V and 240V using DPDT Switch with Center Off

The above two menthoned methods are only swith the circuit between 120V and 240V. To add an addinatl operation i.e. 120V, Switch Off and 240V, we may use double pole DPDT switch. 

To do that, just follow the wiring configuration as given in the below wiring diagram. Use a 30A circuit breaker for 240V and 20A circuit breaker for 120V.

Working of the Circuit:

  • Up Position of DPDT Switch = L and Neutral are connected (through B1, L1 and B2, L2 terminals) to the Water heater = 120V AC
  • Center Position of DPDT Switch = Switch OFF – No supply to the water heater.
  • Down Position of DPDT Switch = L1 and L2 are connected (through A1, , A2 and L2 terminals) to the water heater = 240V AC.

How to Toggle water heater between 120V and 240V using DPDT Switch

Toggle water heater between 120V and 240V using T106 Timer

To do that, just move clock motor wire from COM to NO (Normally Open) terminal. This configuration will work for both 120V and 240V whereas 120V will consume 25% of the total wattage rating of 4500 Watts which is 1125 Watts in case of 120V.How to Toggle water heater between 120V and 240V using T106 Timer

Additional methods for 120V to 240V water heater and vise versa are using automatic or manual transfer switch (Change over switch), using timer (T106, T1906, T1975 ect) or SPDT timer with dry NO NC relay setting the DIP switch for 120V. 

Note: Perform a continuity test for switch terminal before wiring and installation. Use 10 gauge wire for 240V and 12 Gauge for 120V in the above wiring tutorials. Follow the user manual guide or contact a licensed electrician for proper installation. All the above wiring diagrams are for non-simultaneous (non-contentious) heating elements and thermostat operation.

Note: Red color illustrates the Line or Phase wire and Black color shows the Neutral Wire in the above figures. You may follow your regional wiring color codes i.e. IEC or NEC.

Wiring Color Codes – 120V and 240V

We have used Red for Live or PhaseBlack for Neutral and Green for Earth Wire. You may use the specific area codes i.e. IEC – International Electrotechnical Commission (UKEU etc) or NEC (National Electrical Code [US & Canada] where;

NEC:

  • Single Phase 120V AC:

Black = Phase or LineWhite = Neutral and Green/Yellow Earth Conductor

  • Single Phase 240 – 208 AC:

Black = Phase 1 or Line 1Red Line 2, Blue = Line 3, White / Gray = Neutral and Green/Yellow Earth Conductor

IEC:

  • Single Phase 230V AC:

Brown Phase or LineBlue Neutral and Green Earth Conductor

Warning:

  • Disconnect the power before replacing, repairing, troubleshooting, maintenance and installation electrical appliances and equipment.
  • Use the proper size of cable and wire by following this simple calculation method (How to determine the suitable size of cable for Electrical Wiring Installation)
  • Failure to do so can result in electrical shock, serious injury, fire or even death.
  • Please follow the manual instruction, local area codes or contact a licensed electrician for proper installation.
  • The author will not be liable for any losses, injuries, or damages from the display or use of this information or if you try any circuit in wrong format. So please! Be careful because it’s all about electricity and electricity is too dangerous.

Related Water Heater Wiring Diagrams

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4 Comments

  1. MasterPlumber says:

    12ga wire isn’t allowed on a 30amp circuit outside the appliance. You should used 10ga to the water heater. There’s no real saving by wiring it this way, the water heater will only take longer to heat.

  2. Mahendra Subandh says:

    I am BE ( Electrical) Engiineer from COEP Pune passed in the year 1971,
    May I request you to kindly send me the updates from time to time
    Regards,
    M B Subandh
    M : 9824210661

  3. Les Murphy says:

    That makes no sense to do this. 1/4 power will use the same energy to heat tank. It will just take 4 times longer. May as well just shut it off completely…

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