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How to Wire 120V Simultaneous Water Heater Thermostat?

Single Phase 120V Continues Dual Element Water Heater Thermostat Wiring

In our previous water heater wiring and installation post, we discussed about the non-continuous (non-simultaneous) water heater wiring connection. In today post, we will be showing the the continuous or simultaneous 9kW dual element water heater thermostat wiring.

Simultaneous or Continuous Operation of Water Heater

Continuous or simultaneous heating elements are not dependent on each other i.e. both elements are “ON” at the same time. In other words, both of the upper heating element and lower heating element will turn “ON” at once and this operation is needed at more conservation and hot water consumption points like restaurants. This is the automatic process due to thermostat wiring connection where the lower thermostat / element and upper thermostat / element is fed up by the separate power supply.

This way, both elements operate at the same time. Keep in mind that the same thermostats can be wired for non-simultaneous or non-continuous operation where both elements are not operational at the same time in case of low hot water consumption.

Simultaneous 120V AC Single Phase Dual Element Water Heater Thermostat Wiring

In simultaneous water heater thermostat wiring, the phase or Hot “L” line is connected to the left terminal L1 of upper thermostat and second wire is outgoing from T2 to the heating element. The Neutral “N” is connected to the right terminal of thermostat L3 and outgoing from L4 to the heating element.

The lower thermostat and heating element is powered up separately through a 50A double pole (you can use single pole) circuit breaker same as the above thermostat heating element wiring. In short, a separate 120V supply wires are connected to the lower thermostat i.e. Line to thermostat terminal L1 and to heating element via T2 and Neutral wire to thermostat terminal L3 and to heating element via L4.

Since there are two heating elements each of rated about 4500 watts, the total wattage rating of this heater is 9000 watts. We will be using the breaker and switch sizes for full rating i.e. 9kW as both elements are operational at once.

Now in case if 120V AC, a 50A circuit breaker is suitable for both wires having 10 gauge size according to the following calculations.

  • Total wattage = 9000 Watts.
  • Load Current = 9000 / 120V = 75A.

We have used separate curcuit brekaer for each heating element, the breaker rating size for single element would be:

  • Wattage: = 9000W / 2 = 4500 Watts.
  • Load Current = 4.5kW / 120V = 37.5A

Breaker Size should be 1.25 (125%) of the load current:

37.5A x 1.25 = 46.875A

Another way, the safe limit of circuit breaker is 80% (0.8), this way 50A x 0.8 = 40 Amp which is safe for load current.

This way, a 50A circuit breaker for over current protection is suitable in case of 9kW dual element continuous water heater circuit for 120V AC.

120V AC 9kW Electric Water Heater Dual Elements Simultaneous Wiring

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


  • 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.

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