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Electrical Question from Martin about 240 Volt Wall Heater Wiring

Will 12-2 wire and a 20 amp breaker handle a 4000 watt 240 volt wall heater?

Electrical Question from Martin about 240 Volt Wall Heater Wiring

Received from Martin a Homeowner in Tacoma, WA

Question: Will 12-2 wire and a 20 amp breaker handle a 4000 watt 240 volt wall heater?

Dave’s Reply:
Thanks for your electrical question.
Hi Martin – A 4000 watt 240 volt wall heater will require about 16.6 amps. Although a 12/2 wire is rated at 20 amps it would be best to install a 10/2 wire for a 4000 watt 240 volt heating unit. This would ensure that the circuit is safely loaded at 80 % of the capacity of the wire.
When using a 2-wire Romex cable with a ground understand that a 2-pole circuit breaker will be required and the white wire will need to be identified with red or black electrical tape as a power conductor at both ends of the circuit.

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4 Responses to “Electrical Question from Martin about 240 Volt Wall Heater Wiring”
  1. Del says:

    Water Heater Wire
    Will 10-2 Romex work with a 240 volt water heater?

    • Dave Rongey says:

      Wire Size for a Water Heater
      Hi Del,
      240 volt Water heaters come in several different sizes, therefore it is essential to identify the specific water heater you have before determining the wire size.
      Please Note: The wire size is based on the amperage or watts, as well as the voltage.
      I hope this helps,

  2. TJ says:

    I have a Fahrenheat 240 volt wall heater,
    10/3 romex (Black, red, white and bare)
    The heater has 2 wires and the ground nut (green) on the back box.
    Black to black, bare ground to the ground nut on the back box,
    instructions (and code) says white gets changed to black (electrical tape),
    doesn’t say connected to what?
    what happens with the red as i thought that was the other hot leg?

    • Dave Rongey says:

      Hi TJ
      Here is the confusion:
      This type of 240 volt circuit for this wall heater does not require a 3-wire (with ground) cable, so 99% of the time there would be a 2-wire (with ground) cable installed, which would not have a red wire. So in the case of a 2-wire cable the white is colored red.

      In a case like yours where there is a 3-wire cable, I would normally cap off the white wire as a spare, and connect black/black, red/red ground/ground as instructed.
      HOWEVER – this may only be done if it can be verified that the black and the red are in fact terminated at the 2-pole circuit breaker for the 240 volts, and it normally is, you just want to be sure.
      If the white wire of the 3-wire cable is terminated to the neutral buss then that may remain as it is, just make sure it is capped off at the heater end.
      Also verify that the ground wire is terminated to the ground buss.
      And that’s it – you will be set.
      Enjoy the heat and Be Careful!

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