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Wiring a new hot water heater

Posted by dunkinidaho (My Page) on
Mon, Feb 4, 13 at 13:49

I need to replace an electric water heater and need some questions answered. It looks like my current heater is a single element one. I can't tell if it's 120 or 220 volt. The circuit breaker is 40amp but the wiring going to it looks like regular house wiring, ie: white and 14-2 maybe 12-2. The heater I'm going to use will be a dual element but both elements are never on at the same time. I thought when using 40amps and 220 volt you need 10ga or larger wiring. Can a water heater be 120volt and need 40amps?

please advise and thanks


Follow-Up Postings:

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RE: Wiring a new hot water heater

How many poles does hte CB have?

2-pole breakers are used for 240 V and are twice as wide as 120 V breakers.

Some are made up of separate 120 V breakers and have two handles to create a common trip, others have a single handle but two poles internally.

All are wider to pick up both legs in the panel.

A 30 A water heater needs #10 wire.
A 40 A would use #8.

A straight 240 V load only needs three wires, hot, hot, ground.
Two conductor cables with ground is often used with the white wire re-marked as a hot.

A turn of black tape is all that may be there, or even just coloring the white wire black with a permanent marker at each end.


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RE: Wiring a new hot water heater

What is the data nameplate wattage of the new heater? What is on the data plate of the old heater? Any chance the #8 is aluminum? How did you determine that it is #8?


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RE: Wiring a new hot water heater

I have a dual element heater, each is about 1500w, perhaps a bit more. It uses 12ga wiring, I think (since I'm not looking at it right now).

As suggested, look at the name plate for the wattage and current specifications of both the old and the new.


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RE: Wiring a new hot water heater

I doubt you'd find a 40 amp, 120v breaker in residential wiring (actually, I'm not sure if they even make such a thing).

Double pole breakers for 240v circuits typically look like this:

It's very important to follow the manufacturers requirements for circuit size. Typically they will be listed in the installation manual. Also, do not guess as to the existing wire size. It should be printed on the jacket of the cable or the insulation of individual wires.


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RE: Wiring a new hot water heater

Eh? SquareD makes 1 pole 40A breakers in both the QO and HOM lines.


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RE: Wiring a new hot water heater

Eh? That's what I get for not looking. ;-)

So what's a single pole 40 amp circuit used for in residential construction?


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RE: Wiring a new hot water heater

Not sure about what 40a single poles are for, but last week I replaced a 1 pole 50a QP Siemens breaker that was feeding a receptacle circuit in a warehouse office. It was ran in #10awg THHN.


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RE: Wiring a new hot water heater

Thanks for all the help. I have made a few errors in my first posting. The current heater is 3800watt 240V and the replacement will be 3500watt 240V. The breaker is not 40amp. It is 2X20amp breakers. I still don't know if the current heater is one or two elements. I think I'm OK with the #12 600V wire in this installation.


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RE: Wiring a new hot water heater

20A should be sufficient for 3800W @ 240. Yes #12 is fine.


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