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How to Wire an Electric Water Heater

December 1, 2010

I’ve been getting lots of questions lately about how to wire an electric water heater. I’m guessing this is because a lot of you are switching your water heater from gas to electric due to the extremely high natural gas prices. Some people have also asked how to wire an electric hot water heater. I’ve always gotten a kick out of this. In my opinion, you do not heat hot water; you heat cold water. Perhaps is should be called a cold water heater. At any rate, call it what you will, I think we all understand what it is.

Unfortunately, I do not have an electric water heater in my home and I haven’t installed one in a while either. So. I had to “borrow” some pictures from the internet. I found a site with all of the images that I need. The article is entitled Chicago new condo water heater inspection.

This article is going to assume that your electric water heater is 240 volts and this is a residential application.

You need to install a 10/2 NM cable (romex) with ground from your breaker box to the water heater location. At the water heater location you need to install a disconnect. I recommend a 30 amp, non-fused, air conditioning disconnect like the one on the right side of the water heater in the image below.

Electric water heater with disconnect on right

When choosing a location for your disconnect, it needs to be within sight of the water heater and have 3 feet of working clearance in front of it. It also needs to be firmly mounted to the wall no higher than 6 feet 6 inches above the floor to the top of the disconnect.

From the disconnect you need to install a 10/2 NM cable with ground to the water heater.

If any of the 10/2 NM cable is on the surface below 8 feet, then it needs to be protected from physical damage. What we usually do is install 1/2 inch flexible metal conduit (flex) as a sleeve over the cable. This only needs to be done to the cable where it is below 8 feet. You will need flex connectors to terminate the flex to the disconnect, water heater and breaker box if it is needed in these places. When installing the flex from the disconnect to the water heater, be sure that the flex does not touch the copper pipes; like in the image below. The dissimilar metals will create a reaction and eventually burn through the copper pipe causing a water leak.

Electric water heater conduit

The NM cable or flex needs to be supported within 12 inches of the breaker box, disconnect or water heater and every 4 feet thereafter. Sometimes it is difficult to support the cable or flex within 12 inches of the water heater. Just get it as close as you can here. To support the NM cable, I recommend plastic romex staples. To support the flex, I recommend 1/2 inch, 1-hole straps.

If you need flex as a sleeve at your breaker box or disconnect to a point of 8 feet or higher, then place a connector over the end of the flex here as well. This will prevent the flex from scraping the NM cable and creating a future problem; like a short or fire.

At the water heater, connect the ground wire to the ground screw on the water heater. Connect the black wire in the 10/2 to the black wire on the water heater. Wrap some black tape around the white wire to re-identify it as a hot wire and connect it to the red wire on the water heater.

Electric water heater wiring connections

You also need to ensure that all of the connections are made within the water heater’s wiring compartment.

Electric water heater wiring

At the air conditioning disconnect connect the 10/2 which goes to the water heater to the load side. The ground wire will connect to the ground bar, the black wire will connect to one load terminal and the re-identified white wire will connect to the other load terminal.

The 10/2 which comes from the breaker box will connect to the line side of the air conditioning disconnect. Again, the ground wire will connect to the ground bar, the black wire will connect to one line terminal and the re-identified white wire will connect to the other line terminal.

At your breaker box, you need a 2-pole 30 amp circuit breaker to protect this circuit. The ground wire will connect to the ground bar, the black wire will connect to one side of the breaker and the re-identified white wire will connect to the other side of the breaker.

Do not turn on your water heater until it is full of water, If you turn it on before it is full, then you will burn out the heating elements.

The image below shows the components of an electric water heater.

Electric water heater components

Which type of water heater do you have in your home?

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