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Question:

How do you properly connect the electrical wiring in a hot water heater?

Coming from my electrical box I have a white wire and a dark grey wire and coming from my water heater I have a red wire and a black wire. What colors go with what? Thank you in advance!

Answer:

Put the black wire to the grey wire, and the other two wire together. Before you do this, verify that the old wires from the fuse box or breaker panel are attached to a 240 volt source. The description you give indicate a possible 120 volt hook up. If it is 120 volt, your heater will not work properly and be very expensive electrically to operate,
White is neutral Grey is power for 120V, but seems to me that your water heater is 220V if you have a red and black wires. check this before you connect anything
You will need to wire it with 220v. The white, if properly wired, is, as previously noted, is neutral, and is not the correct wire to use. Start with another wire from the opposite leg in your panel that the dark grey is hooked up to. If you look at your panel, you will find two large black wires coming into the panel from the meter, hooking up to each buss. You will have to look at your diagram on the back of the panel cover to see if they alternate, or continue in one row. Use a wire from each leg to create a total of 220v going to the hot water tank and hook one of these to the red, and the other to the blackA volt meter would be the best tool to verify. Also, If the grey wire is 14 gage, it is too small, and should be increased to either #10, or #12 wire. #10 is good for 30 amps, and #12 for 20.Not sure of your amperagemaybe someone else can tell.
this is typical i have installed a lot of water heaters. under normal circumstances it does not matter what color goes to what wire on 220 the only wire that matters is your ground. make sure the bare wire goes to the green ground lug on top of your water heater. the 2 colored wires from the breaker panel can connect to either of the colored wires from your water heater.
If you replace it. . . Replace all of it. i.e. the tank and a new Ball shut off valve at the water inlet. Do not use a Gate valve they corrode away in a short period of time. A multi turn vale will work fine also but I prefer a Ball valve. If it’s not on a stand to raise it up off the floor check with the place where you purchase the heater and see if it is code where you live. Most times the safety pop-off valve comes with the heater but if it doesn’t purchase one of those also. If your pop-off valve does not have a drain pipe from the safety pop-off valve to the drain you can purchase PVC pipe to make that connection and most states require a drain pan under the tank. Check with the place where you purchase your tank for all the local codes such as the drain pan, stand to raise the tank off the ground and piping from the pop-off valve to the drain. Buy some copper tubing the size that you need and a couple of extra collars / connectors, use these to practice sweating the joints. It’s not hard but does require a little practice. Draw your installation out on paper to get a feel of what you’re going to do. You’ll need to turn the water off at the meter before you start. Ask lots of questions, watch some videos on line. it isn’t difficult but your first one can be a little taxing. Purchase the tank by quality not price

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