Electrical – Piggyback power to dual water heaters

I really question the wisdom of switching just the lower heating element. Doing so is going to overwork the upper element. I assume you are trying to conserve electricity, but this tactic is not going to be a great benefit to you. The upper element is used as the "quick recovery" unit that gives the cooling water from the lower part of the tank a boost as cold water is added as the hot water is used. If the water cools too much in the tank and you call for hot water, the upper element will not be able to keep up until the temp of the majority of the water warms up. I would suggest an inline switch or timer controlling both elements to help conserve energy and insulate the tank as much as you can. If you are looking for short, small quantities of hot water , you may consider a 1 or 2 gal point of use electric tank, in line at the specific appliance. There are several models to choose from at the box stores for under $300.00 I might add, if you do try this switch idea, you will need to switch both 120VAC lines with a DPST,(double pole, single throw) 30amp switch wired with 10AWG copper. When you open the thermostat cover, you will find the lines are connected to screw terminals on the thermostat switches.

For a typical 240V tank style water heater you will need a 30A/240V circuit. No neutral is required.

You CAN use the white wire as a hot going to the breaker for a straight 240V circuit such as this. You just need to mark it as a hot conductor with a permanent marker or such. You can use any typical hot color such as black, red or blue. You DO NOT need to use a red wire.

Now....you ask "Can I install the red line to a second hot breaker". This implies there is a red wire in the cable. If this is the case then yes, you must use the red and you can just cap off the white in the water heater since it is not used.

You CANNOT simply use a "second hot breaker". You MUST use a two-pole breaker for a 240V circuit as previously mentioned.