I installed my water heater 10 gauge wiring to the breaker box 30 am breaker. The breaker only has a white common, black hot and ground.The second breaker box has all 30 am breakers with only red hot lines. The house is 70 years old. Can I install the red line from the second breaker box and the black line and ground from the 1st breaker box to the water heater? If not then will I have to have another sub panel installed because there isn't any more room in the old breaker box.
On the main panel the is a 100 amp double breaker and two 60 amp double breakers. The previous owner ran line off one of the 60 am double breakers for the washer and dryer. I guess I will have to run 10 gauge line to a new breaker box off of the main panel and then in to the hot water heater. Thank you for your advice.
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.
A 240V hot water heater, like any 240V load, needs a 2-pole breaker.
I mean a 2-pole breaker. (Not a duplex or tandem breaker, which are 2 single breakers double-stuffed into a single space, these are useless because they can only access one pole.)
They do make hybrid 2-pole breakers which are 2-pole and also double-stuffed. They take 2 full spaces and have 4 handles.
Edit: What makes it a 2-pole breaker is that it's 2 spaces wide, and it has a "handle tie" so that both "sides" of the breaker trip together. This obviously requires it be connected to the same panel. In your comments you seem to be describing two separate panels each serving 120V only. There's no legal way to serve a 240V load from those panels, because the handles must be tied.
If the old water heater took 60 amp breakers, you could install a new water heater that takes the same size breakers. If you downsize you need to change breaker which sounds like it would be a problem.
Also, note what Speedy has to say above. Most water heaters do not need a neutral, so if you have a white wire in the cable, it is "free" to be re-assigned to be a hot. Do this by marking every accessible area (where it's not in sheath or conduit) with a wrap of black electrical tape. Make sure the wires are big enough for the load (10 AWG for a 30 amp circuit). If the wires are bigger than necessary, that's OK.