Using a Reliance 10 Circuits, 125/250 Volts, 30 Amps, 7,500 Watts, Model# 30310A unit to my panel. I was interested in running my water heater which in my panel has a two 30 amp breakers. Can I run this breaker to the dual pole 20 amp breaker on the transfer switch? Only concern is in my panel it is labeled as a 30 amp but there are no 30 amp breakers on the transfer switch so want to make sure that is not an issue.
With having 4500W elements, the 2-pole 20A breaker will not handle the load. Based on 4500W at 240V the heater would be drawing close to 19 amps and will likely trip. You could swap the elements for 3800W which would draw closer to 16 amps. Your water will just take a little longer to heat up to temperature.
I'm no fan of those transfer switch panels. They are very expensive, very weak, not particularly well-built or safe, and the wiring is a spaghetti nightmare. It's far simpler, safer, and more versatile to use an actual $75 subpanel with a $30 generator interlock. Further, the transfer switch breaks mandatory AFCI/GFCI requirements on some circuits. The subpanel handles that correctly.
You are running into the "very weak" limitation. Read the water heater's manual to see what breaker it requires. If the manual calls out a 30A breaker, then you cannot connect it through a 20A breaker on the panel. I most definitely expect it will require 30A; a 4500W water heater requires a 30A breaker.
If you're committed to this water heater, and you have spaces in your main panel, I'd start by shopping for a generator interlock kit for your existing main panel.
If that is not feasible, then go subpanel. If you have AFCI or GFCI breakers on circuits you want to serve with the generator, then get a subpanel that is compatible with the breakers you already have, and for which you can get an interlock kit. Those range from $20 to $120 depending on panel and arrangement. Otherwise go Siemens/Murray main-lug or no-lug, as they have the best interlock kit ($23 and it ties down the breakers and works with the panel cover off).
As far as panel size, the interlock needs 4 spaces. To support 6 single + 2 2-pole circuits, you need 10 more spaces, but hey, spaces are cheap. We'd go with a 30-space or even a 40-space.
For the two interlocked breakers, I'd use a 60A (to generator) and 70A (to utility) just to keep costs down. I'd run #3 copper feeder (which can support 100A) from the sub to the main panel (70A breaker there) and #6 copper to wherever you put your generator inlet.
Now you just reroute any circuits you want on generator so it's served out of the subpanel instead of the main panel. No spaghetti, just normal wiring. AFCI/GFCI are supported.
This gives you a clean, tight and proper installation that will pass inspection with flying colors, and won't be a troubleshooting nuisance in the future. At half the price, all-in.
It certainly won't have any trouble supporting the water heater, or even two of them.
Can you use a double pole 30 amp breaker for a water heater, sure. I would if the breaker is not in use. If the breaker is in use you will run into tripping problems when the other loads are used while the heater is working. Bathroom receptacles are supposed to be on a 20 amp circuit. code for many years will allow 2 bathrooms to have the same circuit but not 3. I don’t follow the last question about the switch, read the site guidelines , there should only be 1 question. Since it is unclear edit that out and ask a new question.
It sure sounds to me like your transfer switch is limited to 20 A. If so, you will not be able to power an electric tank water heater through the transfer switch because the heating element in the tank will draw 30 A at 240 V.
Is this transfer switch for changing from the grid to a generator? What is the size of the generator? The generator you have may be able to supply only a small portion of the loads and an electric water heater may be one load you would not be able to supply by the generator.