Water – What metal is most suitable for a ladder submerged in an underground water tank

I suspect you have a set-up like this?

You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?

The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)

In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.

So for anyone finding this page, none of the answers are incorrect or be qualified electrician. please for any electric services needed find and ask a qualified electrician.

Basically the breaker size protects the wire, So the breaker should not be higher amperage than the wire rating, however can over size the wire to meet voltage drop requirements. load calculations are for what minimum size panel/service needed but not the actual service rating if the breaker from the service side is a 100amp breaker feeding this then wire rated for 100 amp, etc. Most ampacity charts are up to 100 feet and go up one size of additional 100 feet. So, if using a 100amp breaker at the service side, then need to minimally size the wire accordingly and there are different rating for non burial, burial and in conduit Aluminum sizes will be larger than copper. For that service would use URD Aluminum 4 conductor cable (need 2 hot, neutral and should have separate ground to shop, grounds isolated from neutrals in shop panel. for 300 feet for 100 amp rated service I would use Aluminum direct burial 1/0-1/0-1/0-1/0, the forth can be as low as #4 for the ground (but also in conduit, even if in conduit must still be rated underground wire and required by code also) Also note the size wire the breaker can handle, cannot cut strands to make fit, no no no, so check the breaker specs first. So if not really needing full 100 amp load, can go down smaller wires sizes, but use smaller breaker to meet the maximum amperage rating for that wire. so for example could use a popular 2-2-2-4 URD Aluminum direct burial wire, but would only use a 60 amp breaker for that distance, 80 amp if needed but could experience some voltage drop at high load, bad for running motors any length of time. Copper would allow smaller wire size, bigger wire still fit into breaker but is way more expensive. best to google look at a few wire size charts better contact an actual qualified electrician.