Backstory: I'm considering replacing my current tank water heater with a tankless whole-house unit. The units I see generally require a 120-amp breaker circuit.

Currently, my panel has two ganged switches marked "50" supplying the water heater. My understanding was that the two breakers, each calibrated for 50 amps, are wired in parallel to produce a 100-amp circuit to the water heater. So, I probably need to have an electrician swap out that breaker for a slightly beefier one (and make sure the installed wiring can handle the extra power) as part of the upgrade.

But, when looking thrrough the electrical section, I saw ganged circuit breakers numbered with "60" on each switch labelled as a "60-amp breaker". This would imply to a relative layman like myself that the entire package will trip at 60 amps of draw. If that's true, I may need a second input panel just for the tankless, 'cause the full panel for my house is only rated 200A, which seems to be the average for a single-family home when looking at breaker boxes in that same aisle.

Can anyone unconfuse me here?