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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?

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Check the documentation for the heater you are looking at. This one says you need 4 30A double pole breakers. This one says 3 50A double pole breakers. –  Tester101 Jun 22 '11 at 18:36
How much water are you heating? 120A seems like a lot for a residential water heater (though I could be wrong, I have not done much research on tankless heaters). –  Tester101 Jun 23 '11 at 3:14
@Tester101: tankless heaters have very high peak loads, even though their total Wh usage is lower than tank models. –  Jay Bazuzi Jun 23 '11 at 4:46
your question is a little mixed up. Part of it is "What is a ganged circuit-breaker?" and part of it is "How do I wire a tankless whole-house water heater?" –  Jay Bazuzi Jun 23 '11 at 5:00

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A double-pole breaker with a "60" on each half will trip at 60 amps. The reason it's two connected breakers is to give the circuit 240V potential, by using two opposing AC phases. These do not "add up" to more current -- you're using them to double the voltage.

So if the heater requires 240V power at 120amps, you will need double-pole breakers that can provide a total of 120amps, e.g. 4 30A double-pole breakers as Tester101 suggested.

An electrician will be able to tell you if your service panel can accommodate this extra load -- it's a lot, but your 200A service might be adequate, depending on the normal load elsewhere in your home.

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That makes all the sense in the world. So if I wanted tankless, we're probably talking about an upgrade to a 300A main breaker, or a second parallel panel (if that's legal in a SFD). I don't think I have space in the existing panel for 4 more 30A DP breakers. –  KeithS Jun 22 '11 at 21:34
@KeithS my neighbor just built a super-energy-efficient house but still needed 400A service because there's a tankless heater for the radiant floor, and another for the domestic hot water, and both could be on at the same time. –  Jay Bazuzi Jun 23 '11 at 4:48
Running multiple breaker pairs instead of a properly rated double pole breaker makes me nervous. Good advise from Shimon, get an electrician to check this one out. You are really maxing out that 200amp panel. –  shirlock homes Jun 23 '11 at 11:04
Adding to what @shirlockhomes mentioned: circuit breakers are not like resistors that you can put in parallel to achieve a higher current carrying capacity. Instead, the one with the poorest connections will hog almost all the current, and then trip, and pass the current along to the next-poorest-connected one, where the process will repeat. Unless you're lucky and they happen to share nicely, you'll end up with just an expensive under-rated breaker. –  Bernd Jendrissek Jul 15 '12 at 22:53
A 240V device (in the US) may look like two circuits, but they are not independent. Any time power is disconnected from one it must also be disconnected from the other. If power is disconnected from only one side, a 240V device may without warning bridge power from the powered side to the other. Someone working on the "unpowered" side at the time could be electrocuted. The only safe ways to protect a 240V device are to use a purpose-designed double-pole breaker, or else two adjacent breakers which are designed so they can be locked together to work as a single double-pole breaker. –  supercat Jul 18 at 16:35

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