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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
    Commented Jun 22, 2011 at 18:36
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    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
    Commented Jun 23, 2011 at 3:14
  • @Tester101: tankless heaters have very high peak loads, even though their total Wh usage is lower than tank models.
    – Jay Bazuzi
    Commented Jun 23, 2011 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
    Commented Jun 23, 2011 at 5:00

1 Answer 1

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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
    Commented Jun 22, 2011 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
    Commented Jun 23, 2011 at 4:48
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    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
    Commented Jun 23, 2011 at 11:04
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    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
    Commented Jul 18, 2014 at 16:35
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    Big tankless heaters are built with several heating elements each of which gets its own breaker. The size of the elements is chosen by the manufacturer which decides the size of the breakers you'll need. So if you buy four 30s you may get the heater and discover you need three 40s.
    – Harper - Reinstate Monica
    Commented Jan 14, 2016 at 10:06

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