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Replacing tankless water heater electric

I'm replacing my electric hot water tankless unit. Can I use my existing 2 - 60 amp circuits for a seisco 28 when the new ecosmart 18 says to use 2 - 40 amp circuits

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Posted 2018-05-20T14:53:31+0000  by Bcinny Bcinny
Hi Bcinny,

Each circuit breaker has a specific rated amperage, or amount of current. When that amperage is exceeded, the circuit breaker shuts down the flow of current to prevent damage to the wiring.

If your 40 amp rated water heater is on a 60 amp circuit, and a failure occurs increasing the amperage
above the rated 40 amps the water heater could be destroyed and a fire could result because the 60 amp breaker is too high of a rating to shut off the power before the damage occurs.

Always use the power size wire and circuit breakers for the appliance you are installing.


Posted 2018-05-21T23:34:35+0000  by Mike_HD_OC
Hello Bcinny.

Circuit breakers are there to protect the internal circuit wiring from overheating and causing a house fire.  Since your existing wiring is, I assume, compatible with a 60 amp load, there should be no problem wiring in the 40 amp load from your new heater.  The load drawn will be less than the circuit can easily handle.  We do this all the time.  Most of the things we plug into a wall socket take far less than the minimum 15 amps that a common 120 volt house circuit can deliver.

The advent of ground fault and arc fault circuit interrupters has extended the role of circuit breakers to also provide some protection for both people and those products that connect to that circuit.

A GFCI senses the difference between line in power and the neutral tap power, tripping when any is discovered.  With 120 volt power, this works by assuming that any difference means power is being sent to ground outside the circuit.  This protects both people and things at a power level far below the rated breaker capacity.  Unfortunately, with 240 volt power, GFCI protection does not work, as a perfectly balanced 240 volt load will have no return power through the neutral.

Arc fault breakers do exist for both 120 and 240 volt circuits.  I have seen both 15 and 20 amp models by various brands.  They work by sensing the power sine wave for high frequency "noise" which can indicate arcing, and can react at current loads far less than the breaker rating. 

To date I have not seen them for dedicated appliances like water heaters, dryers and ranges though.  These require their own 240 volt circuit from 30 amps and higher.  Conventional double pole breakers are used.  Normally the circuit capacity is matched to the load's maximum draw.

What I would do is to simply take the 60 amp double breaker out and switch it for a 40 amp double breaker.  The circuit wiring will still be protected.  You can go down in amperage rating on a breaker, but not up without making sure the wiring is sufficient.  A circuit does not "push" current into a load.  The load "takes" what amperage it needs.  Matching the breaker size to the appliance load rating will give the best results.  The fact that the circuit wiring is now "oversize" is actually a good thing, as the voltage drop along the circuit is reduced.  All is well.


Posted 2018-05-22T15:01:43+0000  by Chris_HD_CHI
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