Tankless Water Heater

[jar546]

If an electric, tankless water heater has a tag showing it is listed by an approved agency with the following information on the label: 240v, 10kw, 42A, what size wire and circuit breaker would be required by code in the absence of manufacturer's installation instructions?

Please don't turn this into a discussion about manufacturer's instructions. This is a legit code question. You have all the information you need.

 

[mark handler]

Distance from Panel?

 

[jar546]

Distance from Panel?

20'

 

[chris kennedy]

I would use a circuit breaker that is
2” x 2” x 2 1/2”

 

[BLangley]

Spent lunch reading and looking at a bunch of tables...

6 AWG and a 55-amp breaker

We don't have a conductor to carry 45-amps, so we step it up.

I got my head spun around a few times on the breaker sizing, so I've got two reasons (or one 2-part reason) to use a 55 vs a 45:

1) no manufacturer instructions saying a 100% uptime 45-amp is okay
2) 42*125 = 52, so to get a breaker that isn't loaded at more than 80% if the heater runs over 3-hours continuously, we go 55

Some of the reading I was doing mentioned over-sizing the conductor the same way, but I don't know if that is required here...don't have the actual code, just snippets and parts in articles

 

[mark handler]

Wire Size
6 Copper 4 Aluminum
https://www.electricaltechnology.org/2014/04/electrical-wire-cable-size-calculator.html

 

[cda]

Non electric-an answer

8 on the wire

And 40 on the breaker

 

[jar546]

I would use a circuit breaker that is
2” x 2” x 2 1/2”

You are an ass. Lol

 

[Mr. Inspector]

I would use a circuit breaker that is
2” x 2” x 2 1/2”

How about a black one?

 

[jar546]

How about a black one?

Or white breakers.

 

[jar546]

I would like if those interested would take the time to read NEC 422.11(E) and see how this applies to this situation.

 

[BLangley]

Applying 422(E)(3) suggests a 65 amp is okay (42*1.5=62, step up to a 65), but doesn't that get into potentially enough amperage to overheat the conductor without tripping the breaker?

Still reads to me like the 55 is better- doesn't exceed the 150% max.

 

[jar546]

Applying 422(E)(3) suggests a 65 amp is okay (42*1.5=62, step up to a 65), but doesn't that get into potentially enough amperage to overheat the conductor without tripping the breaker?

Still reads to me like the 55 is better- doesn't exceed the 150% max.

Yes, I too am having issues with applying 422.10(E)(3) but it is legit

 

[jar546]

resurrect

 

[wwhitney]

Yes, I too am having issues with applying 422.10(E)(3) but it is legit

Not seeing any issues: if you say that a tankless water heater is an appliance, then 422.11(E)(3) just says the maximum OCPD on the 240V/10kW/42A heater is 42*150% = 63A, and permits rounding up to the next highest standard size OCPD, or 70A.

So the OCPD for the tankless water heater could be 45A, 50A, 60A, or 70A. The respective minimum ampacity conductors required would be 42A, 46A, 51A, and 61A.

If there's no ampacity adjustment or correction required, the most likely choice would be a 50A conductor on a 50A OCPD. I.e. #8 Cu if using a 75C rated wiring method (and the equipment is so rated), or #6 Cu if using a 60C rated wiring method. For aluminum if the equipment terminations permit, #6 Al or #4 Al, respectively.

Cheers, Wayne