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Water heater queries #48040 02/02/05 08:20 AM
Joined: Dec 2004
Posts: 329
IanR Offline OP
This may be a dumb question, but I'll ask anyway.
I've noticed that most residential electric water heaters are rated at 4500W, so why are all of the branch cicuits that I have seen for them 30A @240V? Being that they are just resistive heating elements there is no inrush current that I know of. Just seems overkill running a 7200W circuit for a 4500W load. Comments?

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Re: Water heater queries #48041 02/02/05 08:27 AM
Joined: Jan 2003
Posts: 4,391
iwire Offline
422.13 Storage-Type Water Heaters.
A branch circuit supplying a fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall have a rating not less than 125 percent of the nameplate rating of the water heater.

4500 / 240 = 18.75 x 1.25 = 23.4 amps.

This puts you into a 10 awg, you could use a 25 amp breaker but most just use a 30 amp breaker.

Bob Badger
Construction & Maintenance Electrician
Re: Water heater queries #48042 02/02/05 08:32 AM
Joined: Dec 2004
Posts: 329
IanR Offline OP
Thanks Iwire. Figured there was a code requirement, any idea of the logic behind that rule though?

Re: Water heater queries #48043 02/02/05 04:25 PM
Joined: Jan 2005
Posts: 354
pdh Offline
Consider that there may be as much as 5% (or even more) variability in manufacturing tolerance, as well as 5% variability in your supply voltage. If the element is exactly 4500 watts and the voltage is exactly 240 then your have 18.75 amps (1.25x = 23.4375). Now let's raise the voltage by 5% (252 volts). The same resistance (12.8 ohms, not accounting for change due to heating) gives you 19.6875 amps (1.25x = 24.609375). Then if the element itself is 5% lower in resistance (12.16 ohms) you'd have 19.7368421 amps at 240 volts (1.25x = 24.671052625) and at 252 volts you'd have 20.7236842 amps (1.25x = 25.90460525). You may need that 30 amp breaker. And even the breakers themselves have some variability (why the trip curves have some width to them).

Re: Water heater queries #48044 02/02/05 04:58 PM
Joined: Dec 2004
Posts: 329
IanR Offline OP
Good points

Re: Water heater queries #48045 02/07/05 07:46 AM
Joined: Aug 2001
Posts: 329
Steve Miller Offline
The last one I did the AHJ required a 25A breaker. We put in a disconnect with 25a fuse and he was happy. Breaker is still 30.

Re: Water heater queries #48046 02/07/05 11:12 AM
Joined: Sep 2003
Posts: 649
winnie Offline
Another factor to consider is that most circuit breakers are only rated continuous operation at 80% of their trip rating.


Re: Water heater queries #48047 02/07/05 07:04 PM
Joined: Jun 2003
Posts: 681
PCBelarge Offline
That is an interesting statement, as the '05 NEC now states that 120 gal water heaters 422.13, are considered as "continuous duty".


Pierre Belarge


2020 National Electrical Code
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