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#48040 - 02/02/05 04:20 AM Water heater queries
IanR Offline
Member
Registered: 12/06/04
Posts: 326
Loc: Palm Bay FL USA
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?
Ian
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#48041 - 02/02/05 04:27 AM Re: Water heater queries
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Quote:
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.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#48042 - 02/02/05 04:32 AM Re: Water heater queries
IanR Offline
Member
Registered: 12/06/04
Posts: 326
Loc: Palm Bay FL USA
Thanks Iwire. Figured there was a code requirement, any idea of the logic behind that rule though?
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#48043 - 02/02/05 12:25 PM Re: Water heater queries
pdh Offline
Member
Registered: 01/20/05
Posts: 354
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).
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#48044 - 02/02/05 12:58 PM Re: Water heater queries
IanR Offline
Member
Registered: 12/06/04
Posts: 326
Loc: Palm Bay FL USA
Good points
Thanks
Ian
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#48045 - 02/07/05 03:46 AM Re: Water heater queries
Steve Miller Offline
Member
Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
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.
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#48046 - 02/07/05 07:12 AM Re: Water heater queries
winnie Offline
Member
Registered: 09/15/03
Posts: 652
Loc: boston, ma
Another factor to consider is that most circuit breakers are only rated continuous operation at 80% of their trip rating.

-Jon
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#48047 - 02/07/05 03:04 PM Re: Water heater queries
PCBelarge Offline
Member
Registered: 06/08/03
Posts: 657
Loc: Dobbs Ferry, NY, USA
Jon
That is an interesting statement, as the '05 NEC now states that 120 gal water heaters 422.13, are considered as "continuous duty".

Pierre
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Pierre Belarge
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