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#48040 - 02/02/05 08:20 AM Water heater queries  
IanR  Offline
Member
Joined: Dec 2004
Posts: 328
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 08:27 AM Re: Water heater queries  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
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.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#48042 - 02/02/05 08:32 AM Re: Water heater queries  
IanR  Offline
Member
Joined: Dec 2004
Posts: 328
Palm Bay FL USA
Thanks Iwire. Figured there was a code requirement, any idea of the logic behind that rule though?


#48043 - 02/02/05 04:25 PM Re: Water heater queries  
pdh  Offline
Member
Joined: Jan 2005
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).


#48044 - 02/02/05 04:58 PM Re: Water heater queries  
IanR  Offline
Member
Joined: Dec 2004
Posts: 328
Palm Bay FL USA
Good points
Thanks
Ian


#48045 - 02/07/05 07:46 AM Re: Water heater queries  
Steve Miller  Offline
Member
Joined: Aug 2001
Posts: 325
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.


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

-Jon


#48047 - 02/07/05 07:04 PM Re: Water heater queries  
PCBelarge  Offline
Member
Joined: Jun 2003
Posts: 681
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


Pierre Belarge


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