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#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?

#48041 02/02/05 08:27 AM
Joined: Jan 2003
Posts: 4,391
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
#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?

#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).

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

#48045 02/07/05 07:46 AM
Joined: Aug 2001
Posts: 335
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
Joined: Sep 2003
Posts: 650
Another factor to consider is that most circuit breakers are only rated continuous operation at 80% of their trip rating.


#48047 02/07/05 07:04 PM
Joined: Jun 2003
Posts: 681
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

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