ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Violation?
by renosteinke - 01/27/23 09:52 PM
Does NEC 551.71 (F) apply to dwellings?
by BigB - 01/20/23 10:46 AM
Power submeter connections
by HotLine1 - 01/19/23 09:09 AM
AFDD's coming to the UK
by Texas_Ranger - 01/17/23 07:22 PM
New in the Gallery:
Burger King crown sillyness
Burger King crown sillyness
by wa2ise, December 11
240/208 to a house
240/208 to a house
by wa2ise, October 9
Who's Online Now
0 members (), 27 guests, and 16 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#48040 02/02/05 07:20 AM
Joined: Dec 2004
Posts: 329
I
IanR Offline OP
Member
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

#48041 02/02/05 07:27 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
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 07:32 AM
Joined: Dec 2004
Posts: 329
I
IanR Offline OP
Member
Thanks Iwire. Figured there was a code requirement, any idea of the logic behind that rule though?

#48043 02/02/05 03:25 PM
Joined: Jan 2005
Posts: 354
P
pdh Offline
Member
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 03:58 PM
Joined: Dec 2004
Posts: 329
I
IanR Offline OP
Member
Good points
Thanks
Ian

#48045 02/07/05 06:46 AM
Joined: Aug 2001
Posts: 329
S
Member
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 10:12 AM
Joined: Sep 2003
Posts: 650
W
Member
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 06:04 PM
Joined: Jun 2003
Posts: 681
P
Member
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

Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
 
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
pcsailor
pcsailor
Minneapolis, MN, USA
Posts: 21
Joined: September 2019
Top Posters(30 Days)
BigB 9
Popular Topics(Views)
302,563 Are you busy
231,912 Re: Forum
216,547 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5