ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
You will never guess
by grich - 02/24/21 07:18 PM
Lock-down Thread
by Bill Addiss - 02/24/21 04:40 PM
New tool
by SMOKEYBOB - 02/15/21 04:59 PM
Split Bus Breaker Panel
by gfretwell - 02/15/21 02:30 PM
Sierraplex recpts
by gfretwell - 02/15/21 02:27 PM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
1 registered members (Scott35), 15 guests, and 19 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 3 1 2 3
Ampacity of Branch circuit #200211 03/25/11 02:50 PM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline OP
Member
Here's my question - If the ambient temperature of the conductors installed on the roof is 129 degrees and the run is 100 foot long with 10 feet on the roof and 90 feet installed inside the building at an ambient temperature of 86 degrees what if any adjustment has to be made in the way of correction factor to the conductor ampacity?


George Little
2017 / 2014 NEC & Related Books and Study Guides
Re: Ampacity of Branch circuit [Re: George Little] #200215 03/25/11 03:01 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
Member
George:

May this be the answer to your inquiry:

2008 NEC 310.15 (A) (2)

"(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."


John
Re: Ampacity of Branch circuit [Re: George Little] #200219 03/25/11 04:09 PM
Joined: Jul 2004
Posts: 9,568
G
gfretwell Offline
Member
I bet that is why he picked 10 feet wink

I do think it is funny that they give you a break on ampacity for a 10 foot section in this article but bust you for 1.5" if it is going through insulation in 334.80


Greg Fretwell
Re: Ampacity of Branch circuit [Re: George Little] #200220 03/25/11 04:34 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
Member
You think George is giving a test??

The logic behind this is the roof "heat' will disapate in the remaining 90'????

Me, I would derate the run! Not that I would have to, but....


John
Re: Ampacity of Branch circuit [Re: George Little] #200222 03/25/11 04:46 PM
Joined: Jul 2004
Posts: 9,568
G
gfretwell Offline
Member
I believe the example in the handbook was a pipe running through a warmer room than the rest. I am with you, on the roof I would derate. If for no other reason, the heat in the pipe will rise to the highest point and that is probably the roof.


Greg Fretwell
Re: Ampacity of Branch circuit [Re: George Little] #200225 03/25/11 06:40 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
Member
I think that George is going to lurk around and see what (hopefully) the lot of us say.


John
Re: Ampacity of Branch circuit [Re: George Little] #200228 03/25/11 07:18 PM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline OP
Member
John- How in the world could you even think that I would ever do such a devious thing as that?.

This roof top heat thing is all over the map. What if the roof is white what if the roof is black or maybe a metal roof. What if the conductors are on the roof but in a shaded part of the roof due to structural features?

Yes the code reference is 310.15(A)(2) and I don't think it solves the problem.

In the mean time I hope John will humble himself and apologize for thinking poor little ole George was trying to be crafty.


George Little
Re: Ampacity of Branch circuit [Re: George Little] #200229 03/25/11 07:32 PM
Joined: Jul 2004
Posts: 9,568
G
gfretwell Offline
Member
This roof thing is still in it's infancy. I assume there will be adjustments to the language as we go along but I am happy that they are referencing it. Down here in sun country, walking around on a roof with an IR thermometer will really open your eyes. Metal is usually too hot to touch and 140-150f is not unusual. A built up "tar and gravel" roof will be gooey.


Greg Fretwell
Re: Ampacity of Branch circuit [Re: gfretwell] #200231 03/25/11 07:49 PM
Joined: Jul 2004
Posts: 783
L
LarryC Offline
Member
Quote
If for no other reason, the heat in the pipe will rise to the highest point and that is probably the roof.


RANT ON Heat does not rise! Heat travels from high temperature to low temperature. Gravity is not involved! Hot air rises because the denser cold air is pulled down by gravity. If the roof was colder than the building, would the cold fall to breaker panel? RANT OFF

Sorry, thermodynamics and nuclear power are triggers for over explinations.

Larry

Re: Ampacity of Branch circuit [Re: George Little] #200235 03/25/11 08:52 PM
Joined: Jul 2004
Posts: 9,568
G
gfretwell Offline
Member
OK at night time the cold air from the roof would be pulled down by gravity displacing the warmer air in the building. Happy now wink
Air is still going to be the working fluid and it will be affected by gravity. The warmer air will be at the top.

When the sun is shining the roof will be the hottest spot unless this is a smelting plant.


Greg Fretwell
Page 1 of 3 1 2 3

Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

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

 

Member Spotlight
The_Lightman
The_Lightman
Orlando, Fl, USA
Posts: 49
Joined: August 2001
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
275,352 Are you busy
209,224 Re: Forum
196,526 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3