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rooftop raceway #5218
11/07/01 10:23 PM
11/07/01 10:23 PM
D
dom  Offline OP
Junior Member
Joined: Jun 2001
Posts: 8
how is the temperature determined for a raceway on a roof, for the purpose of derating the conductors?
dom

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Re: rooftop raceway #5219
11/08/01 12:24 AM
11/08/01 12:24 AM
sparky66wv  Offline
Member
Joined: Nov 2000
Posts: 2,236
West Virginia
Oh, Man... Good Q...

Why do I have a feeling I'm gonna end up with 500 KCmil THHN Cu service conductors for a 200A service after this thread?...

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-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI
Re: rooftop raceway #5220
11/08/01 08:41 AM
11/08/01 08:41 AM
S
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,360
Do we ask a weatherman? http://www.wunderground.com/cgi-bin/findweather/getForecast?query=chester%2C+vt

Re: rooftop raceway #5221
11/08/01 01:15 PM
11/08/01 01:15 PM
Joe Tedesco  Offline
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
310-10 Temperature Limitation of Conductors.
No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way with respect to type of circuit, the wiring method employed, or the number of conductors that the limiting temperature of any conductor is exceeded.
FPN: The temperature rating of a conductor (see Table 310.13 and Table 310.61) is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Annex B, the correction factors at the bottom of these tables, and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities, ampacities, ambient temperatures, and number of associated conductors.
The principal determinants of operating temperature are as follows:
(1) Ambient temperature — ambient temperature may vary along the conductor length as well as from time to time.
(2) Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.
(3) The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.
(4) Adjacent load-carrying conductors — adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.


Joe Tedesco, NEC Consultant
Re: rooftop raceway #5222
11/08/01 05:56 PM
11/08/01 05:56 PM
W
WARREN1  Offline
Member
Joined: May 2001
Posts: 176
Greenville, SC, USA
Very good answer, Joe.
But?
I would contact my local HVAC engineer for an ambient temperature to use. That's where we get those things from here in the office. Even in a residential attic, the temp's are probably up in the 150-160 F range. How many people derate for the lighting conductors installed there? For 90 Deg C wire, the temp derate is at 0.41 for 160 Deg F. That means your #12 wire is good for about 15 Amps., with a 20 Amp breaker ahead of it. Legal? As Joe pointed out, any part derates the entire length of conductor.
Just a few thoughts, and the kind of things we in the design end of things have to consider also.


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