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#36885 - 04/17/04 11:02 AM derating ampacties of service conductors ?
mj Offline

Registered: 06/10/02
Posts: 188
Loc: meriidian, ms
The average temp in my area in the summer is about 90- 105. as per sec, 310.16 table, if the temp exceeds 86 F ,a correction factor is required, as to derate the ampacity of the conductors. most of the electricians in my area do not apply the rules of 310.16 and 310.15(2)b tables. do you apply the rules of derating ?

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#36886 - 04/17/04 12:27 PM Re: derating ampacties of service conductors ?
DougW Offline

Registered: 06/08/03
Posts: 1083
Loc: North Chicago, IL
Wonder if your AHJ has allowed the non-derated temp to remain as the "accepted" one, since the temp is not year-round, and (hopefully) it gets cooler during the evening. I couldn't find anything specific for this during a (very) brief Code check.

You know the saying..."when in doubt, ask your AHJ"

#36887 - 04/18/04 08:11 AM Re: derating ampacties of service conductors ?
Obsaleet Offline

Registered: 04/05/03
Posts: 361
Loc: Pa
In my area I see seu/ser installed on commercial buildings. The conductors in the cable assembly are derated for residential. Inspectors seem to just let it slid?

Temp rating is only considered indoors?

Choose your customers, don't let them choose you.

#36888 - 04/18/04 08:37 AM Re: derating ampacties of service conductors ?
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Obsaleet I do not follow your post.

SE is not restricted to dwelling units only.

As to the original question, I see the ambient temp derating issue as pretty much ignored.

If you ask an AHJ they should tell you that you must apply it.

In my area outside temps hit 100 F about once a year and temps in the 90s F are typical during August.

Say we have a 200 amp service that we choose 3/0 copper XHHW-2 (common for SE cable) for with a rating of 200 amps @ 75 C

Now if we have an ambient temp of 96 to 104 F that will be a .91 adjustment.

3/0 XHHW is rated 225 Amps @ 90 C

225 x .91 = 204.75 amps, still fine for the 200 amp service.

As a matter of fact any value above 175 amps and you could still protect the wire with a 200 amp breaker, as long as the derated ampacity was greater than the connected load.

Bob Badger
Construction & Maintenance Electrician

#36889 - 04/21/04 04:00 PM Re: derating ampacties of service conductors ?
Sparkeee32 Offline

Registered: 03/23/04
Posts: 32
Loc: Federal Way WA USA
Keep in mind, in places with uncommon temperatures, that the equipment within the service itself is raited for certain temp. For instance, if lugs are not supplied with the switch gear, you must make sure they are rated the same temp as the switchgear... lugs come in several temperature ratings, and if they don't say, then they are asumed bye the AHJ to be the lowest rated. just something to add to the mix! Brian


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