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Old 07-08-2007, 02:10 AM   #1
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Default What is the ampacity for 8 AWG wire? 50 or 40

Every electrician I ask tell me different story
This is important since wire are very expensive
I am need to connect an air-condition rated 240 50 Amp 3 wires (2 hot and ground)

According to the NEC
Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60C Through 90C (140F Through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)

Types RHW, THHW, THW, THWN, XHHW, USE, ZW

For 60C (140F) is 40 amp

And for 75C (167F) is 50 amp

What is this temperature? Is it the temperature in the attic where I put the wire?

Also from the NEC 2002 book
8 AWG THHN, 90C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60C. This same 8 AWG THHN, 90C wire is limited to 50 Amp

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Old 07-08-2007, 10:36 AM   #2
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These are the temp. ratings of the connection points, devices, etc. as well as the wire. It's the temperature that the wire insulation can tolerate.

You rate the wire from the LOWEST temp rating on a circuit - If you use 90 deg wire (THHN, for example) but the terminations (lugs) are only rated 75 deg, you must use the 75 deg column for wire ampacity.

Read 110.14(C) (2005 NEC) I don't have my 2002 NEC at home

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Old 07-08-2007, 10:47 AM   #3
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Need to add a few things.

The attic (or wherever you run the wire) is your AMBIENT temperature. See the correction table at the bottom of table 310.16 (for example)

You apply correction and adjustment factors from the highest rating of the wire (90 deg for THHN, for example) but the final result is still restricted by other factors, I couldn't excede the 60 deg column if that was what the lugs were rated for.


I have 12 gauge cu THHN. This is a 90 deg conductor. I have 4 current carrying conductors in a conduit. I have to adjust per table 310.15(B)(2)(a)
30 amps x 80% = 24 amps.
ambient temp is 50 deg c. Correction of .82
24 amps x .82 = 19.68 amps.
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Last edited by JohnJ0906; 07-08-2007 at 10:53 AM.
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Old 07-08-2007, 10:48 AM   #4
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There is almost nothing, except for conductors, that is rated for 90c.
Most all terminations and breakers are rated for 75
c, so that is typically what is used for figuring ampacity for other than NM cable. For NM cable we are limited to 60c due to NEC 334.80 .
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Old 07-08-2007, 11:14 AM   #5
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Among residential electricians, even good one's, I find that using #8 romex for 50 amps is a very common violation. As Speedy points out, Romex has that code limitation to the 60 degree column on the ampacity chart. I guess it kinda messes with your head if you normally work with MC, pipe and wire, as well as romex in your day to day job. Guys are quick to whip out "ampacity charts" that they might have picked up at the supply house over the years or whatever to show you that #8 copper is okay for 50 amps, but they're forgetting about romex being limited to the 60 degree column. I think that #8 is the only size that causes this problem, but I could be wrong. (Maybe #2-#3 too, I forget). There, now you have homework. You can look that up.
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Old 07-08-2007, 11:28 AM   #6
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Actually I didnt copy past the whole paragraph from the NEC book
The whole paragraph is:

Section 110.14(C)(1)(a) requires that conductor terminations, as well as conductors, be rated for the operating temperature of the circuit. For example, the load on an 8 AWG THHN, 90C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60C. This same 8 AWG THHN, 90C wire is limited to 50 amperes where connected to a fusible switch with terminals rated at 75C. The conductor ampacities were selected from Table 310.16. Not only does this requirement apply to conductor terminations of breakers and fusible switches, but the equipment enclosure must also permit terminations above 60C. Exhibit 110.5 shows an example of termination temperature markings.


My question:
What is the switch with terminals rated at 75C ?
Is it the disconnect near the air-condition? Or the breaker on the panel?
I guess I dont understand why the rating of a terminal would have any effect on the ampacity of the wire?
By the way I am not running an NM cable I am running 3 single conductors THHN (2*8 awg and ground 10 awg) in flex. And I already run the wire and preferably dont want to change the wire to 6 AWG
What do you mean by terminations (lugs)
Is it the connection to the breaker and the lugs on the disconnect box?
Also what is the ambient temperature for an attic in sant monica ca

My contact

http://www.amorelectric.com
gil@amorelectric.com

Thank you
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Old 07-08-2007, 04:47 PM   #7
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Quote:
Originally Posted by gilamor View Post
Also what is the ambient temperature for an attic in sant monica ca
I think Freakin HOT! is the technical term.
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Old 07-08-2007, 08:58 PM   #8
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Santa Monica? Shoot, it's really nice down their right now. I just got in from Palm Springs where at 11 AM was 113. It's been really hot in L.A. this past week. In fact, tomorrow I have an installation with about 2 hours of attic time. I will be leaving my house at 4 am (hour drive). It's the strangest thing to drink a large cup of coffee and about a gallon of water / Gatorade and still not pee come 5 pm.

Back on topic: Is it me, or do you smell handyman?

Quote:
I guess I don’t understand why the rating of a terminal would have any effect on the ampacity of the wire?


Quote:
Also what is the ambient temperature for an attic in sant monica ca
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Old 07-08-2007, 09:57 PM   #9
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Quote:
Originally Posted by JamesINla View Post
Back on topic: Is it me, or do you smell handyman?
Nah. I smell novice.

The thing that bothers me is this:
http://www.amorelectric.com/html_site/index.html

If Gil is a licensed C10 electrician in Ca, why is he asking these types of newbie questions???
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Old 07-09-2007, 07:16 PM   #10
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After re-reading the OP, I noticed that you want to wire an A/C unit. Keep in mind that you don't have to size wire to the breaker, you size wire to the "minimum circuit ampacity" on the nameplate.

Unless, of course, your jurisdiction has an amendment stating otherwise.
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Old 07-10-2007, 08:41 PM   #11
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Quote:
Originally Posted by gilamor View Post
Every electrician I ask tell me different story
This is important since wire are very expensive
I am need to connect an air-condition rated 240 50 Amp 3 wires (2 hot and ground)

According to the NEC
Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60C Through 90C (140F Through 194F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)

Types RHW, THHW, THW, THWN, XHHW, USE, ZW

For 60C (140F) is 40 amp

And for 75C (167F) is 50 amp

What is this temperature? Is it the temperature in the attic where I put the wire?

Also from the NEC 2002 book
8 AWG THHN, 90C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60C. This same 8 AWG THHN, 90C wire is limited to 50 Amp
Gil,

This temperature you are speaking of ( the 60 and 75 degree ) is based on the terminals and conductor ratings. As I believe speedy stated most terminal ratings will be at 75 degrees but if you are using NMC for this installation you are limited to 60 degrees as he also stated.

The ampacity of 8 AWG is 40A if you are intending to use NMC cable for this application.

Now even if the terminals are rated for 75 degrees you would be sizing based on 60 degrees because of if being NMC.

Now be it other than NMC and you have some options.....be it NMC you are limited to the 60 degrees maximum.

334.80 Ampacity.
The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60C (140F)
conductor temperature rating. The 90C (194F) rating
shall be permitted to be used for ampacity derating purposes,
provided the final derated ampacity does not exceed
that for a 60C (140F) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11.
Where more than two NM cables containing two or
more current-carrying conductors are bundled together and
pass through wood framing that is to be fire- or draftstopped
using thermal insulation or sealing foam, the allowable
ampacity of each conductor shall be adjusted in
accordance with Table 310.15(B)(2)(a).

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Old 10-09-2008, 06:15 PM   #12
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60,75 ,90' ,why not make all terminals rated at 90c ,this is one big joke ,why play with #'s ,up,down ,over there, over here, not that one, this one ,low ,high ,what ever, be consitenent ...
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Old 10-09-2008, 07:03 PM   #13
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Quote:
Originally Posted by ce2two View Post
60,75 ,90' ,why not make all terminals rated at 90c ,this is one big joke ,why play with #'s ,up,down ,over there, over here, not that one, this one ,low ,high ,what ever, be consitenent ...
You obviously don't know the NEC then. They live for this crap!
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Old 10-09-2008, 09:54 PM   #14
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Yeah, there's nothing user friendly about it.
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Old 10-10-2008, 12:19 AM   #15
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Sorry about that ,i'm a perfectionist..better to do right the first time ,or why bother doing it. I believe your mistaken ,i understand code just believe it could be a little more organized ........ no huh?
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Old 10-10-2008, 07:06 AM   #16
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Quote:
Originally Posted by ce2two View Post
Sorry about that ,i'm a perfectionist..better to do right the first time ,or why bother doing it. I believe your mistaken ,i understand code just believe it could be a little more organized ........ no huh?
I don't think it is a question of doing it right. It is a question of figuring out HOW to do it right.

I am also a perfectionist. When ever I hear "Can't see it from my house" I lay into the guy. For anyone working for me that comment is taboo!
I have become somewhat obsessed with getting the code right.

Of course my statement was facetious. I can only assume you know the code to have gotten where you are.
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Old 10-10-2008, 01:11 PM   #17
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also, see 240.4(G) and 440.52 ......05 NEC

i think i got that right?
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Old 10-17-2008, 10:19 PM   #18
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If you are a novice and don't know how to do all of the calcualtions alluded to here, rely on the 75 degree column in article 310-16. You should be fine (if the environment is not excessively hot).
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Old 10-19-2008, 04:15 PM   #19
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Quote:
Originally Posted by Nodoggie View Post
If you are a novice and don't know how to do all of the calcualtions alluded to here, rely on the 75 degree column in article 310-16. You should be fine (if the environment is not excessively hot).
Maybe it is time you put down the mouse and pick up the code book.

The enviroment ambiant temp is only part of the issue.
If you are going to dumb down the NEC, go to the 60 degree column at least that way you won't be creating as many violations. In a circuit you are limited to the temp. rating of the lowest rated part of the circuit, and many parts can have a 60 degree rating not just the conductors. As far as an AC unit go by the name plate rating "wire to the Minimum fuse to the maximum" and for NM the cable will be in the 60 degree column.
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Old 10-19-2008, 05:56 PM   #20
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You ever worked on high-rises? what about indsutrial work? power plants, nukes? I have. I just gave general advice to an uninformed kid, is all. It's sunday...go watch some football.


Last edited by Speedy Petey; 10-19-2008 at 08:40 PM. Reason: Remove unnecessary remarks
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