# What is the ampacity for 8 AWG wire? 50 or 40



## JohnJ0906

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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## JohnJ0906

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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## Speedy Petey

There is almost nothing, except for conductors, that is rated for 90ºc. 
Most all terminations and breakers are rated for 75ºc, so that is typically what is used for figuring ampacity for other than NM cable. For NM cable we are limited to 60ºc due to NEC 334.80 .


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## MDShunk

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. :laughing:


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## gilamor

Actually I didn’t 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, 90°C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60°C. This same 8 AWG THHN, 90°C wire is limited to 50 amperes where connected to a fusible switch with terminals rated at 75°C. 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 60°C. Exhibit 110.5 shows an example of termination temperature markings. 


My question:
What is the “switch with terminals rated at 75°C “?
Is it the disconnect near the air-condition? Or the breaker on the panel?
I guess I don’t 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 ¾ flex. And I already run the wire and preferably don’t 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
[email protected]

Thank you


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## Speedy Petey

gilamor said:


> Also what is the ambient temperature for an attic in sant monica ca


I think Freakin HOT! is the technical term. :laughing:


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## JamesINla

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?




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


 


> Also what is the ambient temperature for an attic in sant monica ca


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## Speedy Petey

JamesINla said:


> 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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## JohnJ0906

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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## TheElectricalGuru

gilamor said:


> 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, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)
> 
> Types RHW, THHW, THW, THWN, XHHW, USE, ZW
> 
> For 60°C (140°F) is 40 amp
> 
> And for 75°C (167°F) 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, 90°C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60°C. This same 8 AWG THHN, 90°C 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 60°C (140°F)
conductor temperature rating. *The 90°C (194°F) rating*
*shall be permitted to be used for ampacity derating purposes,*
*provided the final derated ampacity does not exceed*
*that for a 60°C (140°F) 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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## ce2two

*Ce2two*

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 ...:yes:


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## Speedy Petey

ce2two said:


> 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 ...:yes:


You obviously don't know the NEC then. They live for this crap!


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## gilbequick

Yeah, there's nothing user friendly about it.


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## ce2two

*ce2two*

Sorry about that ,i'm a perfectionist..better to do right the first time ,or why bother doing it.:yes: I believe your mistaken ,i understand code just believe it could be a little more organized ........:yes: no  huh?


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## Speedy Petey

ce2two said:


> Sorry about that ,i'm a perfectionist..better to do right the first time ,or why bother doing it.:yes: I believe your mistaken ,i understand code just believe it could be a little more organized ........:yes: 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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## billsnuff

also, see 240.4(G) and 440.52 ......05 NEC

i think i got that right?


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## Nodoggie

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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## acrwc10

Nodoggie said:


> 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. :blink:

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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## Nodoggie

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.


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## william1978

Nodoggie said:


> 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.


:laughing:


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## acrwc10

Nodoggie said:


> **** 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.


It's never ceases to amaze me how the ignorant always resort to name calling when they are taken to task for something. 
Just be cause you worked in a HIGH-RISE or have done INDUSTRIAL WORK doesn't mean you know anything. 
As far as being qualified goes, I'm not the one giving "*<bad> ADVISE*" that would be you. So if you want go making stupid statements don't get your panties in a wad when you get called on it. 
If you are going to give advise to the uninformed maybe you could try giving accurate advise rather then bad advise. By the way "Dumb ass" is 2 words not one. :laughing::laughing::laughing:


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## KayJay

My state has had an amendment for many code cycles modifying NEC 334 which allows applying 310.15, A, 2, Exception, regarding ampacity of NM cable not embedded in thermal insulation. 
This amendment to 334.80 allows for the ampacity selection of NM cable at the 75-degree C column of Table 310.16, as long as the NM cable is not embedded in insulation for more than 10-feet or 10 percent of the circuit length. NM cables #10 and smaller are still normally protected per 240.4, D.
So, at least here in my state anyway, #8 NM cable can often be protected at 50A and likewise, #6 NM cable can often be protected at 60A, similar to when individual conductors are pulled in conduit.

I’m surprised to find that more states don’t have similar amendments, as this has been common practice in my area since the 1980’s, when 90-degree C NM-B came about.


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## Speedy Petey

Guys, I can this not going in a good direction. Let's try to keep it civil.


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## West.D

I was going to ask the exact same question before I found this thread. Thanks for this info. My client is going to be pleased.


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## readydave8

Speedy Petey said:


> Guys, I can this not going in a good direction. Let's try to keep it civil.


hi speedy petey


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## MotoGP1199

West.D said:


> I was going to ask the exact same question before I found this thread. Thanks for this info. My client is going to be pleased.


Keep in mind most if not all of these answers are per the NEC(or refer to nec code sections) and not the Canadian code. Not sure how it works up there.


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## ryelecserv

gilamor said:


> Actually I didn’t 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, 90°C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60°C. This same 8 AWG THHN, 90°C wire is limited to 50 amperes where connected to a fusible switch with terminals rated at 75°C. 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 60°C. Exhibit 110.5 shows an example of termination temperature markings.  My question: What is the “switch with terminals rated at 75°C “? Is it the disconnect near the air-condition? Or the breaker on the panel? I guess I don’t 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 ¾ flex. And I already run the wire and preferably don’t 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 [email protected] Thank you


 LOL: That question is a fundamental one I think that any certified journeyman would nt take the time out to answer rather you need to go back to the beginning and study the basics.🤷‍♂️ Just saying


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## macmikeman

Thanks for bringing up a fifteen yr old thread which highlighted just what a bunch of argumentative Mike Holt wannabes that used to frequent this place........ And all these years I just have stuck to #8 is forty amps to play it safe anyway. As for attics, I consider myself pretty much the resident expert on spending time in hot attics and never yet come across a melted nm cable melted up there due to the attic heat. What I have found several times is melted nm strapped to ceiling trusses in commercial establishments being used in rather code shakey ways to run excessive numbers of fluorescent lights, metal halide lights , sometimes motor operated equipment, and other "tarded" ideas. Seen my share of steaming hot emt conduits as well. Wire=fire, no chance em bra.


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