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

[gilamor]

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

Can someone tell me if I can use 8 AWG THHN

Moderators note: Personal contact information removed, please use PM to exchange this info


Thank you

 

[sceepe]

The answer is is already in your post. It depends on the temperature rating of the disconnect terminals. If you don't know the terminal rating you are supposed to use the 60 degree column if less than 100 amps.

The temp of the attic is the ambient temp. In your post you wrote that you were looking at a table for an ambient temp of 30 deg. If the attic is going to be hotter than that, you have to derate the wire for ambient temps. There is a table in the nec to tell you how much to derate.

 

[iwire]

Lets not forget 334.80 when using NM....it's always 60 C.

 

[Pierre C Belarge]

Lets not forget 334.80 when using NM....it's always 60 C.

This is a good example of a casual reader of the NEC. The occupancy can make a big difference. If you do not spend enough time in the NEC (casual reader) you may be unaware of the different requirements of a topic, located in different parts of the NEC.

That is one of the really good aspects of this and other forums, someone who is a student of the NEC will always chime in and let you know.
And, on a Sunday morning, you have your answer in 4 hours...thats impressive. Try doing that with some techie on the phone for any other service.

 

[augie47]

rating

rating

"I am need to connect an air-condition rated 240 50 Amp 3 wires (2 hot and ground)"


I'm curious as to this rating. Is this the Min Ckt Amps on the nameplate ?

If not, remember your wire size can be based on MCA and not OCP.


"Try doing that with some techie on the phone for any other service. "
and some of us almost type english.

 

[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 in ? flex (2*8 awg and ground 10 awg). And I already run the wire and preferably don?t want to change the wire to 6 AWG because I would lose money on the job

 

[jim dungar]

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 in ? flex (2*8 awg and ground 10 awg). And I already run the wire and preferably don?t want to change the wire to 6 AWG because I would lose money on the job

Cable acts like a heat sink which prevents the device from overheating.

The rating of the terminal is not important, what counts is the rating of the termination. This is not just a choice of words, you can not change out the just terminal (lug) to a higher temperature.

When the termination temperture is not known you must assume it is 60C if the device is 100A or smaller. For larger devices you can use the 75C values. But there is no device termination rated for the 90C values (even though they may require 90C insulation).

 

[tryinghard]

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?

In this case it's the disconnect, in my opinion you are correct at 50A for #8's so leave the #8's

Something to check: if running through an attic depending on your area you may use 114-122f and if thats the case 75% of the 50A = 37.5A, with this result check the nameplate of the unit to make sure it's not more than 37.5A

 

[jim dungar]

In this case it's the disconnect

Actually it is both the disconnect and the breaker terminations. You cannot exceed either one.

 

[bob]

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 in ? flex (2*8 awg and ground 10 awg).

You need to consider the terminals and the conductor a circuit. The max load is limited by the weakest link. In you case the terminals are likely rated at
75C. Therefore the entire circuit is limited to the 75C. Looking at the 310.16 table you see that #8 ampacity is 50 amps at 75C and 55 amps at 90C.
Under the conditions listed at the top of the table, a continuous of 55 amps
will heat the conductor to 90C in 3 hrs and 50 amps will heat it to 75C in 3 hrs. If you allow the 55 amps, the conductor could reach the 90C and overheat the terminals. Therefore the conductor must not exceed 50 amps.
There is a question regarding derating the conductor in an attic that is 130
degrees. Using the 90 rating of 55 amps, the derating factor is .76. The resultant ampacity is 0.76 x 55 = 42 amps. This question has been asked before. Some say that there is no derating required. The thinking is the the conductor is only operating in a hot environment for say 6 hrs a day for 3 months and a cooler environment for 9 months. I don't think there was ever
an agreement on the answer.

 

[gilamor]

Thank you all
I also talked to several electricians with 30 years experience
They told me that 8 AWG is OK

Gil Amor
Amor Electric

 

[augie47]

oh ?

oh ?

Thank you all
I also talked to several electricians with 30 years experience
They told me that 8 AWG is OK

Gil Amor
Amor Electric

that conviences me..

lets throw out the code books and set up a panel of electricians with 30 yrs experience.

Gil, not knocking experience...about 50 years of it here...but (a) codes change (b) a lot of answers are based on lack of input, and (c) trust me, sometimes exprience can mean you've just been doing it wrong a long time.

I think you can see from the answers here...there are many varibles that make the determination of wire size and allowed ampacities.

 

[iwire]

I agree with Augie experience has nothing to do with it.

1 year or 50 years in you should own a current code book and know how to look the answer up.

 

[LarryFine]

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

Look at it like this: Its more the other way around. The ampacity of the wire is not what changes, it's the terminal's ability to withstand the heat contributed by the conductor when used at the various amperage levels.

Obvioulsy, a #8 loaded at 50a will generate more heat than will the same conductor when loaded at 40a. The equipment may or may not be able to survive when subject to the extra heat from the more-heavily-loaded conductor.

 

[Mr. Bill]

Gil, not knocking experience...about 50 years of it here...but (a) codes change (b) a lot of answers are based on lack of input, and (c) trust me, sometimes exprience can mean you've just been doing it wrong a long time.

Something I had heard once. 30 years of experience = 1 year of experience followed by 29 years of repeating the same thing over again.

 

[petersonra]

One o the things used to irk me at a previous place of employment was the justification for doing something a particular way was often "we have been doing it that way for 40 years". My response was generally along the lines of "about time we started doing it right then".

 

[JohnConnolly]

I had been "doing it that way" for years also.

I only recently learned that while #8 SE is good for 50 amps, # 8 Romex is not.

Lame.

 

[George Stolz]

...#8 SE is good for 50 amps...

How do you figure?

 

[infinity]

How do you figure?

If both the SE and NM were copper the #8 SE could be used at 75 degrees C or 50 amps. The #8 NM at 60 degrees C or 40 amps.

 

[JohnConnolly]

If both the SE and NM were copper the #8 SE could be used at 75 degrees C or 50 amps. The #8 NM at 60 degrees C or 40 amps.

Yeah....what he said.

A customer called me on it once about 5 years ago and I looked it up. He was right as far as I could tell. That code lingo really confuses me.

When I started wiring, we didn't HAVE #8 NM, it was all SE style. I assumed for years that #8= 50 amps.