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

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

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

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.

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

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 .

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.

__________________
I'm an electrician, not a magician.

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
gil@amorelectric.com

Thank you

Speedy Petey

I think Freakin HOT! is the technical term.

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?

Quote:

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

Speedy Petey

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???

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.

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

TheElectricalGuru

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.

__________________
Paul W. Abernathy,CME,RMS,CMI
Master Electrician/Electrical Educator/NEC Consultant
ICC & IAEI Certified Electrical Inspector/Plans Examiner
www.TheElectricalGuru.com

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 ...

Speedy Petey

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

gilbequick

Yeah, there's nothing user friendly about it.

ce2two

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?

Speedy Petey

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.

billsnuff

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

i think i got that right?

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).

acrwc10

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.

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.