Replies

1. 
   brownbagg | May 28, 2003 03:17pm | #1

   It is not a concern. The temp on romex is for the insulation to begin melting creating a short. They have a standard of 90 degreess but thats for the wire it self to be. air temp and conduction temp will be highly different. Think , most romex are running through attic of temp 120 and above, no problem. The 90 of romex is a safety factor.

1. 
   Mattohio | May 28, 2003 03:36pm | #2

   Thanks for your response,

   I understand the temperature in an attic is often 120-140 degrees, but doesn't the temperature in a sealed rafter bay (finished with insulation and drywall) get to be much warmer than that? Maybe I just worry to much, but better safe than on fire!

   Thanks again,

   Matt

2. 
   GHR | May 28, 2003 03:56pm | #3

   Centigrade and farenheit (I should learn to spell those words) are different. You should learn the differences.

   You should also learn to adjust wire ratings for ambient temperatures.

1. 
   Dinosaur | May 28, 2003 04:35pm | #4

   You said what I was going to. That 90-degree figure is indeed what you called Centigrade, which means the wire is rated at 194 of those under-sized Farenheit degrees you use down there in the 'States.

   By the way, 'Centigrade' has been replaced by Celcius, which is the name of the dude that invented that scale (as Farenheit is the name of the guy who invented the other one. Then there's degrees Kelvin, but let's not get into that...).

   In any event, if you've got temps over 90C in a rafter bay, overheating your NMD might just be the least of your problems....

   Dinosaur

1. 
   NickNukeEm | May 28, 2003 07:51pm | #7

   You forgot Rankine, which is the Kelvin equivalent for Fahrenheit (F + 460 = R)

   I never met a tool I didn't like!

1. 
   Dinosaur | May 28, 2003 09:16pm | #8

   You're right, I forgot that one. But if you've got to measure the temp in a rafter bay in either Kelvin or Rankine, I've got a feeling the value of the house is gonna wind up at Absolute Zero.

    Dinosaur

1. 
   NickNukeEm | May 28, 2003 11:44pm | #9

   Well said.  Pun intended, I assume.

   I never met a tool I didn't like!

2. 
   Dinosaur | May 28, 2003 11:56pm | #10

   "I never met a pun I didn't like."

   Dinosaur

3. 
   CPopejoy | May 30, 2003 07:28am | #11

   Anyone wonder how old Mr. Farenheit cooked up his temp scale?  I mean, the two easiest reference points in nature--the freezing point and the boiling point of water--are 32 and 212 degrees F.--weird, eh?  Why not zero and 200 degrees or some other round numbers?

   It all made sense when I learned what Mr. F. used as points to fix the scale--

   --human body temp was set at 100 degrees;

   --the lowest temp reached by a saturated brine (salt) and ice solution was set at zero degrees; and,

   --the range between those points was divided by 100.

     It turned out that the woman he chose to set the 100 degree point of the scale (a housemaid, I think) happened to be running a slight fever--so the ref point was higher than normal temperature.  That's why normal human body temp is 98.6 degrees F.

   Extrapolating the scale up to the boiling point of water results in a value of 212 F.

   The derivation of the Centigrade scale makes more sense to me.  Oh, and the two scales cross at -40.

   Just call me Mr. Science trivia.

   Cliff

4. 
   jhausch | May 30, 2003 01:11pm | #12

   Inch by inch, we're going metric

   -Dean Kamen

   I love Celcius for its clarity, but I prefer Farenheit for its finer resolution.

   Steelkilt Lives!

   Edited 5/30/2003 6:12:00 AM ET by Jim

5. 
   Dinosaur | May 30, 2003 05:26pm | #13

   Yep, at -40, both Farenheit and Celcius are the same: distinctly refreshing!

   My mother, who lives outside Pittsburgh, just can't get her head wrapped around the Celcius scale. When I talk to her on the phone in the winter and tell her that it's, say, -18, she still asks, "How cold is that in real degrees?" Typical American Philistine, but there you are.

   You gotta try skiing at minus 40 sometime; absolutely gorgeous. Nobody else on the hill but some poor ski patroller (me) who's gotta be there, and he's wondering why the heck you didn't stay in the chalet.

   It's actually not that bad, as long as you don't let your face get wind-burn and you're dressed with enough layers. You're better off putting on some tele skis and heading off into the backwoods than out on the downhill trails, though. Less wind, and you move slower.

   Here's a pic of the top of Johansson Peak, which is the highest of the 4 summits on Mont-Tremblant. The temp that morning was -33C; it was -27C at noon when we left the summit of 'The Edge' to climb up to Jo Peak. By the time we'd gone about a half kilometre, both of us had stripped down to shirts and sleeveless vests and headbands. That mound of snow behind my buddy Fried is a buried igloo one of the boys had built earlier in the winter; there's about 13 feet of snow on the ground here. The bird eating out of my hand is a geai gris, or 'Gray Jay' (like Blue Jay, I guess); it had stolen a good 6"-long piece of dried sausage I'd been eating and tried to fly away with it, but it was too heavy. We decided it should be rewarded for its efforts and so I cut a piece off and let him have it.

   Sure beats roofing at that temperature, no matter what scale you use.

   Dinosaur

   'Y-a-tu de la justice dans ce maudit monde?

   
3. 
   BillHartmann | May 28, 2003 05:22pm | #5

   "

   I would like to run some wires in a cathedral ceiling, but I am concerned that the temperature in the rafter bays will be too warm for the romex cables."

   It is not too hot for to use Romex.

   But it MIGHT be too hot for this APPLICATION.

   First is this NM-B. That is rated 90 C, but if this is retro-fiting it might not be and that is rated at 75.

   And you did not indicate the application.

   Wiring does start to get derated above 30C (86F).

   At 120 F the wire derated to 82%.

   BUT #14/#12 90 degree NM the ampacity rating used for temp derate is 25/30 amps. So after derating it is 21/25 amps. Which is fine for common 15 amd 20 amp lighting and recptacle circuits.

   However, if you have a feeder or high amp cable, say to an AC, you need to do some more checking.

1. 
   Mattohio | May 28, 2003 06:07pm | #6

   Bill,

   Thank you for the information. My cable is new NMB and is being used for receptalces and lights. I appreciate your advice.

   Thanks,

   Matt