Replies

1. 
   BillHartmann | May 15, 2003 01:40am | #1

   I don't think that you can get NM is that size, but maybe.

   But you can get SE type R. That is what you is used that for that application.

1. 
   dayoff | May 15, 2003 03:16am | #3

   Thanks, does the SE R cable need to be in a conduit? 2271

2. 
   MisterT | May 15, 2003 01:45am | #2

   4 guage wire!!??

   You gonna be welding in this bedroom?

   Mr T

   Do not try this at home!

   I am an Experienced Professional!

1. 
   dayoff | May 15, 2003 03:24am | #4

   I actually used to arc weld in that room (was a shop), but am not an electrican.  I did wire this house 17 years ago and it hasn't burned down yet.  :-) 

   The code appears to ask me to overrate the wire becasue it needs to be derated because of the attice heat.   I am still only going to use 50 amp breaker at the head of the wire.

   and am not designing an overload.

   Any more help/ideas?   2271

1. 
   fireball | May 15, 2003 05:47am | #5

   Type SE cable is Service Entrance cable.Like NM it has a thermoplastic jacket around insulated conductors.Some SE cable has bare strands for a neutral wrapped around two insulated conductors.For a subpanel get the cable with three insulated conductors(two hots and a neutral) and use the bare strands for an equipment grounding conductor.The ampacity depends on the insulation on the individual conductors,but on a 50 amp breaker I'm sure your # 4's would be comfortable.The lowest category of insulation is rated at an ambient temperature of 140 degrees,does your attic get that hot?Next level up is 167 degrees.If your attic gets that hot it's time to move North.

1. 
   joeh | May 15, 2003 08:11am | #6

   If the attic's that hot it's time to call the Fire Dept....

   Joe H

2. 
   getgo | May 15, 2003 11:14am | #8

   Maybe the wife needs more power.   [email protected]

3. 
   4Lorn2 | May 15, 2003 09:33am | #7

   If your attics are anything like the ones around here, Florida, they can get Very hot. I used to take a digital thermometer into attics with me. Bragging rights on the misery index I guess. The hottest I recorded was 157 degrees Fahrenheit. After reading this temperature I stopped carrying the thermometer preferring to just call it too damned hot. At those temperatures a 20 minute visit brings me to the point of not caring is I come out on my feet or on my head. Longer visits can be a real hazard. Know your limits.

   The derate for temperatures of 141F to 158F is (.58). SE and NM cables have to be used as rated at 60C but, because the actual insulation as opposed to the jacket is rated at 90C, the difference in amperage between a 60C and 90C of any given size is allowed to be applied to the derating. 

   A 90C #4 copper wire is, I'm using table 310.16 from the 2003 NEC,  rated for 95A. This 95A has to be multiplied by the .58 derate factor giving you slightly more than 55A. So placing it on a 50A breaker is fine.  The #4 is a good choice. and I have seen cables in this size. 

   If you could run the cables in a conditioned, or generally cooler, space the derate would be much less but derating for temperature starts at 30C (86F) so most places south of the Mason-Dixon line have to deal with this.

   While agree with your interpretation, I think it is what a conservative interpretation of the code calls for,  some local authorities don't see it this way. They claim that if you  tuck the cable under the insulation, as if the plumber and AC man won't pull it up at their leisure, the highest temperature that must be calculated in is the highest outside air temperature. So, if you wish to have a legitimate reason (The AHJ always has the final say) to cut corners, you might want to talk to the local authority having jurisdiction. 

   The idea that you can use a conductor at a temperature anywhere near the temperature rating of the insulation is a popular misconception. Just because the insulation doesn't melt while the circuit has no load on it does not mean that  it won't when it heats up, as all wires do, under load in a hot environment while buried in insulation.

   As with any construction work be careful. Know your limitations. Take care and be safe. A days work by an electrical contractor is still cheaper than a trip to the hospital or a funeral.

1. 
   dayoff | May 15, 2003 02:52pm | #11

    

   Thanks much for your help.

    That’s what I was looking for – verification of derating and other nuances that the code doesn’t seem to have. 

    I no longer trust the local AHJ to have the correct answer, although I do know they have the authority.

   I asked them about vapor barriers in 1987.  And even the code was wrong about vapor barriers in the South – since they now recommend that vapor barriers go on the outside of the wall here in Houston or not used at all.(ref FHB No. 151, page 53, insert box and alsohttp://doityourself.com/insulate/vapordiffusionandairbarriers.htm).2271

4. 
   getgo | May 15, 2003 11:18am | #9

   Try these sites,

   http://iguana18.securesites.com/store/merchant.mv?Screen=CTGY&Store_Code=bes&Category_Code=y

   http://www.kingwire.com/inet/invsrch2.cgi?sid=69fd56d7&select2=PortableCord.htm

   [email protected]
1. 
   dayoff | May 15, 2003 02:18pm | #10

   Thanks!