90 degree C supply conductors

Reply

  #1  
Old 10-30-05, 07:57 PM
Bob B.
Visiting Guest
Posts: n/a
90 degree C supply conductors

Need to replace a ceiling light fixture in a 1983 house. Virtually all fixtures on market require 90 degree C supply conductors and state most pre-1985 houses have 60 or 75 degree conductors--first time I've seen this. From research I learned that NMB wire is 90 and NM is 60. From what I can determine all my original wiring is NM--I used NMB on many circuit extensions, etc, over the years.

Advice I've gotten ranges from "just pigtail some NMB and install the fixture" assuming I can get it all back in the box, to "you need to replace the entire branch circuit". Help on this please.

Also, I have no clue what my main 200 amp panel supports, but a newer sub-panel states on the circuit breakers that they only support 60/75. So is the panel still a fatal weak link regardless if I pigtail or replace circuit?
 
  #2  
Old 10-30-05, 11:39 PM
C
Member
Join Date: Oct 2004
Location: Yorba Linda, CA
Posts: 338
Received 0 Votes on 0 Posts
Originally Posted by Bob B.
Advice I've gotten ranges from "just pigtail some NMB and install the fixture" assuming I can get it all back in the box, to "you need to replace the entire branch circuit". Help on this please.
Neither suggestion is correct. One won't work and the other isn't necessary.

The purpose for the rule is to keep the wire in the fixture box from getting all crispy from heat and shorting out. To keep that from happening, you need to use 90 degree C wire in the fixture box, but it can be 60/75 degree wire elsewhere. The most common way to do this is to pull the end of the old NM out of the fixture box and land it in another box is some accessible location - usually the attic. Then run NMB from this junction box back to the original fixture box and hook up the fixture.

Also, I have no clue what my main 200 amp panel supports, but a newer sub-panel states on the circuit breakers that they only support 60/75. So is the panel still a fatal weak link regardless if I pigtail or replace circuit?
Panel is fine. The 60/75 marking indicates that you can only use the breaker to protect wire that is sized according to the 60 or 75 degree rating for that wire, not sized to the 90 degree rating. This is not significant in 14, 12 and 10 gauge wire - the ampacities of these sizes is fixed at 15A, 20A and 30A regardless of the temp rating of the wire.

As a practical matter, it's easy enough to pull in the right wire if there is an attic above. On the lower floor of a multi story house or in houses without attics it gets a bit trickier, and plaster removal becomes a distinct possibility. The best way around this problem is to select fixtures that do not have this restriction - notably pendants and other fixtures where the fixture hangs down a bit from the fixture box. There are lots of them that look great, even if they are a bit more expensive or not just exactly what milady had in mind.

If milady insists however, get out the hammer...
 
  #3  
Old 10-31-05, 08:03 AM
Bob B.
Visiting Guest
Posts: n/a
Thanks Cheyenps!

I'm quite sure I can get to it from above and not have to get milady involved.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: