It's all about safety.

60 deg. wiring are wires that are not protected against the elements, and thus have no physical barrier.

90 deg. wiring are protected against the elements and do contain a physical barrier.

Additionally, an electrician needs to upsize a wire size depending on the location due to ambient exposure. For example, a #12 is good for 20A with no ambient exposure, BUT it is only good for 17A with ambient exposure.

Azlighting -

What is the 'physical barrier' you are talking about? Is this just Romex, where the 'barrier' is the outside plastic liner, and then each wire with the individual insulation?

I know that I'm just finishing up a huge retro fit of the home my wife grew up in... Originally had only two circuits in the entire house... Fuses. Started this about 5 years ago, and we both agree we never want to do it again...

Anyway - All knob and tube now removed. All new grounded circuits installed, (standard romex of course...)

Someone else earlier mentioned something about the 90 degree wire, so I'm truly curious. Since I know that I wouldn't really be thinking that installing some 60watt (or even 120watt) light fixture was going to cause some problem...

On our Retro-fit we are LONG past wiring... we're finishing up finishing touches... built a bathroom, bedroom installed flooring, sheetrocked walls, built two porches, framing of roofs, installed all new plumbing, etc.

But I would like to know about this wiring thing... probably best thing would be for me to take my own common advice - get me a current book - and read about it...

NM wire - pre 1985 - is 60ºC wire
NM-B wire is 90ºC wire. The -B is the indicator. If you just finished you house wiring then you should not have any 60º wire to worry about.

The ambient temp would need to be over 70ºC/158ºF before derating kicks in for #14 or #12 cable.

Joed & Azlighting -

Thanks very much for the info.

Sometimes the problem of heat-damaged conductor insulation is caused by over-lamping, using wattages greater than fixture design. Sometimes the heat is great enough to damage the drywall surrounding the ceiling box. I see this often in rental properties. One solution to both problems is to use a surface extension box, such as Wiremold
V5738. Splice short lengths of THHN to the wires in the existing box and have only the THHN in the extension box. The Wiremold box also acts as a spacer to create some distance from the heat source. The solution is not good esthetically, but is good technically.

Here is a link that might be useful: Wiremold

I can second the disasters of trying to work with previously-over-lamped fixtures and 60deg wire.

Some of the worst offenders are the box-store "fixtures" which hold a couple bulbs and glass over them, in one way or another. Many say "60W MAX" or the like, but most people ignore those warnings. The result can be cracked, disintegrating insulation (at best).

If you don't want to rewire, I'd:
* Religiously adhere to the wattage limits
* Consider CFLs
* Avoid fixture designs that hold bulbs close to the junction box or contain the bulbs' heat in a way that heats the junction box
* Make sure you leave the foil/insulation in the base of the fixture

this is most timely as I am enountering this problem now. I Recently replaced an original to the house porch light (on the ceiling) and the new light came with that warning about the wiring. I asked about it here and was told I should have the wiring replaced. I have contacted two electricians now to do the work and both have said it wasn't necessary. They acted as if this was not a problem at all. That it was perfectly safe. Then why the warning label? The electrician who installed the light also removed the foil part. I used the correct watt bulbs, but I still worry. Is it a huge job to rewire a light? Should I keep trying to find an electrician who is willing to do the work? Better safe than sorry I suppose but it is frustrating when you want to hire someone to do the work and they seem to think you are overreacting to the problem.

Abbey

The "foil part" should NOT have been removed. Do not allow that person on your premises ever again. The insulation/foil sandwich is designed to lessen the transfer of heat from the fixture to the mounting box. What was that idiot thinking? Have this corrected immediately.

thank you Bus Driver, for confirming my worst suspicions when I found the foil/insulation covering on top of my garbage can. I have been using this particular company for some time, and this particular electrician put in 3 other new lights for me. None of them required the 90 degree wiring however. I will try and find another electrician who will be willing to replace the wiring for me. I don't understand why the two I have contacted felt it wasn't required, but if necessary I will insist upon it.

Abbey