I am currently in the process of remodeling my basement. I want to add hydronic radiant heat above the slab and also joist cavity hydronic radiant heat in the floor joists to heat the main floor. The slab installation appears pretty straight forward. However I have a question I cannot seem to find an answer to anywhere...Since this house is an existing construction home and the house already has wiring completed, is it even possible to install radiant heat in the joist cavity? Would the heat after the reflective barrier is installed increase the temperature in the joist enough to degrade or damage the existing wiring's insulation? There is curently electrical, telephone, lan and cable lines already running through the joists. Lastly, if it is safe to use this type of heat in the joist cavity with existing wiring, is there a minimum distance the tubing must be kept away from the wiring? Your help is greatly appreciated. I am at a stopping point with my installation until this question is answered.

The other wires make it more difficult to install as a retrofit. Usually radiant is installed to the flooring or just below it and the other wiring is done below that. The only space needed for the floor radiant is 2 inches or less below the flooring. The space below the radiant tubing insulated, or else the radiatn heats the lower floor as well.

"Stetson explains that prior to 1984, the National Electric Code required the insulation on wiring used in farm buildings to be rated for 140[degrees]F. (60[degrees]C.). After that date, the code required a rating of 194[degrees]F. (90[degrees]C.).

"It's important that the temperature rating for the wiring in the building match or exceed the rating for the fixture. If the temperature rating of the wiring is exceeded, the insulation around it becomes brittle and may break away. This allows arcing between bare wires, which causes heat that melts the fixture," says Stetson.

Because most farm buildings built before 1984 have wiring rated at 140[degrees]F., Stetson used that as the maximum in his study. Amazingly, he found that with some bulb and fixture combinations, the maximum temperatures shot to 204[degrees], 261[degrees] and 262[degrees]F." http://www.findarticles.com/cf_dls/m1204/11_98/66498183/p1/article.jhtml

Usually rdiatn that is applied directly to the floor material does not run over 140F, so wiring well below that whould remain unaffected. Some installers suspend tubing a few inches below the flooring and use 180F temperatures to convect upward. The affect on wiring below that tubing, especially if insulation separates themn is unpredictable.

I think the link you are referring to is mainly discussing the curent carring characteristics of wire in conjunction with appropriate wire sizing for it's application. In such uses, the result of using too small a wire with a larger current draw is increased heat. I am referring to ambient temperature. While Additionally, musch of the wiring i am referring to has no current draw to speak of eg..telephone, lan and cable lines. The lan wire i have is rated at 75 degrees C. This temperature rating is on the outer insulation of the wiring. However, the telephone and RG6 cable tv wiring does not have a temerature rating. Additionally, this house was built after 1984 and there is also no temperature rating on the 12/2 grounded electrical wiring. (either existing or newly purchased)I see on the link you referred me to it stated that as of 1984 the electrical wiring must be rated at 194 degrees F for farm building. It does not specify if that code was also carried over to residential buildings. But since this document is referring to non-residential wiring codes and is referring to the current carrying characteristics of wire, i am not sure if it really answers the question at hand. Thanks for the assistance, if there is any further information you or anyone else has, please let me know.

Only your local code official can tell you what he will accept. The information given is to discuss the concept. An amperage rating temperature limit would be a limit, IMO, because if the temperature in normal ambients is the rating, the actual temperature would limit the amperage further.

The important information is whether you will place insulation above the wire to protect it, or are you trying to keep it inside a hotter space.

"My rule of thumb is to go up to 100 ft. at the 60 deg C amp rating (20a for 12ga.) of the wire for 3% or less voltage drop, 1 phase. That's for normal residential wiring where you're allowed 5% total drop from the service entrance. It could put you a hair over 3% in some cases. Also, it's for 120v. For 12v, use 10% of that (10ft., 1 way).
Most of these formulas on the internet are for house wiring or DC applications. Note that DC wiring usually has higher amperages for ea. wire size (often 105 deg C rating) & therefore higher voltage drops. Residential wiring should never use more than the 75 deg C amperage rating (you can't get 90 deg C connectors) AC or DC. The NEC tells you how to allow for ambient or conduit (as opposed to "in air").
" http://www.fieldlines.com/story/2003/7/20/155947/196

"General purpose wiring forlighting and power - residential, commercial, industrial buildings in accordance with the NEC, maximum conductor temperature of 75°C in wet or dry locations, for circuits not exceeding 600 volts" http://www.thomasregister.com/olc/17044454/sectionb.htm