There is a term known as DE-RATING FACTOR of cables.
What are the reasons for this de-rating of cables.?

Anything which causes the cables to heat up more than the nominal conditions under which they are rated. For example, stacking multiple cables together in close proximity causes them to heat each other (similar to the way that two people snuggling together in bed are warmer than one person alone). Also, if the ambient conditions are warmer than that of the nominal rating. For example, if a cable is rated for burial in the earth at 25 degrees C, but the soil temperature is actually warmer than that, the cable will be derated. Likewise, if it's in air that's warmer than the typical rated temperature of 30C. Also, if the cable is exposed to solar heating or other heat sources, it must be derated as well.

These charts (http://thecablediretoryblog.com/2008/03/18/electric-power-cables-%E2%80%93-whatever-happened-to-the-factor-of-safety/) will give you some good examples for the various factors that cause derating.

Every cable has the ambient temperature for example 40*c if the temperature of the surrounding is greater than the ambient temp of the cable then the conductor of cables become heat up which increases the resistance of cable and decreases the current flow so the cable will derate

BS7671 gives several methods of grouping and methods of laying cables which affects the current rating.