Cable Runs and Current Rating

→ British Standard 7671.

The "current" carried by a cable is not the answer to your question. The process of rating a cable for its safe current handling capacity looks closely at the ability of the cable to dissipate heat. Overheat a cable, you stress its insulation, stress it far enough and the insulation breaks down and fails. When it fails, the "magic smoke" (as one forum member calls it) gets released to the atmosphere.

So... the mutual heating effect caused by the bundling of multiple cables is what you have to prevent. The Canadian Electrical Code rule 4-004(11) speaks to this issue. How we regulate this issue in Canada is by controlling the maximum length of the "bundled" section of "multi-conductor cables". The length specified is 600mm.

If the length of the bundled section is less then 600mm, we do not care how many cables you bundle together. Why? Because in a distance less then 600mm, the mutual heating effect is considered negible. If the length of the bundled section is over 600mm in length, here in my jurisdiction, we do not allow multi-conductor cables to be bundled together.

What does "multi-conductor cables" mean? We allow no more then 6 current carrying conductors (3 x 2 conductor cables) to be bundeled together for a distance longer then 600mm.

It really depends on the electrical standard you are working to, and the application.

The actual de-rating value (not current as current varies depending on numerous de-rating factors applicable to your application and the way you mount or enclose the cable(s)) for parallel cables sharing current should be indicated in the applicable electrical standard you are working to. You may also find it in cable manufacturer literature available of their website.