What is formula for calculate size of cable?

Answer 1

CABLE and WIRE are not the same thing.

Cables are sized by the diameter of the longest side (romex isn't round, it's an oval, compared to CAT 5, which is round)

Wires are sized based on the type of insulation and gauge of copper.

Then you take that info to the code book. A chart tells you the answer. BEWARE: multiple cables or wires in a conduit changes the equation.

Whether or not more will fit, the only correct way to size cables or wires is to use the Code Book.

For UK Installations

This topic is covered in Appendix 5 of the On-Site Guide supplement to BS 7671:2008 Requirements for Electrical Installations (the 'IEE Wiring Regulations'), although this Appendix only relates to the ease with which cables can be drawn in, and does not take into account the effect of 'grouping' (i.e. the effect that the temperature of each conductor will have on the rest). The number of cables permitted depends upon (1) the total cross-sectional area of all the cables drawn in, (2) the length of run, and (3) the presence of bends.

The effect of 'grouping' is to reduce the number of cables actually allowed within a conduit (or other enclosure), because, as the number of circuits increases, the current-carrying capacity of the cables reduces. Guidance on this is provided through 'space factor', which is the ratio of the (total csa of all the cables) to (the internal csa of the conduit), expressed as a percentage, with the maximum permissible space factor being 40%. The remaining 60% space is required to allow the dissipation of heat from the cables. (As the first answer indicates, the csa of a flat cable is based on the maximum width of that cable).

Answer 2

The width of the cable tray is determined by the size of the cable to be installed into the duct system plus the total insulation needed to meet regulation 392.80(A)(2)(d) UL standards.

Answer 3

cable sie for DC = (current x loop length x 0.019)/ voltage drop

where 0.019 is conductivity of copper cable