Cable Sizing

The common practice here is British Standard 7671. Oversizing is wasteful.

If you have taken ALL the parameters needed into consideration, then you NEED NOT oversize.

Quite.

It takes more electricity to fill those larger cables, so you'll always have a little reserve juice left over.

But, if it stays in the conductor too long it will get stale.

Does one of these parameters also include expected future increases in load current due to expected plant expansion, future equipment upgrade, etc?

If so then using a larger cable size is much better than having to re-run a larger cable later on, if not then there is no reason to use a larger cable. Copper ain't cheap.

...in which case the cable would be correctly sized and protected for the larger duty, ready for when the duty comes on. To describe it as oversized would be inappropriate.

It is not a common and general practice to select the next higher size cable if all considerations of cable sizing are appropriately considered. However, many engineers do it to be more safe. In addition to the excess cost of the cable, selecting higher size cable may cause some other issues, such as cable termination, cable bending, space in raceway, excess weight etc. When you select higher size cable, make sure not to arise these issues.

Believe me or not, sometimes cost saving happens with selecting higher size cable. For example, there are five motors to be installed, three of them require 4 AWG, each 400 feet (total 1200 feet) and other two require 2 AWG, each 400 feet (total 800 feet). The cable vendor has minimum order quantity 2000 feet for each size of cables. If we use both size cables, we need to buy 2000 feet 4 AWG cable and 2000 feet 2 AWG cable. But, if we use all 2 AWG cable (it is higher size cable for three motors, 2 AWG instead of 4 AWG), we can completely avoid buying 4 AWG cable and it saves almost 50% of the cable cost.

- MS

Check this for complete Detail
Conductor and Cable Sizing