I have a 900VA/480Watt UPS/surge unit, and it doesn’t quite reach the wall where I need to plug it in. I'm using it to power an iMac. The unit has a 3x16 AWG cord. Is there any harm or difference, in using a heavy gauge extension cord that matches or exceeds the original cord on the unit itself? Say a 10,12,or 14 AWG cord to connect it to the wall? Or even a 16 AWG cord (like the actual gauge with the unit)?
It's a heavy load like an air conditioner, so use an air conditioner extension cord.
Manufacturers can be greedy/cheap about sizing extension cords, using the absolute minimum UL requires for safety. Bumping to the next size could cost them 25 cents a unit wholesale, which on 100,000 units, is real money.
Using a same-gauge extension cord is not a good idea, then you would be substituting 6 feet of 16AWG for 12 feet of 16AWG, and that is worse.
900vs is only 7.5 amps a standard 16 gauge cord would be plenty. Unless you want to go +50' . cords have different ampacities than house wiring. Per table 400.5.a.1 , 16 gauge 13 amps.