what program could i use to create a diagram for a Power Distribution of a factory?

No easy software, but in the uk generally you would use autocad, autocad LT is only £500.00 or use excel/powerpoint or visio (word isnt too good for this type of thing. may be able to get a freeware cad program that will do a dxf file?

amtech or hevacomp are expensive and although intended for electrical design have poor quality output.

There were no "as-built" circuit diagrams made when the factory was built? If not (that would be a code violation in the USA), there is no easy way. You will need to trace and sketch all of the circuits from the utility entrance down ward and ID what loads are on each circuit breaker/fuse. Then use AutoCad which has a library for common symbols including panels, switches, breakers, fuses, receptacles, etc.

A simple diagram will not work. The electric/power requirements should be balanced.

Lets make this simple:

We have a simple factory and it is all in one building.

90% of all the machines are going to be operating all of the time. Th other 10% only as needed.

You need to count the total load of each machine in watts (Ohms Law).

Lets say that you have 100 amp breakers and you have 5 of them in the main panel.

You now have to make sure each breaker is used equally.

AutoCAD would be the first one id use, or Visio (Microsoft office) which also has electrical symbols.

AutoCAD electrical costs around £4,500 in the UK..

i think you want draw a single line diagram?

for a very basic SLD you could use Microsoft word..

I would use a Flow Charting program.

Excel allows creating something like a flow chart, plus you can insert computations.

I use Excel for almost anything I do.