Module Wire Recommendation

Hi,

Your question is more general then related to Siemens modules. Wire diameter is calculated according the currect which will be used. On analog modules you normally use 20 mA, on digital modules it depends on the module (the currect). I can't tell how to select cable diameter according to currect, but you can find much of that info in the internet. Normally we take 0.5 mm^2 copper wire and its ok for mostly common used.
Also please pay attention to cable lenght also. Maximum alowable cable lenght depends on module used and you can find this info here SIMATIC S7-300 S7-300 Automation System Module Data