The short answer, if using copper, is 0 AWG, commonly referred to as 1/0 or "one ought".
Type of wire depends on location & temperature, but in my common scenario I am using 1/0 THHN inside a 2" schedule 40 PVC conduit (3 - 1/0 wires plus a 4 ga bare ground) to run indoors, a distance of about 25 feet from my main panel to a sub for upstairs.
I could go into more detail regarding outdoor, direct burial, grounding, aluminum wire, etc but would need more details.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
2 copper
A 200 amp service panel with a 60 amp sub-panel.
Yes, the wire size has to be increased. The existing 100 amp service will now probably have a #3 conductor with an insulation factor of 75 or 90 degrees C which is rated for 100 and 105 amps respectively. Increasing to a 150 amp service will use a #1/0 wire size. If you are thinking of an upgrade, skip the 150 amp service and go to a 200 amp panel. New homes are installing 200 amp 42 circuit panels. The extra cost is only reflected in the materials as the labour costs will be the same regardless whether a 150 amp or a 200 amp panel is installed. The wire size for a 200 amp panel is #3/0. With an insulation factor of 75 or 90 degrees C the rating is 200 or 210 amps respectively.
For Service Entrance Cable 600 kcmill (Thousands of circular mils).
For a 125 amp service, a ground wire size of 8 AWG copper or 6 AWG aluminum is typically sufficient. However, it's important to consult with local electrical codes and regulations to ensure compliance with specific requirements in your area.
Yes, no problem at all going to a larger ampacity of wire. Larger size wire yes, smaller size wire no.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
AWG # 1/0 copper would normally be used.
2AWG.
2 copper
A 200 amp service panel with a 60 amp sub-panel.
10 guage
Yes, the wire size has to be increased. The existing 100 amp service will now probably have a #3 conductor with an insulation factor of 75 or 90 degrees C which is rated for 100 and 105 amps respectively. Increasing to a 150 amp service will use a #1/0 wire size. If you are thinking of an upgrade, skip the 150 amp service and go to a 200 amp panel. New homes are installing 200 amp 42 circuit panels. The extra cost is only reflected in the materials as the labour costs will be the same regardless whether a 150 amp or a 200 amp panel is installed. The wire size for a 200 amp panel is #3/0. With an insulation factor of 75 or 90 degrees C the rating is 200 or 210 amps respectively.
For Service Entrance Cable 600 kcmill (Thousands of circular mils).
#10 wire is used on a 30 amp service.
AWG 12 will suffice.