A 250 MCM copper conductor will limit the voltage drop to 3% or less when supplying 100 amps for 300 feet on a 120 volt system.
Three feet of free and clear space in front of panel.
Depends on the size of the sub-panel in that garage. If you are installing a 60 amp sub-panel 400 feet away from the main service panel then use AWG # 4.
A 1/0 MCM copper conductor will limit the voltage drop to 3% or less when supplying 100 amps for 250 feet on a 240/120 volt system. This calculation is for a full 100 amps at the panel. If exact amperage is known the conductor size can be reduced to match the known amperage.
AWG # 4
This is a voltage drop question. The sub panel's voltage needs to be stated to answer the distance part of the question. A #3 copper conductor with an insulation factor of 75 or 90 degree C is rated at 100 and 105 amps respectively.
Yes, as long as it has the necessary ampacity for the load and is properly fastened at each terminal. For example, we have a 4/0 aluminum cable feeding our 200 A subpanel 120 feet away.
Three feet clearance is needed in front of electrical service equipment.
Need to know what the load amperage is on the 100 amp panel. If interested, open discuss question page.
Buried or conduit #2/0 aluminum will exceed the NEC recommended 3% or less and provide a 2.72% drop Increasing to 3/0 provides a 2.26% drop Make sure you resize your ground.