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.
20Sqmm
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.
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.
An electrical sub panel is a smaller version of the main panel. It is a way to move many circuits to a point further away from the main panel instead of taking many single circuits from the main panel board. The sub panel has just one supply feed from the main panel. By using a sub panel there is a cost savings in labour and materials. On larger buildings there may be multiple number of sub panels. The amperage rating of the sub panel board governs the size of the wires and the breaker used in the main panel to feed it.
It depends on where the sub panel is situated. If the panel is under the same roof as the main distribution panel then no ground rods are needed on the sub panel. If the sub panel is in a different building then a ground rod might be needed. There has been an electrical code rule change on this subject. Check with your local electrical inspector before installation so that it is done correctly.
20Sqmm
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.
AWG 2/0 copper.
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.
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.
A #1 copper conductor is rated at 140 amps with an insulation rating factor of 90 degrees C at 240 volt single phase, will give you a full 100 amps at 200 feet. #4 wire should give you 95 amps. Anything over 200 feet you will start loosing more amps.
A 125 amp panel or subpanel requires # 2 gauge feeders and neutral. (# 2 awg is rated for 130A) However, depending on the existing load in your main 150A panel you likely don't have enough power to operate a 125 Amp subpanel.
Normally you would use AWG #6 but for this long run and voltage drop I would recommend you use AWG # 4.
An electrical sub panel is a smaller version of the main panel. It is a way to move many circuits to a point further away from the main panel instead of taking many single circuits from the main panel board. The sub panel has just one supply feed from the main panel. By using a sub panel there is a cost savings in labour and materials. On larger buildings there may be multiple number of sub panels. The amperage rating of the sub panel board governs the size of the wires and the breaker used in the main panel to feed it.
It depends on where the sub panel is situated. If the panel is under the same roof as the main distribution panel then no ground rods are needed on the sub panel. If the sub panel is in a different building then a ground rod might be needed. There has been an electrical code rule change on this subject. Check with your local electrical inspector before installation so that it is done correctly.
The main items required for an electrical panel include the panel box itself, fuses, and wiring. The wires are fed into the box and attached to a series of circuit breakers to allow electricity to be directed as needed.
if it is the main panel outside yes, you could have a sub panel inside that come from the main panel