I have a 150 amp service feeding a 100 amp main lug sub panel. I need to move the sub panel to another location during a remodel. My question is what size copper wire is needed to lengthin the panel feeds, including the ground. The total distance from the main is approx. 40 feet. Planning to use insulated bugs on the mains.
My plan is to pull all necessary circuits from existing panel location into the attic and use a getter box to make up joints to lengthen the circuits. This box being a 10"x10" galvanized should be grounded. Is this correct?
In additon to these concernes, I have investigated the existing sub panel and found whoever installed it landed the ground from the main to the neutral buss bar. This should have its own ground buss bar. Am I correct on this?
Any helo would be greatly appreciated.

#4 THHN/THWN copper is the minimum size wire allowed for a residential 100 amp feeder.

What do you mean that you plan to bug the main? A 100 feeder for a subpanel needs to be connected to a 2 pole 100 amp circuit breaker. It cannot be bugged onto the main lines feeding the main panel.

All metal boxes in an electrical system needs to be grounded.

Yes, the equipment ground must be separated from any neutral bar in a subpanel. The neutral bar is to be insulated from ground, and equipment ground bar is bolted directly to the panel box.

I do in fact intend to bug onto the 100 amp main panel feed to the sub panel. Didn't mean to scare you. What is the correct size ground wire required? Is this also a #4?

So you plan to connect directly to the 150 amp rated main incoming feeder ahead of the main breaker? With this plan there will be no overcurrent or short circuit protection at all.

If you plan to connect to main lines downstream of the 150 amp breaker, you would have both protections, but the 100 amp feeder line will not be protected properly for overcurrent.

Neither plan is allowed, or a safe practice.

Is the "100" amp in the second post a typo? First post stated a 150 amp main.

I assume the ground wire you refer to is the equipment ground that is needed for the new feeder. This would be a min of #8 copper.