Old wires too short to reach new main lug connections

EndlessGrowth

right, typically only connect neutral & ground at main.

yes 3 new 20A GFCI'S on 12/3 mc/bx, but two will only be on 15A breakers for now (20A 3-pack was on sale! and bigger is no prob.). other 20A circuit will power existing old outlets and shop lights (not gfci there). i just set everything for 20A just in case later i need to run bigger feed wire/breaker than this 30A #10.

but now that i think about it, can't i just install two twin 20/20 breakers instead of these two 15/20 in main lug? from what i've seen in various homes, total sum of breaker max's DOES exceed feed/main/box rating (i assume since everything being energized at max, at once is rare).?

EndlessGrowth

actually there is a smaller main lug i saw at HD yesterday that has a rear penetration a few inches closer to the lugs than this one i have now. i'm going to go look at it again tomorrow. i didn't get that one initially because the ground bar they sold would not seem to fit in there right unless it blocked the right KO entry, but i'll try again. if i can connect w/o pigtails, i'd much rather do that.

actually, do i even need a ground bar in a main lug, or can i just wire nut the two grounds coming in from the two 12/3 cables (feeds 3 GFCI's and 1 circuit to lights & old outlets) to the main feed ground?

thing is that panel is only a 60A box instead of this 70A one. so now i question; what determines the rating of the box? the bus bar size? i'm assuming not the sum of the breaker maximums in the panel? because every panel i've ever seen, the total breaker amps far exceeds the main.bkr/panel rating.