How can I install standard outlets on a 30A #10 multi-wire branch circuit?
Subpanel Time!
You can't run any 15A or 20A devices (e.g, 15A or 20A receptacles or light fixtures designed for 15A or 20A circuits) on the 30A breakers. But you can install a subpanel. This will let you put as many things as you want on the circuit. From a practical standpoint, you could definitely put a subpanel with 4 x 15A circuits (15A x 2 = 30A on each leg) but you could actually install more - e.g., 6 x 15A or 4 x 20A or some other combination. That is because the 30A limit is based on the total simultaneous usage. Since most of the time most of the smaller circuits will have very little use (e.g., a lighting circuit might pull only 1A for LEDs to light up the whole garage), this can work very well. About the only thing you wouldn't want to put on this subpanel is a heavy 240V load like a water heater or air conditioner.
The subpanel can be any size 30A or larger. As I understand it from other Q&amp;A, in an attached garage you generally don't need a main breaker in the subpanel, but it is OK (and often convenient) to have one, and it can be any size - larger than 30A is perfectly fine as it functions here as a switch and protection for the subpanel but not for the wires back to the main panel. Ground and neutral not bonded in the subpanel. And don't forget GFCI - most garage circuits will need to have GFCI protection.