Electrical Panel – How to Route #8 THHN Wires into an Existing Panel

I'm amending this answer to correct a misconception.

There is nothing wrong with this installation

I realize there's a trend these days in society where people want to destroy or conceal all utility space. As if they're shameful or something. Fine, then, go be Amish. For the rest of us... utilities are a miracle.

Surface mount is particularly correct for a garage, because stuff in garages changes - that's precisely why you got such a large panel and surface mounted it in the first place. Since the garage is utility space, burying the panel in a wall is just dumb.

The problem, really, is the Code requirements to physically protect wire/cable from damage. And this is where you've "split the switch". You adopted the surface mount/accessible/maintainable philosophy which is correct, except you are still "married to" concealed wiring methods (NM/Romex) which rely on the walls for physical protection.

The pro way to proceed from here is multiple conduits. (Because of the "4 circuits per conduit" rule). Normally I'd run 1/2" or 3/4" conduit, but this is where that NM/Romex wiring method gets in the way. Cable takes a lot of room in conduit, because it's treated as a single wire of the widest dimension (since it twists). So NM-B cable is 0.40" wide as far as conduit fill is concerned, and that requires fat conduit.

Keep splitting the switch: run a hybrid THHN/Romex setup

One very elegant answer is to run several smallish conduit (1/2" or 3/4") to locations of your choosing, and proceed from those locations with normal Romex/NM. The box becomes like a "mini panel" - you don't have to reach the panel with the Romex, only one of these boxes.

Inside the pipe, use wires made for pipe - THHN individual wires. This is a fantastic way to work, as the individual stranded wires are flexible and supple, not the gator-wrestling of working with Romex (can you imagine trying to thread Romex through a hole in a stud!? Ha ha, who would put up with that?) You do have to "do the pipe-fitting" to get the conduit up there in the first place, but the wiring is easy street.

And from a novice's perspective, it's easy to fit the pipe, check it out, shoot photos/have it inspected, revisit if needed, and have the physical installation be perfect. Then add the wires, which is straightforward work.

A 1/2" conduit can fit 4 common circuits (that's the max anyway due to thermal limits) and 3/4" can fit big stuff (and other circuits too). Metal conduit also serves as the ground "wire" so no need for wires proper.

EMT metal conduit is also a great way to deal with outlets or devices that are "right next to the panel" e.g. on the same piece of plywood. A short EMT nipple, steel junction box, two THHN wires and you're done!

On certain upgrade-friendly runs, run conduit/THHN all the way

Especially if you wind up walling the wiring off behind drywall.

You would do this where you might have a variety of shop tools, e.g. a saw that you might upgrade to a 240V saw someday, or an EVSE (go big on that conduit - Tesla chargers can draw 80A!) Then, upgrading the wires to that location will be a snap.

Working with the Romex

Your method #2 will require a way to guard the NM cable as it exits the panel and goes in to the plywood.

Your method #3 will require special cable clamps that can affix from one side (since you can't access the backside). My concern is the cable clamp won't work properly unless the hole-behind is larger than the knockout, and how can you accomplish that??? (I imagine 10 minutes of grinding with a Dremel or something).

I think you have the right idea with the conduit. There's only one snag: You're only allowed 4 circuits in a conduit before a thermal derate becomes necessary. The thermal derate means bumping one wire size for 5-10 circuits in the same conduit, and two wire sizes for 11-20 circuits.

The cure to this is to run multiple smaller conduits. NM-B cable in conduit takes a lot of space, though, as it counts as a single wire of the wide dimension.

14-2 NM-B is 0.36" across. 14-3 NM-B is only 0.31" across (it's basically round).
12-2 NM-B is 0.40" across. 12-3 NM-B is 0.37" across (again rounder).

Punch those numbers under "custom" in a conduit fill calculator and it'll tell you what pipe sizes will work. Generally it looks like 1-1/4" is the "univeral donor" and 1" will work with three /3 or #14 (since those are smaller).

Ground yes. Neutral no.

You can do that with safety ground.

You cannot do that with neutral. Neutral needs to come over to the new panel that the hots are served out of.

This is easier to justify when using a type of metal conduit which itself is a valid grounding path, which is typically NOT the flexible kinds. EMT is a little more work but takes care of the grounds for you.

There are also good reasons to try to keep inter-panel conduits shorter than 24". Otherwise you are only allowed 4 circuits in them and must significantly derate the large feeder if there are other circuits in the pipe.

There is nothing wrong with using a service panel as a junction box to hold the wire-nuts that extend the circuit. Note this is a NEC interpretation, that may not work in Canada.