[RiverRock]

I am finishing up a bathroom addition and need to add a subpanel next to the existing 200A main panel to clean-up several 'doubled-up circuits I found in the main panel as well as provide breakers for the new bathroom installation.

I plan to install a 100A sub-panel within a foot of the existing panel and although the sub-panel I'm purchasing has its own 100A main breaker I will be feeding the sub-panel with a 2-pole 60A breaker from the main panel. From what I read on this forum, that's okay to do, correct?

This also means that my actual load capacity for the 60A breaker is 48 amps, right?

Is a length of 4 wire #6 copper the correct feed wire size for this application?

If, when I remove the doubled-up branch circuits from the main panel and attempt to move them into the sub-panel I find that they are too short, do I simply add a junction box outside of the two panels to add on to the wiring?

Since I am using a main panel as a sub-panel, I also need to ensure that the neutral bus is isolated from ground - is that correct?

Looking at the existing main panel, the branch circuit wiring is a mess. ( some of the wires barely make it to the breakers, others are long enough to go around the box twice and are all coiled or scrunched up.) what is the general rule for determining the length of wiring inside the panel box?

What about the main feed wire - how long should it be inside the box?

Thanks for taking the time to help me out. I'm working in a new community and interacting with the building/electrical inspectors for the first time. As they say - "You only get one opportunity to make a good first impression".

[KISS]

I am finishing up a bathroom addition and need to add a subpanel next to the existing 200A main panel to clean-up several 'doubled-up circuits I found in the main panel as well as provide breakers for the new bathroom installation.

I plan to install a 100A sub-panel within a foot of the existing panel and although the sub-panel I'm purchasing has its own 100A main breaker I will be feeding the sub-panel with a 2-pole 60A breaker from the main panel. From what I read on this forum, that's okay to do, correct?

It's OK to leave the 100 A main breaker in place. You will likely have to purchase a ground bar kit and make sure the screw that bonds the neutral to ground is removed. The Ground and neutral buses must be isolated in the sub panel.

This also means that my actual load capacity for the 60A breaker is 48 amps, right?

Not exactly. It means that if all the loads were on more than 3 hours a day they would be counted as "continuous". 48 A is the maximum the panel can sustain forever. Continuous loads are multiplied by 1.25 to arrive at their derated capacity.

Is a length of 4 wire #6 copper the correct feed wire size for this application?

Yes. Conduit and THHN might be a good choice to use here. You can tape the ends of the wires the proper colors if they end up being all black.

If, when I remove the doubled-up branch circuits from the main panel and attempt to move them into the sub-panel I find that they are too short, do I simply add a junction box outside of the two panels to add on to the wiring?

You can't use a panel as a junction box, but you can extend wires within the panel. Two wires on the same terminal are not allowed, but they can be pigtailed. There are breakers which can be used that will allow two 120 V circuits to occupy one space. They will not have the bar connecting them. The "#of pole" specification for the panel cannot be exceeded. Short answer - the number of terminals must be less than the # of poles allowed.

You can splice outside the panel in a junction box or there is one specific TAP/splice connector Modular Splice & Tap Kit - 194130062 that may be allowed.

Since I am using a main panel as a sub-panel, I also need to ensure that the neutral bus is isolated from ground - is that correct?

Yes. I answered it earlier. You will likely have to purchase an optional ground bar kit as well.

Looking at the existing main panel, the branch circuit wiring is a mess. ( some of the wires barely make it to the breakers, others are long enough to go around the box twice and are all coiled or scrunched up.) what is the general rule for determining the length of wiring inside the panel box?

I don't think there is one. Just remember that once a cable enters the panel, the outer jacket should be removed. Make it look neat and labeled and you'll probably earn points.
Even include labeling the Panels as Main and Sub.

What about the main feed wire - how long should it be inside the box?

Enough that if you have to restrip the cables somewhere down the road there won't be an issue.

Thanks for taking the time to help me out. I'm working in a new community and interacting with the building/electrical inspectors for the first time. As they say - "You only get one opportunity to make a good first impression".

And "if you don't have time to do it right, when will you have time to do it over?"