J Box Sizing and Wire Derating

[Paulski]

I'm putting in a 100 amp sub panel to free up space in my main panel (kitchen is all electric)

What I have right now:
200A Main panel -> 2" FMC -> 16"x16"x4" box in the crawl space

What I would like to do for the feed is run 1G wire from the main to the 16" box, run 1.5" FMC from the 16" box to the subpanel

For all the appliance feeds (currently in 1/2" or 3/4" FMC conduits to main panel) I want to run 2" FMC from the sub panel to a pull box directly under it (in the crawl space) than tie in all the appliances to the new pull box to feed up to the sub panel. This way I can move over each appliance as time allows and have room for expansion later on that is easy to add.

What size does the new pull box need to be? I was thinking about using a 6"x6"x12" raceway box, but I would need to run the 2" FMC conduit to the 6"x6" face to allow for the 8x conduit size straight section. But since the pull box can be mounted directly under the sub panel, I think it would be cleaner/neater to mount the box directly under the sub panel and avoid a bend in the conduit, but than I only have a 6" straight section in front of the 2" conduit fittings. Is that ok since I am only running max size 6G wire in the 2" FMC?

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Edit:

So I just learned about derating wires when multiple current carrying conductors are in the same conduit so my plan to run everything through a 2" up to the sub panel may not work without upsizing every wire due to needing to be derated to 45-50% according to table 310.15(B)(3)(a)

Now my concern falls on the 2" from the main to the existing 16"x16" box, which will have (All THHN):
Table 310.15(B)(16)
2 6G on a 50A Breaker, wire run is ~40'
3 4G on a 70A Breaker, wire run is ~55'
3 1G on a 100A Breaker, wire run is ~20'
1 8G Ground

That's 8 CCCs that will need to be derated to 70%

6G rated for 75A derates to 52.5A
4G rated for 95A derates to 66.5A
1G rated for 145A derates to 101.5A

So everything is good except the 4G on the 70amp breaker, it will need to be either upsized to 3G or a 60A breaker installed

Does my math check out or is there anything else I'm missing?

 

[exranger06]

You don't need to derate conductors if the conduit is less than 24" long. 310.15(C)(1)(b)
I don't understand the point of the pull box under the subpanel. Why not run the circuits directly into the subpanel instead? You can still connect one conduit or cable at a time to the subpanel if you want to gradually move circuits over.

 

[wyliesdiesels]

i wouldnt bother with FMC. its a pain to pull through. just use some SER to tie your subpanel to your main panel.

 

[Paulski]

You don't need to derate conductors if the conduit is less than 24" long. 310.15(C)(1)(b)
I don't understand the point of the pull box under the subpanel. Why not run the circuits directly into the subpanel instead? You can still connect one conduit or cable at a time to the subpanel if you want to gradually move circuits over.

Unfortunately the 2" conduit is ~3'-4' long. I will re-measure it, but I think the vertical section just off the main is already close to 2', then it bends 90* and runs another ft to go to the 16" pull box

The sub panel and main panel are both flush mounted in finished walls, I was thinking the pull box under the sub would make it easier to change the existing circuits over without having to fish through the knock out holes and wood sole plates, also easier in the future if I needed to add another circuit.