Conduit fill
The 110 NM terminates first
14-2 NM cable is 0.36" across in the wide dimension. Whenever a flat cable is used in a conduit, it must be treated the same as a round wire of the wide dimension.
Three such cables just fit in 1" sched 40 PVC.
The first 220 NM terminates shortly after. It is 0.75" conduit in the area with 2 NM cables.
Two 14-2 NM cables require a conduit with ID of 0.914". 3/4" PVC is much too small. This installation is illegal.
Two cables in a conduit are the worst-case scenario: the round swept area of the cable are permitted only 31% conduit fill.
The second 220 NM terminates shortly after that. It is 0.5" conduit in the area with only 1 NM cable.
A single 14/2 cable requires conduit with an ID of 0.497". 1/2" PVC is barely adequate.
Use of THHN wires
Note that THHN requires to be inside conduit its entire length. You cannot throw random loosy-goosy THHN wires inside a wall. This is one minor advantage of NM cable in conduit; it can hop in and out of conduit at will. The disadvantage is it requires ginormous conduit, and you see where one, two or all three of your conduit segments are too small for the cables therein.
However, you are able to transition between THHN and NM/Romex at any junction box with enough space for the splices. So it may be possible to run NM-in-wall up to the head of the conduit run, then THHN inside the conduit. You may need to simply fit a super-large junction box at the head point of the conduit, and call it good.
GFCI on 240V circuits is coming
My understanding that NEC updated for 2020 saying that all circuits in a garage must be GFCI protected - is this true? If so - I will use a GFCI breaker for the new 220 circuit when I replace it.
It will be true when your state adopts NEC 2020. Only MA has done so yet. So pull your permit right now and you should be in 2017 Code.
How wire ratings actually work
My understanding is that I will have 6 conductors running through the conduit so must derate to 80%. This should make no difference for the 110 15 amp and 220 15 amp with 14 gauge THHN wire (25*0.8=20). But will make a difference for the 220 30 amp circuit as I cannot use 12 gauge THHN (30*0.8=24). So I will step up to 10 gauge THHN for the 30 amp circuit (40*0.8=32). If this is all correct, should I use 10 gauge ground? Or do I have to derate the ground as well?
You seem to correctly grasp that you use the 90C column for 310.15(B)7 derate purposes. The reason is that THHN actually is rated 90C, and NM and UF are rated at 60C but are allowed to derate off the 90C column for 310.15(B)7 purposes.
However, you seem to think you are at liberty to use any of the columns for your actual ampacity, e.g. 30A on #12 wire since you saw that in the 90C column. That is wrong in two ways. First, 240.4 limits your ampacity to 20A @ #12 no matter what. Second, even if your wire size was too large for 240.4 and you had 90C wire, you are still limited by the allowed temperature of the terminations. A 60C termination puts you firmly in the 60C column. You won't find any terminations hotter than 75C.
That said, derates for #14-10 wires are a nothingburger. At <= 4 circuits per conduit, the derate has no effect. At 5+ circuits per conduit, the derate is crippling and you are better off not allowing that to happen at all.
What to do
As far as running new wire through the conduit - any advice? I understand attaching my new 10 gauge wire to the old 14 and pulling will not be easy - especially in the section where there are 3 cables. Should I pull the old NM cable with a pull cord attached to it, then use that pull cord to pull the new cable?
Pulling the old cable out will not be easy. Putting cable in conduit is the act of a madman. Pulling the new wires in will be a cakewalk, especially if you select stranded wire, which you should. Pigtail receptacles; directly attaching stranded wire to receps and switches is an acquired skill. Stranded works fine on breakers and in wirenuts.
Seriously, once you work with stranded you'll wonder what possesses anyone to tolerate solid wire.
I would replace at least the new 30A run with THHN wire - 2x #10 hot (same color is fine), 1x neutral if needed (no need to include it if not), and 1x #10 ground.
- Four #10 + one 14/2 NM-B will fit inside 3/4" sch40 PVC @ 36.1% fill. (allowed 40% given 3+ "wires").
- Four #10s alone will fit inside 1/2" sch40 PVC. @ 28.61% fill (allowed 40%).
So that takes care of your problem either way.
You also have the option to yank all the #14 wires and replace with THHN. I wouldn't bother buying #14 THHN, because why own any of that? Just use #12. 3 or 4 of of either size will be net-smaller than a #14 NM-B.
Grounding/splicing
You can share one ground wire for all the circuits. What you cannot do is splice in a conduit body. There are 2 separate rules:
- Conduit bodies (and junction boxes which have only pass-thrus with no splices, therefore effectively are conduit bodies): Must have internal cross-section 2x the pipe cross-section. Not so bad.
Junction boxes (and conduit bodies used for anything put passthru, making them effectively junction boxes): 2.0 cubic inches per #14 wire and 2.5 cubic inches per #10 wire, with all grounds counted as 1 (largest) wire.
- You don't get to split these rules and count only the spliced wires. All-in, or all-out.
- This is a deal-killer for conduit bodies not wildly oversized for the pipe.
So you might have to run 3 ground wires due to the conduit body issue.
I have no idea what you're looking at, but no, the NEC does not demand a pipe that fat for a 200A feeder
I have absolutely nary a clue what sources you are looking at, but according to Chapter 9, Table 5A of the 2017 NEC, a 4/0 Al XHHW-2 compact stranded conductor takes up 176.3mm2 of fill. Multiplying that by four, which is quite conservative as you will see, gives us 705.2mm2 of fill used by the entire feeder, whereas a 2", Schedule 80 PVC conduit has 742mm2 of usable fill area in it as per Chapter 9, Table 4 of the 2017 NEC. So, clearly, a 2" conduit will suffice according to the NEC; you can upsize to 2.5" if you wish for ease of pulling, but no, you don't need to faff with reducers just to get your conduit to mate to your box.
Note that in all likelihood, you will not be running 4 4/0 wires for this. The largest Mobile Home Feeder (MHF) quadruplex available is a 4/0-4/0-4/0-2/0, and there's also a 4/0-4/0-2/0-4 configuration available; while one could get 4/0-4/0-4/0-4/0 URD, there's no reason to given that Code permits a smaller ground wire and cables with the smaller ground wire are readily available in URD and MHF styles. If you're actually pulling individual wires, a typical bundle would use 4/0 XHHW-2 Al wires for the hots and neutral, with a 6AWG bare or green (THHN) copper grounding wire in the conduit.
Best Answer
Honestly, I would affix a chunk of marine plywood to the pole, prime it, paint it with exterior paint, and attach the RV panel to that. That will give you more freedom to position it for a "back to back", "out the bottom of the RV panel and then in the back of the main" with a single bevel 90, or down, over and up with two 90s.
Given the very short distance, the cost of wire is negligible, so I would use #3 copper. I'm not a huge fan of copper for feeder, but it's the most compact, so it'll fit in the smallest conduit.
Here's the thing about those "50/30/20" RV stands. Generally, you're expected to install one of those per RV site. And then, the RV is expected to use the one socket that is compatible with its power.
You keep talking like using all three of them at the same time, you're not required to provision for that. You only need to provision for the largest socket available to any given site, and you ignore all the smaller sockets.
If you follow the "One 50/30/20 panel per site", then it's pretty simple. You calculate based on 50A/12,000 watts per panel/site, and then apply the favorable derate that Ed Beal mentioned from 551.73.
So again, you don't have to provision more sockets than you have RV sites. You just count your RV sites and provision that many sockets, largest first.
And follow what Ed Beal says about how 50A RVs take a lot more power than 30A RVs. That is because 50A RVs take 240V power, not 120V. They take over triple the power of a 30A.
And then apply the derate as Ed says.
Like I say, given your 100A provisioning, I would just go ahead and install two 50/30/20 RV subpanels at wherever you expect will be most useful. Because the subpanels are good for 100A busing, you don't actually need to give them separate breakers - you can simply daisy chain from one subpanel to the next. But the wire needs to be 100A.
Does it matter how many sites you support with two 50/30/20 RV stands? Actually it doesn't -- no combination will exceed 100A.