Changing 220 15 amp to 220 30 amp. Present circuit is in conduit with two other circuits. Need some advice
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 &lt;= 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.