[b]Goal[/b][color=#141414]: Replace existing 10-30 (8/3 Al) receptacle with a 14-50 (6/3 Cu). Ive been reading up and down the interwebs (including NEC) trying to get a concrete answer for this. And based on everything Ive gathered it would be to code to do the following.[/color] [color=#141414]First thing I want to say is that I know some will say to just run THHN, but NM-B is much more convenient for me, the cost is the same for the run as THHN, but the conduit is existing, I am upgrading NEMA 10-30 dryer outlet to NEMA 14-50 (to use as a car charger, will pull 32A but id like to be capable of pulling the full 14-50 allotted on 50A breaker because who know what this outlet will be used for if I ever sell the house, its just the right thing to do).[/color] [color=#141414]So the conduit exists, its a 3/4 inch metal flex conduit (with 8/3 Aluminum feeding the 10-30 outlet, I dont think this is to code anymore? This wire is thick! no way it passes NEC's fill rate, its thicker than the copper 6/3 NM-B, but its probably been there since 1980, was probably to code back then?).[/color] [color=#141414]The main reason NM-B is what I want to use is because the run starts in a wall (behind dry wall from the breaker panel), and ends in a wall (behind a drywall where 14-50 is). The FLEX conduit is running along the crossbeams at the top of the garage (exposed beams) from one wall (where breaker panel is) across garage to the other wall (where 14-50 is). Its about 10 feet inside one wall (two 90 degree turns), and another 20 feet or so inside of the outlet wall (one 90 degree turn). Section inside the flex metal conduit is about 30 feet (width of 2 car garage) In order to use THHN id have to rip up the drywall to install conduit inside of both walls, I really dont want to do that.[/color] [color=#141414]Everything Ive read here and elsewhere points to NM-B not being allowed inside conduits to being an 'urban legend' and misinterpretations of NEC. But from what Ive read you can not run NM-B in conduit only in the following specific scenario. Wet location, (any location outdoors in a conduit is considered wet location). My entire run is inside of a garage, the Flex conduit is at least 8 feet off the ground, not a wet location.[/color] [color=#141414]Ive come across people saying to take off the sheath, but thats a no-no because the conductors arent labeled inside, and that will definitely break NEC. And I dont want to do that.[/color] [color=#141414]Ive also come across people saying that the over-sizing of the conduit would be impractical, and derating of wires is required. But I thought derating is required only for more than 3 conductors? But even then, its automatically derated being NM-B to 60C even though conductors are actually 90C rated. And at 60C you can pull 50A through 6/3. So it doesnt seem like heat is a concern and derating debate is a moot point since its automatically derated to 50A.[/color] [color=#141414]So then fill would be and issue, right? NEC says to treat NM-B as one conductor and take diameter of the entire thing (At thickest point in case its oval, mine is round). I measured it, its 0.5in diameter. I used an online fill table calculator [/color][url]https://www.southwire.com/calculator-conduit[/url][color=#141414] with following settings:[/color] [color=#141414]Conduit: Flex FMC 3/4[/color] [color=#141414]Conductors: User Defined - 0.5 - 1[/color] [color=#141414]This comes out to 37% fill. Within NEC spec.[/color] [color=#141414]So it seems like I am good to go?[/color] [color=#141414]I guess my questions are. Am I not understanding something? Specifically regarding the fill calculation? It seems like a lot of people make the mistake of calculating individual wires inside of NM-B for fill table, but NEC specifically says to treat the whole thing as 1 conductor.[/color] [color=#141414]Any advice/help/suggestions are welcome.[/color]

Hi all, sorry for starting off with asking a question but I'm hoping the community can help me from making a potentially expensive mistake. I recently purchased a 22kw Generac Guardian series air cooled standby generator. It will be running off propane and is located approximately 75' from my home's service entrance. The generator came with a service entrance rated automatic transfer switch. The generator and transfer switch are rated in the 75C column according to their labeling, and the generator has a 100 amp circuit breaker and a max output of 91.7 amps when running on LPG. Looking at both voltage drop calculations and the 75C column of 310.15 it would appear that #3 Copper would be sufficient for my purposes. #3 can carry 100 amps in the 75C column and voltage drop for an 85' run (added a few feet on for bends and whatnot) is 2.79%, just shy of the max 3%. Generac also conveniently sells installation cable for their 17-22kw generators that contains 3x #3awg + ground, + the 6 control cables in #18awg. They say that cabling is sufficient for installs of up to 113' away. With all that said I was feeling pretty good about buying that convenient all in 1 cable until I started reading here and other places where people were saying the under no circumstances can #3 carry 100amps. I've read some people saying you have to go up to #2 and others saying you have to go as big as 2/0! My plan was to order the cable and do my own trenching and conduit install and then have a licensed electrician come and help me with the final hook ups. I don't want to buy this cable if it won't end up passing an inspection after the fact. Opinions? I l live in an unincorporated area of King County in Washington state if that makes a difference. Thanks in advance for the help.