I'm wiring my workshop (standalone 200-amp service) and running wires in 1-1/4" PVC conduit under the slab from the breaker panel on one wall to another wall about 50 feet away. I have one 50-amp breaker feeding a 240v outlet and also want to run wires for 120V outlets on four 20-amp circuits, all with copper THHN. The 50-amp circuit will use 6 AWG power wires and an 8 AWG ground wire while the 20-amp circuits will use 12 AWG for power, neutral, and ground. All the wires will go through the PVC conduit to a 6x6x4 electrical enclosure and from there through EMT to the individual outlets. I just learned of MWBC's so I'm going to give them a shot. Using an MWBC for the 20-amp circuits (diagram below), I count eight current-carrying conductors. Checking a few tables, it looks like I need to derate to 70% of rated capacity but would still be OK with the wire quantity and AWG listed. Conduit fill seems good, also. Is there anything I overlooked or misinterpreted here?
No, this is much worse. You just degraded yourself from six 120V circuits to four, and now you have foreclosed any possibility of converting a 120V circuit to 240V to support 240V shop tools, which are inevitable.
And if you get a shop tool, heater or mini-split that needs a 25A+ circuit, it's worse, because now you're out cash money for an extra 100' of #10 copper for that long conduit run. You can't just fork the 50A outlet and serve a bunch of 15-30A 240V loads.
I mean compared to your earlier scenario you've sidestepped a thermal derate... but you're still trapped in a situation that a) degrades your capacity and you'll regret that later... and worse, b) costs you more.
The bog-standard thing that we recommend (and just about everyone does) is a 2-2-2-4 or 2-2-2-6 aluminum to a subpanel, giving 90A feeder (feel free to breaker at 60A if your panel's 60A breakers fit #2). Nothing wrong with aluminum at these large sizes, and it's going to lugs made of aluminum anyway. Now you price that in XHHW and it's cheaper than your two #6 copper and one #8. And it's cheaper by enough to pay for a subpanel.
The subpanel then crosses off all these problems with thermal derate in the conduit.
Since you can use 2-2-2-4 SER here, that also means you can delete some conduit, because SER doesn't need conduit the entire route from panel to subpanel.
I was able to buy a few thousand feet of 12 AWG still on the spools from a local school auction for $100 so I'm going to use it :)
I know about that. When I say you don't save any money, I'm assuming free #12.
Which is a false dichotomy anyway, because "use it, or throw it in the trash" are not the only 2 options. I would have it up on Craigslist. By comparison, it's 17 cents per foot on 500' rolls, but most DIYers want less than 500', so they're paying extortionate per-foot prices at the store - up to 50 cents a foot for a 50' roll. And Home Depot's "price inflation" on small reels is shocking - pre-COVID it used to be $9.99 for a 50' roll. Now it's $25. (contrast with $52->$85 for 500').
Also if there's one thing I know about bargain/found wire, it's always the wrong color lol. This is where novices think they can tape it, like a white wire in a cable to make it a hot. Nope! That rule only works with cable, and only neutral-to-hot. With THHN there's no remarking amongst hots, neutrals and grounds on wires #6 or smaller. Grounds must be green, or yellow/green natively. Neutrals must be white or gray.
So yeah, perhaps not the value you'd hoped for.
On your new drawing if configured as MWBC's your wire count will only be six. 4 for the 20A circuits and 2 for the 50A.
On a MWBC's you only count the hots, since the neutrals only carry the imbalance. Stepping back let me explain, with a single circuit you (potentially) can have 20A of heat current that disapates heat on both the black and white. Add the red circuit from the other leg of the panel drawing 20A and sharing the white, then you get 20A on black, 0A on white, and the same 20A being routed through the load connected to the red circuit. You shifted the current and heat from white to red, and got a second circuit without any added heat in the conduit. The only time you get current/heat on the white is if you reduce the current/heat on red, black, or both. So since the white only carries heat is when the current is reduced on the black or red you don't have to count the white.
So in your original drawing: You could arrange the top two 20A circuits sharing a neutral as a MWBC and those 3 wires only counts fill as 2 wires. The middle two could be a second MWBC, the bottom two a third and you have a total of 3 MWBC's functioning as 6 circuits with a wire count of 6. Add the two wires on the 50A recept, and you're at 8 wires, still in the 70% category.