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Home Improvement -- electrical quandries [Purp]

I was replying to someone's question on some forum and couldn't get the freaking captcha shit to work and couldn't post it, so rather than wasting all this typing I did on a perfectly good answer, I'm making it a home improvement post.

The dude in question got red tagged on an inspection and wanted to know why. He had a 12-2 Romex and 14-3 Romex in the same box, and a couple of switches feeding lights. His description of his issue was horrible and didn't actually describe the precise situation, but here we go....

"Regarding new construction wiring and running 12/2 and 14/3 wire in the same box."

Are there two branch circuits here or one?

If one circuit. i.e. lights, If the #14 is going to lights and the #12 is the same circuit from the panel powering the lights (power inbound from panel, power outbound to lights), then you'd be REQUIRED to use a 15A breaker on that circuit.

I'll frequently come heavy out of a panel to reduce voltage drop over long runs, or because of derating issues in a hot attic, then drop down in size as it gets near the loads or dives down into walls. General rule for mixing wire sizes on residential work: breakers/fuses get sized to the smallest wire on a branch. If you're bucks down with a bunch of scraps laying around and don't want to invest in a bunch of new wire, yea, you can mix sizes and use them up if you know what you're doing.

Other than blowing out a box fill requirement(3), there's absolutely nothing in the code that prohibits mixing wire sizes on any particular branch as long as the smallest size wire is big enough for the breaker/fuse and doesn't run into derating issues with pipe fill(1), ambient temperature(2) and such.

If there are two circuits (and two breakers/fuses), say a 20A on the 12/2, and a 15A on the 14/3, that too is permitted, BUT if the two branches land on a device (ex. a split feed receptacle, or a stack switch with the hot interconnect tab busted off making it essentially two completely distinct switches), THEN the 15A and 20A breakers MUST be placed side by side and connected with a "handle tie". Separate circuits that do not land on the same devices aren't required to have handle tie on breakers, but its something I usually do anyway so the next guy has a clue about what's going on. The handle tie is a clue that there's multiple live circuits on devices somewhere downstream.

All grounds in a box must be spliced together regardless of wire size(4). The only exception to this is isolated ground circuits...you won't find too many of those in residential situations.

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Another possibility for this fellow getting red tagged is he's become a victim of a new electric code change -- starting with the 2011 NEC, switch loops for lights (which electricians have used for 100+ years) where there's not a neutral wire sent down into the box too have been made illegal. You can thank California and its obsession with occupancy sensors for that one. One state's crazy became a national requirement. The current to run occupancy sensors in a switch loop situation had been using the ground as a way to complete a circuit. The current was only milliamps, but when enough of them got added up on any given circuit, they were causing Arc-Fault breakers to trip. All Arc-Fault breakers include 30ma worth of ground fault protection, so excessive leakage on the ground trips them.

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(1) In situations where there's more than 3 current carrying conductors in a conduit consult the NEC derating charts to figure out how much the wire needs to be upsized.

(2) When running wire through abnormally warm locations (like attics) you need to take the ambient temperature into account when sizing wire. There's extensive tables in the NEC covering all the temperature ranges and insulation types.

(3) You can't just jam as much shit as you want into an electrical box. The NEC has specific fill limits requiring a certain number of cubic inches of volume be available for each wire in the box. Those values vary by wire size. Sometimes something will be "book legal" and you'll have problems jamming all the shit in anyway. This happens a lot with GFCI receptacles and dimmers that take up inordinate amounts of space.

(4) All the grounds in a box count as one wire for box fill calculations -- the largest one. If you had say 3 14ga, 2 12ga, and 210ga grounds in a box, all that shit counts as ONE 10ga wire for box fill purposes and gets spliced together. Even though all the grounds only count as one, they can take up a shitload of volume and make stuffing a box a real challenge.