why do we do this, what are the benefits, and does it make troubleshooting down the road more difficult?

In my new job, one of the jm doesn't explain too many things...

there were 2 3-ways in a 2gang. old wiring with the paper... the neutrals were hots, and there was a red from one and a white from the other merretted together loose in the box. Where was the switch leg??

It's a white as hot - not a neutral as hot, please. We do it because we don't want to use two kinds of two wire cable and two kinds of three wire cable, one with a white and one without. As it is, I can barely move around in my truck with one kind of each in non-metallic and bx both in several sizes.

There is a specific way to connect the wires, which makes troubleshooting easier.

Perhaps someone can provide a link to a 3 way switch diagram?

In commercial work or resi where the power source for the circuit is at the fixture (vs the switch) the conductors going down to the switch are called (drumroll please) switch legs.

In general, these are the ONLY time you can ordinarily expect to see a white wire used for anything other than (neutral) the ungrounded.

This is one of those "get used to it" things. Sorry about that.
The reason this practice is tolerated is mostly about money.

A good practice is to mark these non ordinary use conductors with some phase tape (some color OTHER than what is elsewise found in that box eg orange or yellow).

You are required to use the white in a cable as the hot on a switch leg if you are using it as an ungrounded conductor and since 2002 or so you are required to reidentify it with tape or other method. 200.7(C)(2)
The thinking is the return from the switch will be a "phase" color when it gets to the luminaire. Then the installer will be presented with a white neutral and a phase color for the ungrounded conductor.
It can get a little confusing when you are on the supply end tho. That is why they started requiring reidentifying the wire.

That's an interesting practice... in Europe mostly the black/brown wire in the switch leg is used as the permanent hot and the blue as the leg to the fixture. Usually there's a junction box in the middle though where the "hot blue" changes back to black or brown.

Where conduit is used it's good practice to use a different phase color like brown (if the other phases are black), orange or purple for the switched hot. So either the power source is at the switch or at a junction box, due to the lack of ceiling boxes except for cast concrete ceilings we can't really make any connections beyond the fixture splices at the fixture.

It's a good thing to remember - never trust a wire color. Oer at the violation photo board you can see pictures of green hots and everything... always expect to encounter hack work rather than risk your life by trusting someone you don't even know (the guy who installed the old wiring you're working on).

Green hots?? I can see someone cheating with a white or blue, and the occassional use where it's justified, but a green? Man, that just takes all!

http://www.homeimprovementweb.com/information/how-to/three-way-switch.htm

My Dad taught me this simple rule many many moons ago:

"The neutral is always white, but the white is not always neutral".

A.D

Another time you'd use white as a hot is a 240V circuit.
Unless, of course, you want to run 12/3 (unneeded expense)and have an unused white at the load. You would/should have to land the white at the panel though, if you did that. I have seen 6/2 WG that has black and red wires (there is some in my house), but I have never seen it for sale myself. So, I don't know if it is even available anymore.

You can get any color combination you desire, if you want to make a special order and wait for delivery.

Good point.
Remember this thread is really about saras and her newbie question.

Most common wiring methods are in the MC and NM families so most jobsite and "what is on the truck" methods will utilize these; and most MC and NM is 10/12/14 gauge. I would hope folks doing conduit runs wouldn't choose to use white for an ungrounded.

Sara, do you understand 3 and 4 ways and how switch legs are done in the (2) basic uses they get?

Actually, I believe that this design is newer rather than older. Correct me if I am wrong, but I believe that black and red is required for two-wire "Loomex" (Romex) over #10 in Canada. If that's really the case, it would make sense that cable manufacturers would standardize with one cable design, especially with today's copper prices. Of course, type SE cable has always been black/red and is very common in the US for higher-amperage 240 volt circuits. Besides, who has ever seen a 50 amp 120 volt circuit?

(No need for feedback from folks in New York City where there are 50 amp, 120 volt feeds to apartments in very old buildings. That's different)!

Actually, around here, this is called a "switch loop".
A "switch leg" is usually the wires that leave the switch and go to the load, when the feed is at the switch box

Green hots?? I can see someone cheating with a white or blue, and the occassional use where it's justified, but a green? Man, that just takes all!

Years ago, I worked as a facilities Electrician at OSU, We used to find "Hot Greens" all the time, Per "T" the sup, at one time Facilities didn't have a very big budget, and a previous sup had saved alot of money buying a large quantity of green wire at close out prices............