What is the difference between a switch leg and a hot leg?

[Averagesupernova]

BTW, you will sometimes see junction boxes in attics and crawl spaces, which are accessible. But again, really only in remodeling, I think.

Junction boxes certainly aren't unheard of in attics. Usually a good electrician will put them in close obvious proximity to the entrance to the attic if at all possible.
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But imagine a scenario where there is a ceiling light controlled by a switch. Only one pair of wires in each box. Electrician comes to troubleshoot the non-working light and finds this. First thing that will be questioned is if it ever worked. If the electrician trusts the "yes" answer to that question then will assume there must abother box somewhere. But where? Buried how deep in insulation? In the basement? Buried in a wall?

 

[sophiecentaur]

Junction boxes certainly aren't unheard of in attics.

I hesitate to join this thread because I live in the UK. The terminology is often very different and sometimes the actual philosophies seem to differ. When discussing house wiring there is one very basic principle and that is any switch should be in the live side of the circuit. If it is then, once the switch is Off, then all of the downstream part of the circuit is unpowered. A switch in the neutral will control whether the device (lamp etc) is on or off but will not guarantee you won't get a shock fooling with the circuit because most of it will still be live. The colours of the wires are nearly always limited to two (in UK Brown is Live and Blue is Neutral) but lighting circuits are usually connected 'daisy chain ' around the ceilings and two conductors go down to the wall to each wall switch. If only brown and blue conductors are available then a brown conductor goes down to the switch and the remaining returning conductor is blue. So the lamp will be connected to two blue conductors. Confusing unless you know the rules.

It seems to be the PF style of presenting circuit diagrams which are more like sketches of the devices in the circuit. This is often useless because you cannot 'see' what happens to the conductors inside the 'boxes'. The layout of the circuit components is usually not relevant but a functional circuit diagram shows exactly what's going on with accepted component symbols. What went wrong in ee education?

 

[DaveE]

When discussing house wiring there is one very basic principle and that is any switch should be in the live side of the circuit.

Yes. Same in the US (everywhere, I think).

The colours of the wires are nearly always limited to two (in UK Brown is Live and Blue is Neutral) but lighting circuits are usually connected 'daisy chain ' around the ceilings and two conductors go down to the wall to each wall switch. If only brown and blue conductors are available then a brown conductor goes down to the switch and the remaining returning conductor is blue. So the lamp will be connected to two blue conductors. Confusing unless you know the rules.

Yes. Same in the US. Except that in NM cable you have Black (hot), White (neutral), and a bare ground wire. So, it is extremely common in a switch leg to have Black (always hot) and White for switched (hot sometimes). Good practice, and the building codes, say that in the boxes the white would be marked with black, like a piece of vinyl tape, or marking pen near the ends. But that's usually a bit of overkill since in one box it will go directly to a switch, and in the other box it will be spliced to a Black wire. If you know about house wiring, you'll know exactly what's being done anyway.

It seems to be the PF style of presenting circuit diagrams which are more like sketches of the devices in the circuit. This is often useless because you cannot 'see' what happens to the conductors inside the 'boxes'. The layout of the circuit components is usually not relevant but a functional circuit diagram shows exactly what's going on with accepted component symbols. What went wrong in ee education?

The only people that do circuit documentation right are practicing hardware EEs; and yes, I didn't include most electricians. You can't do it right with one document, but everybody else tries anyway. IRL, there is a document package. Here's the right way, which BTW is a lot of work to make:
1) A schematic which shows the functional (abstract) operation of the circuit. There is a lot of the designers personality in what these actually look like. I can spot a good HW EE just by looking at the style of one of their schematics.
2) A higher level block diagram that show different modules, connectors, cables, etc.
3) A wire list, if there are wires. That lists each wire, color, it's start and end points, and if it's bundled in a cable. Function isn't necessary.
4) An assembly diagram, or now days, assembly methods (like power point, sort of). That tells people how to connect things.
5) PCB layout files, if appropriate.
6) Bill of materials.

Yes, I did this all the time for Lasers and Satellites. No, I never did this for my house wiring.

An example of a wiring diagram for equipment, my style, of course:

 

[Averagesupernova]

So, it is extremely common in a switch leg to have Black (always hot) and White for switched (hot sometimes).

The diagram I posted has the white wire going to the switch as the constant hot. This is the way it should be. The reason is if the homeowner replaced a ceiling light and both wires are white, then what?
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We never want unbalanced currents in conduits or NM cable (romex). They have to be run like a transmission line so fields cancel. So drawing it like a schematic that does not show conductors grouped could lead this in my opinion.

 

[DaveE]

The diagram I posted has the white wire going to the switch as the constant hot. This is the way it should be. The reason is if the homeowner replaced a ceiling light and both wires are white, then what?

OK, that makes sense. Although I'm not clear on how that confuses an observant home owner changing the fixture. Still, your way is more idiot proof. Plus the internet agrees with you.
Also, I'll take back what I said about marking the white wire with a black stripe. I think it is important.

So drawing it like a schematic that does not show conductors grouped could lead this in my opinion.

That's not what schematics are for, IMO. They rarely tell you how to actually build things. But this gets down to semantics and local practice. A set of understandable drawings is required whatever those are or what you call them. More than one for anything that isn't commonplace, IMO.

 

[Averagesupernova]

Although I'm not clear on how that confuses an observant home owner changing the fixture.

If the constant hot were black going down to the switch then the switched wire that ends up at the load is white. The neutral at the load is also white. Still fail to see the problem?

 

[sophiecentaur]

The neutral at the load is also white. Still fail to see the problem?

The problem is that you don't know which supply to the load is live.

There is no 100% solution to this but your scenario at least can make you feel safe if you touch a white wire and when the switch is set to off.
On colours: the modern european standard for brown, blue and green / yellow striped was carefully thought out on the following basis. Dealing with a wire on its own which has been 'weathered', overheated, mistreated if it's got a visible stripe running through it then it's very likely to be an earth conductor; safe to touch. Likewise for a healthy blue wire. If you come across a neutral wire that's been overheated then it could look brown. You have to treat it as a possible brown / live.

50 years ago, on imported equipment the White / Black standard was sometimes the other way round. "Black for death" couldn't be relied on.

Just for fun, I'll introduce the subject of Double Pole Fusing. That's a scary idea for the mains supply in UK.

 

[Averagesupernova]

The problem is that you don't know which supply to the load is live.

That's the point I've been trying to get across.

 

[sophiecentaur]

. I said that I want to install an electrical outlet in my back porch.

To me, your second and third sentences seem to contradict what the trucker told me. That doesn't necessarily mean that you are wrong.

A trucker in a cafe may not be the best source of this kind of information. Even nice blokes can talk BS about EE.

As in: you should not be running a new line out of a switch box like that.

There's a principle that says appropriate fuses should be in any circuit. Plug an electric kettle into an extended lighting circuit and no one dies; they just don't get their cup of tea. Curing the problem by putting a bigger fuse in the source is asking for trouble.

Over my head.

You are not dim (that's official) but the terminology and some of the descriptions in this thread have been well intentioned but inadequate. A schematic diagram is the best description and least likely to be mis-interpreted. But life's too short.