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

[sevensages]

TL;DR Summary

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

How can I tell if my ceiling fan is run off a switch leg or a hot leg?

I would like to get an electrical outlet installed in my enclosed back porch. I am a long distance trucker. Recently I was in the Driver's Lounge at a truckstop talking to another trucker who used to be an electrician about how I would like to get an electrical outlet installed in my enclosed back porch. There is a ceiling fan in my enclosed back porch. I asked this other trucker if I could get the new electrical outlet that I want to install in the wall of my back porch to run off the same circuit as my ceiling fan. He said that i could only get this new electrical outlet to run off the same circuit as the ceiling fan if the circuit that the ceiling fan runs off of is a hot leg. He said that if my ceiling fan runs off of a switch leg, then I cannot have the new electrical outlet run off the same circuit as the ceiling fan. I have two questions about this:

1# What is the difference between a switch leg and a hot leg? My speculation is that if the ceiling fan runs off a hot leg, that means that there are TWO HOT wires going to the ceiling fan. And my speculation is that if the ceiling fan runs off a switch leg, that means that one of the two wires that goes to the ceiling fan is a hot leg, and the other leg that goes to the ceiling fan is a neutral wire. Is my speculation correct?

2# How can I tell if the circuit that my ceiling fan runs off of is a hot leg or a switch leg? I am totally baffled as to how to tell whether my ceiling fan runs off a hot leg or a switch leg. I mean I just don't have the first clue of how to find this out.

 

[DaveC426913]

1# What is the difference between a switch leg and a hot leg? My speculation is that if the ceiling fan runs off a hot leg, that means that there are TWO HOT wires going to the ceiling fan. And my speculation is that if the ceiling fan runs off a switch leg, that means that one of the two wires that goes to the ceiling fan is a hot leg, and the other leg that goes to the ceiling fan is a neutral wire. Is my speculation correct?

Donning your Googles will help.

A hot leg runs directly from the panel to the load
A switch leg runs to a switch first, then to the load.

2# How can I tell if the circuit that my ceiling fan runs off of is a hot leg or a switch leg? I am totally baffled as to how to tell whether my ceiling fan runs off a hot leg or a switch leg. I mean I just don't have the first clue of how to find this out.

Are there any mysterious switches on walls in or near the room?

 

[sevensages]

Donning your Googles will help.

A hot leg runs directly from the panel to the load
A switch leg runs to a switch first, then to the load.


Are there any mysterious switches on walls in or near the room?

There is a light switch for the ceiling fan light in my back porch that is in my dining room. I don't know if I would use the word mysterious to describe the light switch though. It is just a light switch.

 

[DaveC426913]

There is a light switch for the ceiling fan light in my back porch that is in my dining room. I don't know if I would use the word mysterious to describe the light switch though. It is just a light switch.

It is not clear how many fans you have - or will have - or where they are located.

You Say you want to install a ceiling fan in your back porch but it seems you now say there is a ceiling fan in your back porch that has a switch in the dining room.

*the "mysterious" aspect would be having a switch that doesn't seem to do anything at the moment, since - as it was originally understood - there was nothing hooked up to it out on the porch.)


Anyway, are you having this line for the ceiling fan installed by a pro? If so, it won't be a problem. He will likely run a new line from the panel.

If you are planning on installing the line yourself, are you sure that's a good idea?

 

[sevensages]

It is not clear how many fans you have - or will have - or where they are located.

You Say you want to install a ceiling fan in your back porch but it seems you now say there is a ceiling fan in your back porch that has a switch in the dining room.

No. Please read the OP again
I did not say that I want to install a ceiling fan in my back porch. I said that I want to install an electrical outlet in my back porch.

There is a ceiling fan in my back porch that has a switch in the dining room.

Anyway, are you having this line for the ceiling fan installed by a pro? If so, it won't be a problem. He will likely run a new line from the panel.

If you are planning on installing the line yourself, are you sure that's a good idea?

I haven't made up my mind yet whether I want to install the electrical outlet myself or hire an electrician.

 

[russ_watters]

If the circuit was installed correctly the wire to the fan is not hot unless the switch is on. You could extend the circuit to an outlet if you are OK with the switch turning the outlet on and off. Otherwise you'd need to wire it from upstream of the switch.

 

[sevensages]

DaveC, you edited post #4 and added the quote below after I made post #5. I would like to address that.

*the "mysterious" aspect would be having a switch that doesn't seem to do anything at the moment, since - as it was originally understood - there was nothing hooked up to it out on the porch.)

As I said in the OP, the ceiling fan in my back porch has been in my back porch since I bought this house. The light switch in the dining room was never mysterious to me because it has always controlled the ceiling fan light.

 

[sevensages]

If the circuit was installed correctly the wire to the fan is not hot unless the switch is on. You could extend the circuit to an outlet if you are OK with the switch turning the outlet on and off. Otherwise you'd need to wire it from upstream of the switch.

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

Based on the description of a switch leg that Dave told me in post #2, i believe that my ceiling fan light runs off a switch leg (not a hot leg) because the light switch in the dining room turns on and off the ceiling fan light. My understanding of your statement "You could extend the circuit to an outlet if you are OK with the switch turning the outlet on and off" is that you are telling me that I could extend the switch leg that goes to the ceiling fan light to the outlet if I am ok with the light switch in the dining room turning the outlet on and off. But the trucker told me that if the ceiling fan light is a switch leg, then the circuit could not be run from the ceiling fan. Do you see the contradiction? Maybe the trucker told me that if the ceiling fan light was on a switch leg, the circuit would have to run directly from the switch to the outlet because he thought it would be undesirable to have the light switch in the dining room turning the outlet on and off. Do you agree ?

Edited to add: I don't understand why you wrote "Otherwise, you would need to wire it from upstream of the switch."

Couldn't I wire it from right at the switch (not upstream of the switch)?

 

[DaveC426913]

DaveC, you edited post #4 and added the quote below after I made post #5. I would like to address that.



As I said in the OP, the ceiling fan in my back porch has been in my back porch since I bought this house. The light switch in the dining room was never mysterious to me because it has always controlled the ceiling fan light.

Right. Got it now.

So you could install an outlet on the same line, but the outlet would be subject to the whims of the wall switch.

 

[DaveC426913]

But the trucker told me that if the ceiling fan light is a switch leg, then the circuit could not be run from the ceiling fan.

I'd say "could not" was an over-reach.

You could do it, it just might not have the effect you desire.


I have this in my own house. I have an "after-market" sun room (i.e. previous owner built it). It has electrical outlets that are controlled by the light switch inside the back door of the house proper.
This is not desirable because I keep plants in the sunroom over the winter and, if the light switch gets turned off, all the lights and heaters in the sun room go off. It doesn't take long for them to freeze.

 

[sevensages]

Right. Got it now.

So you could install an outlet on the same line, but the outlet would be subject to the whims of the wall switch.

I gotcha. Yeah, that would be somewhat undesirable to run the outlets off of the switch leg to the ceiling fan light because I would like to plug battery chargers for cordless drills and other cordless tools in the back porch and leave them charging there overnight. If I ran the outlet off of the switch leg, charging up the batteries in battery chargers in those electrical outlets would entail leaving my ceiling fan light on all night, which is quite undesirable. I will have the new electrical outlet run off the switch, not the switch leg for the ceiling fan light.

 

[sevensages]

By the way, I edited post #8 at the very end of the post. If you have already read my post #8, please read the edit at the end of post #8.

DaveC, I invite you to answer the question at the end of post #8 in addition to Russ.

 

[DaveC426913]

Couldn't I wire it from right at the switch (not upstream of the switch)?

There's "you could" and "you should".

As in: you should not be running a new line out of a switch box like that. I am almost certain it is against code.

Most of what you are going to encounter here is going to be less about "what will work for you" and more about "what will meet code" - which, by the way, can be considered synonymous with "won't void your insurance if it's ever discovered".

You don't know what might be plugged in to that outlet down-the-line, which is why it ought to be on its own circuit.

But at least you should not split off a switchbox like that; you would install a new octagon - upstream of the switch - and split off there to the two loads. Still not good, but not as bad as splitting from a switchbox.

Caveat: I am no electrician.

I'll leave it to Russ to weigh in.

 

[DaveE]

hire an electrician.

Yes, do this. Your question(s) are pretty basic for electrical work.
If you haven't (yet) taken the initiative to map out your existing wiring and understand where to connect things, then I fear there are other things that you need to learn that you might not.
If you are going to do it yourself, then you need to put more effort into studying this subject instead of asking basic questions on SM and getting answers from people you don't know.

 

[russ_watters]

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

Maybe the trucker told me that if the ceiling fan light was on a switch leg, the circuit would have to run directly from the switch to the outlet because he thought it would be undesirable to have the light switch in the dining room turning the outlet on and off. Do you agree ?

Yes: It's not a contradiction, it's a choice. Your choice.

Edited to add: I don't understand why you wrote "Otherwise, you would need to wire it from upstream of the switch."

Couldn't I wire it from right at the switch (not upstream of the switch)?

On the upstream side of the switch, yes.

 

[Averagesupernova]

The term switch leg I am familiar with and what is described in post #2 contradicts it.
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To source power for an outlet you need to be sure the box you are hooking into has both a hot and a neutral. A switch leg does not. Where I come from a switch leg is a pair of wires coming from the ceiling light box that this pair of wires interrupts the hot. In that case there is a constant hot and neutral in the ceiling box.
-
The term hot leg I'm not familiar with.

 

[DaveC426913]

To source power for an outlet you need to be sure the box you are hooking into has both a hot and a neutral. A switch leg does not. Where I come from a switch leg is a pair of wires coming from the ceiling light box that this pair of wires interrupts the hot. In that case there is a constant hot and neutral in the ceiling box.

Where does the "switch" part come in to that definition?

 

[Averagesupernova]

Where does the "switch" part come in to that definition?

I didn't think I needed to point out the obvious. It hooks to the two wires at the end of the switch leg.

 

[Averagesupernova]

This diagram contains a switch leg:


This one does not:

 

[DaveC426913]

I didn't think I needed to point out the obvious.

It is obvious to someone who is familiar with it. Clearly, it is unfamiliar.

It hooks to the two wires at the end of the switch leg.

OK, so it sounds like you're talking about the same setup as us, just describing it from a different point of view.

But yeah, your diagrams are a good visual aid.

As a non-electrician, visualizing it does not come easy. I've combined the illos into one to highlight the differences.

Is this correct?

Functionally, they are the same thing - inasmuch as the only circuitry difference is that, in the left diagram the black-to-white wire is longer, whereas it is zero length in the right diagram.

Question for clarity: do both diagrams contain a hot leg? i.e. a hot leg describes a part of a circuit, not a type of circuit.

 

[Averagesupernova]

OK, so it sounds like you're talking about the same setup, just describing it from a different point of view.

A switch leg runs to a switch first, then to the load.

No.

 

[russ_watters]

This diagram contains a switch leg:
View attachment 360801

Ahh, I get it. So is it sometimes done that way due to proximity of the ceiling box to the source? If you run the source through the switch box you'd end up with more wiring in that case.

 

[DaveC426913]

No.

I see I was in error at first, or at least I was sloppy in my description.

I've amended post 20 with a diagram based on yours. Do I have it right?

 

[Averagesupernova]

@DaveC426913 yes basically correct. Notice in the first diagram I posted the little circles drawn. They depict the wires going into a cable sheath or conduit. Basically the entrance/exit to the box. The one diagram you posted has a circle out in the middle of nowhere. A bit confusing but the colors are done right concerning a switch leg.
-
Now here's something that throws a monkey wrench into the gears concerning the terminology. Many places now it is necessary to have a neutral at the switch to power occupancy sensing switches. In the case of a switch leg we just run three conductor cable. And of course now we can continue the circuit on from there. So is it still considered a switch leg? If we chose to wire it so the source stops at the switch first we could eliminate the run with three wires and just use two. But in that case there won't be a constant hot in the ceiling and some day we might want a ceiling fan. So, best to run three wires between the switch and load and the source goes to whichever box it is convenient.

 

[DaveC426913]

@DaveC426913 yes basically correct. Notice in the first diagram I posted the little circles drawn. They depict the wires going into a cable sheath or conduit. Basically the entrance/exit to the box. The one diagram you posted has a circle out in the middle of nowhere. A bit confusing but the colors are done right concerning a switch leg.

The circles are electrical boxes. One for the switch, one for the receptacle and one octagon for the split. (In your diagram you have a Marrette connector, but any connection like that would be in a box, not loose in the wall.)

The only functional difference between the two setups is that there is no split, therefore no octagon box.

Now here's something that throws a monkey wrench into the gears concerning the terminology. Many places now it is necessary to have a neutral at the switch to power occupancy sensing switches. In the case of a switch leg we just run three conductor cable. And of course now we can continue the circuit on from there. So is it still considered a switch leg? If we chose to wire it so the source stops at the switch first we could eliminate the run with three wires and just use two. But in that case there won't be a constant hot in the ceiling and some day we might want a ceiling fan. So, best to run three wires between the switch and load and the source goes to whichever box it is convenient.

Over my head.

 

[Averagesupernova]

The circles are electrical boxes. One for the switch, one for the receptacle and one octagon for the split.

I've never seen that in the wild. It would never be done in residential work since each box has to be accessible. Can't bury one in a wall or ceiling. Happy homeowner will not want boxes with blank covers.
-
Also a good way to irritate the drywallers. I've been asked by a drywaller "What are these boxes for!?!?!?!" in less than pleasant tone. Yep, the dumb electricians put in extra boxes for you to cut around just because.

 

[DaveC426913]

I've never seen that in the wild. It would never be done in residential work since each box has to be accessible. Can't bury one in a wall or ceiling. Happy homeowner will not want boxes with blank covers.
-
Also a good way to irritate the drywallers. I've been asked by a drywaller "What are these boxes for!?!?!?!" in less than pleasant tone. Yep, the dumb electricians put in extra boxes for you to cut around just because.

So where would you put the split, if not in a box? Just loose in the wall cavity? Or is that within the light receptacle box?

 

[DaveE]

So where would you put the split, if not in a box? Just loose in the wall cavity? Or is that within the light receptacle box?

NEVER loose in the wall.*

You would run the source wire all the way to one of the boxes that had a function (switch, light, etc.) first. This could also be a box that was present for some other reason, like an outlet.

*In the US there is an approved in wall splice fitting. But they are expensive and hard to buy because no one uses them. You'd only do this for remodeling work, I think. Never in commercial or industrial buildings.

 

[DaveE]

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

 

[DaveC426913]

NEVER loose in the wall.*

You would run the source wire all the way to one of the boxes that had a function (switch, light, etc.) first. This could also be a box that was present for some other reason, like an outlet.

Ah, so this:

It sounds like Russ has hit on it.
You might use the first configuration if the line is nearest the light receptacle, whereas you'd use the second configuration if the line is nearest the switch.

 

[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.
-
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.