Some electrical questions....

I am pulling out a 3way switch and replacing it with a smart switch. The second 3way switch will be dead. Since the load is in the second box, I am using one of the travelers to connect the switch to the load. I have run into a few odd things.

1)A box has 5 black wires nutted together. Two of them go to the same switch. Is there any conceivable reason for doing that?
2) That switch also has 2 other black attached to it. One goes to a second switch next to the first switch, I have no idea where the last wire goes. Essentially they are using the switch as a junction. I am changing that, but was it code compliant? (I vaguely recall that either a switch or an outlet can't do that, but don't really remember)
3) One light is set up so it uses a hot from one box and a neutral from another. They are on the same circuit. I vaguely recall that you can't do that with metal boxes because they set up an induction loop which can get hot and start a fire. There is no metal except at the panel, and since they are the same circuit that should be fine. So, is that setup okay with plastic boxes?

Thanks.

Presumably, the five black wires are hot, supplying current to the switches and downstream locations. This is easy to test. No reason for two hot wires to go to the same switch, lots of wacky things are found, especially with 3 way switches worked on by homeowners. I've never heard of that 'induction loop' issue- doesn't sound like a thing.

Two wires on the common lug (black headed screw and marked on the back of the switch)? Only three wires to the other 2 way switch? Sounds like an old shared neutral system which will cause nightmares if ever converted like it is to Arc Fault breakers.

Some of the numbered questions make no sense right off.

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Originally Posted by Cameron Wood

Presumably, the five black wires are hot, supplying current to the switches and downstream locations. This is easy to test. No reason for two hot wires to go to the same switch, lots of wacky things are found, especially with 3 way switches worked on by homeowners. I've never heard of that 'induction loop' issue- doesn't sound like a thing.

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Yeah, the 5 black wires are hot.
I looked it up in my 22 year old book on electrical residential wiring.
NEC 300.3(B) forbids #3 because of induction heating. BUT it makes an exception for nonferrous wiring methods, so i am good.

A single wire going through a steel hole is like an electromagnet, only AC heating the metal up. Two wires (a hot and a neutral) cancel each other so nothing happens.

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Originally Posted by Tom M King

Two wires on the common lug (black headed screw and marked on the back of the switch)? Only three wires to the other 2 way switch? Sounds like an old shared neutral system which will cause nightmares if ever converted like it is to Arc Fault breakers.

Some of the numbered questions make no sense right off.

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No, there are two switches in box A. The one going to light 1 has 4 black wires to it. Two come from the same wire nut and are hot. The third goes to Switch 2, and I don't know where the forth goes. Switch 2 is the 3way switch I will be replacing. A pair of travelers go from switch 2 to Box B with the 2nd 3 way switch in it. I am abandoning the switch in box B and will be using one traveler to connect Box 1/Switch 2 to the load.

Actually I made a mistake in my original post. I said there were 4 black wires going to switch 1 and I didn't know where one went. I traced it better and it is a traveler... the switch is a 3way. I missed that before. But it really doesn't affect my questions.

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Originally Posted by Wade Lippman

Yeah, the 5 black wires are hot.
I looked it up in my 22 year old book on electrical residential wiring.
NEC 300.3(B) for forbids #3 because of induction heating. BUT it makes an exception for nonferrous wiring methods, so i am good.

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Interesting- I missed that when I read through the code many years ago.

It also says (300-20b):

"When a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass, or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all the conductors in the circuit."

So that may mean cutting a slot between the knockouts that the single wires go through, or have them enter the box through the same knockout? "Insulating wall"- no idea what that means, perhaps a plastic knockout bushing qualifies.

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Originally Posted by Cameron Wood

Interesting- I missed that when I read through the code many years ago.

It also says (300-20b):

"When a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass, or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all the conductors in the circuit."

So that may mean cutting a slot between the knockouts that the single wires go through, or have them enter the box through the same knockout? "Insulating wall"- no idea what that means, perhaps a plastic knockout bushing qualifies.

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Cutting slots means to cut the box so the steel opening is not a complete circle. That prevents the induction loop. Beats me what an insulating wall is.

Insulating wall means a non-conductive material that the wires would pass through. It's typically made from a phenolic or fiberglass. This solution is more frequently done on large electrical enclosures with large conductors (3 phase, high amperage applications). We would frequently make custom ones for the large control enclosures our company builds.

So scaled down to a #14 wire carrying a couple of amps? Pretty much nothing.

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Originally Posted by Cameron Wood

So scaled down to a #14 wire carrying a couple of amps? Pretty much nothing.

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Yes, exactly right. It would be very low risk with a #14 and low current. Codes are written the way they are to cover as many applications as possible and to minimize the risk of fire.

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Originally Posted by Wade Lippman

No, there are two switches in box A. The one going to light 1 has 4 black wires to it. Two come from the same wire nut and are hot. The third goes to Switch 2, and I don't know where the forth goes. Switch 2 is the 3way switch I will be replacing. A pair of travelers go from switch 2 to Box B with the 2nd 3 way switch in it. I am abandoning the switch in box B and will be using one traveler to connect Box 1/Switch 2 to the load.

Actually I made a mistake in my original post. I said there were 4 black wires going to switch 1 and I didn't know where one went. I traced it better and it is a traveler... the switch is a 3way. I missed that before. But it really doesn't affect my questions.

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Is switch 1 a DPST? If so, it's an odd arrangement but the intent could have been to have switch 1 be a "master" that controlled two sets of lights. Flip it on, and two circuits are energized. One turns on light 1, the other goes to switch 2, and eventually to light 2. Light 2 then turn only if both switch 1 and 2 are on, and switch 1 always turns off both of the lights.

1. I don't see any reason for two hot feed or line wires to go to the same switch.

2. Sounds like you later figured a partial answer is that's a three way switch. It would be helpful to know where the other switch is, if it's still active or wired up properly.

2. Connecting the hot terminal of one switch to another instead of from a pigtail was common. I wouldn't wire a new switch like that but I think it was commonly done in the past. I've seen where they just strip 1/2" of insulation in the middle of the black wire and wrap it around one terminal on its way to the next one.

3. That setup sounds like it's functional but I don't think it would be ok freshly installed today. My impression is both current carrying conductors should be in the same sheathed cable (I assume you are using romex, not conduit). Ideally your second box would have a neutral and both the neutral and 'load' or switched hot would go to the light together in the same cable. Old timey wiring didn't always have neutrals in the switch boxes so that may explain it. It's possible if you trace and analyze all the wiring in the boxes and light you can make it compliant, but that might not be possible without rewiring.

From what you say for your project, I would remove the three way switch and replace it with a single pole. One of the travelers can be repurposed as a switched hot, and the other can be capped off with a wire nut. In the second box I would remove the switch, wire nut your switched hots like you intend and cap/wire nut the other end of the unused traveler. If it's a single gang box you can just put a blank cover on it, if it's two gang or more there are blank filler inserts to take up the space of a toggle/decora slot or there are covers that are 1/2 blank and 1/2 switch.

On a good day three way switches are hard to figure out. My favorite site for diagrams is:
https://www.do-it-yourself-help.com/...iring-diagrams

The easiest way to wire them now is with three wire (includes red wire) with ground cable but the old time way just used two wire cable which is harder to figure out.