Two circuits tied together


Old 08-29-01, 07:20 PM
Thread Starter
Join Date: Dec 2000
Posts: 510
Received 0 Votes on 0 Posts
I ran across an interesting wiring problem when replacing a ceiling light fixture for my mother-in-law.

The first problem was finding the right breaker to shut off. There were six breakers in her panel for general lighting and recptacle circuits. I tried each one individually, but none of them killed the light. In frustration, I turned them all off. The light went out. Uh-oh.

I opened up the ceiling box. There were 4 romex cables coming in, all 2 wire w/ground. One was clearly a switch leg, since its black was tied to the light's black and the white was tied to a cluster of blacks. All the other blacks were tied together, as were the whites and grounds.

I separated all the cables and identified them as follows: (1) the aforementioned switch leg, (2) one cold cable, probably connecting to downstream outlets, and (3) TWO hot cables, each one controlled by a different half of a duplex circuit breaker. Obviously this is not good - essentially two circuit breakers in parallel on this circuit.

My fix was as follows: (1) Reconnect all the grounds together; (2) connect black and white of one hot cable to the cold cable; and (3) connect the second hot cable to the light (through the switch leg). Everything works, and the duplex circuit breaker is no longer in parallel.

My question is: did I fix this problem correctly? Specifically, I now have a ceiling box that has conductors from two circuits in it, one for the light and one just "passing through" to the wall outlets. Is this kosher? I remember vaguely something about only one circuit in a junction box, but I don't know if this applies to "pass-throughs".

Comments, please.
Sponsored Links
Old 08-29-01, 08:23 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Received 0 Votes on 0 Posts
Your analysis sounds flawless.
Your solution was innovative.
Everything sounds fine.

One question: when you say "duplex circuit breaker", are you talking about skinny breakers where you have two 120-volt breakers in a 1" space??

Of course, you're not quite sure there aren't other places where these two circuits come together. You might want to shut off the main, disconnect the two black wires from these two breakers, and do a continuity check between them.

But you have definitely made an improvement. You might consider some way to alert a future electrician to the circumstances.
Old 08-30-01, 07:28 AM
Thread Starter
Join Date: Dec 2000
Posts: 510
Received 0 Votes on 0 Posts
Yes, by duplex breaker I mean two independent breakers in the same slot, so the two lines are on the same phase (if they were adjacent breakers on opposite phases, the problem would have been immediately apparent as one phase got shorted to the other when the breakers were truned on!).

Also, I think the fact that the two breakers are now operating independently (no longer in parallel) indicates that the two circuits do not join together anywhere else. (I'd do a continuity check as well, but I am not at home and don't have my meter with me.)
Old 08-30-01, 10:00 AM
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 1,052
Received 1 Vote on 1 Post
Mike, you seem pretty astute regarding electrical stuff. The two breakers in one space is called a tandem breaker. Some give it the nickname piggyback breaker. There is nothing wrong with this arrangement technically. They are treated as two completely independent single-pole breakers, mainly because that's what they are. If they were side-by-side and the handles were tied together it would be a 220v, 2-pole breaker. Nobody would generally want to do this, but someone without a lot of sense could possibly run two independent 110v circuits from a 2-pole breaker, keeping both runs completely separate throughout both circuits. The only drawbacks would be turning off only one of them, and if something on one of them tripped a breaker, two circuits would go off. This might make troubleshooting difficult, since determining which one actually blew would be difficult. Anyway, that's not the case here, I'm just tossing that out for general discussion.

Good luck,


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes
Ask a Question
Question Title:
Your question will be posted in: