why common trip breakers for 120V circuits?

I'm looking at a subpanel that was installed for an addition to my house about 20 years ago (well before I bought it). I'm mapping out the circuits which were never labeled.
I've found that four 120V circuits are tied to two common trip dual- pole breakers. There are no 240V devices or outlets on these circuits. I can see the wires tied to them and they are all black and run into separate runs of 12/2 that lead to the addition.
Why would this have been done this way? And is there any reason I shouldn't swap them with single pole breakers?
Also, one of those common trip breakers is rated at 30A. I've never seen a 30A breaker used for a 120V circuit and that seems wrong to me since the 12 gauge conductor isn't rated for 30A.
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Find first, what each cable is feeding, then it may make sense. Post back

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I don't think it helps. The breaker feeds only overhead lights and 120V outlets. Specifically, I count:
5 overhead lights 1 outdoor flood light 14 120V outlets
Two of those outlets are on the ceiling in a shop area and power 120V fluorescent lights. Two others power the two garage door openers.

4 12/2 cables tied to the two double-pole breakers. Each cable's neutral goes to the neutral bar, and their grounds go to the ground (which are separate since this is a subpanel).
Here's a part I didn't mention yet, but the more I look the more this seems like the answer. What is leading me to investigate these is I see that at some point previously the central A/C installers tied the a/c power onto these breakers by sticking their wires under the terminals with the existing ones (big no-no). Of course these devices are 240V so they need a 2-pole breaker. I already know that these a/c installers were utterly incompetent crooks, so I'm now thinking that they went to the trouble of replacing the single pole breakers with 2- pole so that they could feed the 240V, but of course didn't find breaker space for what they needed and instead double-tapped the breaker. This might account for the 30A rating on that breaker as well, although other A/C units are tied to 20A breakers, so who knows. I could see these guys just putting in a breaker until it didn't trip it and saying that was good enough.
The reason I'm investigating all of this is so that I can fix it all -- move the a/c's to their own 2-pole breakers, consolidate other circuits as needed.
I wanted to ask about whether there was a realistic need for a 2-pole breakers on individual 120V circuits and given your answers, I think my guess above is probably what led to this fiasco. Now I need to find out what the proper amperage rating is for the a/c units and fix all of this.
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Sounds like you've got it under control. You would use a double pole breaker if you had two 120 volt circuits feeding the same outlet, which would be split top and bottom

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Then why did you post without it? If you want answers, give compete information the first time. We can't see what you have, we can't discern what is hiding unless you give ALL the information pertaining to the subject. You wasted the time of a lot of people with your negligence.
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Edwin Pawlowski wrote:

I didn't see you offering help after the first post.
a
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well lets say some circuits are in the same boxes. its done but not common and can be unsafe unless you tie the 2 breakers together.
when one trips or is turned off so is the other.........
its another explnation to why someone did it this way.....
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On Sun, 3 Feb 2008 15:26:11 -0800 (PST), snipped-for-privacy@pookmail.com wrote:

12 guage wire on a 30A breaker a code violation and can be fire hazard.
From here, there is nothing wrong with using a 20A double pole breaker to feed two single 20A circuits. The only problem with it would be that if one circuit becomes overloaded and trips, it shuts power to both circuits. There is also no reason to change it unless you you have the need to turn one circuit off and have the other remain hot.
Also, you should have 4 white wires you haven't mentioned. You have 4-12/2 circuits leaving the panel. Right?
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wrote:

Now that I think about it, the easiest way to find out what you have would be to go ahead and change all the breakers to single pole 20s. :)
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snipped-for-privacy@pookmail.com wrote:

Sometimes this is done when 2 circuits end up down the line in the same junction box. I don't believe it's required but is safer as most people assume that once they turn off one breaker all is well... then come to find, the hard way, that there is another circuit in the same box. Happens alot in switch boxes at the main entrance or garage entrance. I have found a 3 gang switch box with 3 different circuits.
This does not sound like what you have but sometimes 2 circuits are wired with a shared neutral. Usually this is done using 12 or 14/3 cable. If this is the case then the 2 circuits should be tied and must remain on opposite poles else the neutral will be overloaded.

The only thing I know about that uses 120V 30A is an RV outlet. I have one on my house. It needs to be 10ga wire.
It may be that the installer got a good deal on those breakers or just used whatever he had left over from another job.
You will still need to map everything out to see what is going on.
Kevin
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This is usually true, but some breakers are designed and listed for two wires. For example, Square D QO breakers have a metal plate with two channels, one on each side.
Wayne
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This is 98% true. The one exception I know of is for motor loads. Motors can have a very high startup current, so in a dedicated motor circuit an alternate arrangement is acceptable, as follows. The motor itself or a dedicated motor controller or relay provides the overcurrent (overload) protection. The breaker is just for short circuit/ground fault protection. The breaker is sized so that it doesn't trip due to the startup current. So you could end up with a 30 amp breaker on a #12 wire. For further details, see Article 430 of the NEC.
Cheers, Wayne
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snipped-for-privacy@pookmail.com wrote:

I'm guessing it may have been done with Edison circuits (common neutral) as that would require common trip or 240V style breakers. Are half the hot wires red in color?
I don't know why you have a 30A breaker in there, either if the wire leaving is 12AWG. that sounds wrong to me.
nate
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