12-3 Double Circuit Run Length - Should I Split the Circuit?

Hello
I just finished wiring my basement with 12/3 wire. I was planning on using a double pole breaker with two circuits and a shared neutral (alternating circuits between outlets). My run is around the perimeter of my basement inside wood framing studs. After all was said and done with the requried routing, I've used about 250ft of wire.
I'm concerned that I may have too long of a run and that, instead of creating one big 250ft run with alternating circuits on a double pole 20A breaker, I should just have two 125ft circuits on separate 20A breakers. I guess now's the time to make my decision before I go into my panel. I've already run the 12-3 wire but I don't mind using only 2 of the 3 wires if that will make these circuits better performing and safer.
The reading I have done so far and the 13.6% voltage drop at 20A calculated using this website:
http://www.powerstream.com/Wire_Size.htm
has me leaning heavily toward two separate 125ft circuits.
Any feedback is apprecaited. Thanks!
Kevin
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A balanced multiwire circuit can be half the voltage drop because there is no return path. So, there are two things to consider... will you actually be using 20a on each side, and can you get a balanced circuit.
Then you might want to rethink things; 20a is too much for a 125' run of 12/2.
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I really don't plan on using 20A on either side. I was planning on using the 20A mainly because I had the 12 gage wire & I was sticking with the 12 gage because of my run length. I originally thought the length would be about 170ft but I didn't take into account the routing I had to do up and down to my outlet boxes and around things.
I would call the circuits light use with a TV, dehumidifier (spring/summer) and possibly an electric space heater (occasionally) in the winter. Maybe a computer and some lamps will eventually be down there too but I don't plan on using really heavy draw power tools or having a workshop down there or anything.
Lights are on a separate 15A circuit.
Thanks for your feedback!
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Also, could you elaborate on how I can reduce the voltage drop with my 12-3 wire?
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One other thing I forgot to mention...
If I split the circuit into two 125ft runs, most of my electrical loads I mentioned above would end up on one of the circuits. And, naturally, my circuit breaker box is at the opposite corner of my basement from where most of the used outlets will be.
Thanks again for your feedback. Kevin
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If all the heavy stuff will be on one of the 125' circuits, you wouldn't gain anything that way; it will still be too much and too long. You have to get the following right, or you risk burning your house down. No joke; don't do it unless you are darned sure you are doing it correctly. A multiwire circuit is essentially a 240v circuit split between two 120v circuits. If you can get the two circuits reasonably balanced, the only current coming back on the neutral is the difference between the two; so there is little or no voltage drop on the neutral which reduces the total voltage drop. Unfortunately, the dehumidifier and the heater are both big loads and impossible to balance. Unless you can get them at the first part of the run, you are out of luck. You could get a 240v heater (which is automatically balanced), but I haven't seen a 240v dehumidifier.
To be sure you would have to do a detailed analysis with specific loads and distances, but I suspect you are looking at putting in either a #10 circuit for the big loads or a 240v circuit.
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OK. So what you're saying is that the more balanced the two circuits on the double pole breaker are, the less voltage drop I will have. Theoretically then, if I had a 250' run with two outlets at the end of the dual pole 20A breaker (one on circuit 1 and the other on circuit 2) and had a 15A load on BOTH of them I'd be OK. But, if I removed ONE of the 15A loads, I could be in trouble because the voltage drop would go up and drive the load way up. Correct?
I think I could balance the load between circuits but I can't guarantee it would be that way all the time so that may not be the best option for me. Also, 10AWG wire will still have a high voltage drop for a 250' run (based on the voltage drop calculator using 120v).
What if I have two separate single pole 125' circuits using my 12-3 and two 15A breakers, BUT, I combine the two hots of the 12-3 to make it 12-2 inside a junction box just before going into my circuit breaker box (12-2 would be going into the box connecting to the breaker) and, along the run, pigtail the two hots of the 12-3 together to look like 12-2 inside the outlet boxes. I will then have a very low voltage drop on my 125ft run because the two wires of the 12-3 are combined. It seems this option would get me the resistance of 8 or 9AWG wire while at the same time giving me the shorter 125ft runs.
Thoughts? Dangers? I know if one the splices where the 12-3 is combined to look like 12-2 fails then load & resistance will go up driving up the load. But, that would be on an outlet by outlet basis ( I could even check it with a meter) and the 15A breakers would limit risk somewhat. This method seems safer to me than trying to guarantee I have the balanced double pole circuit option described above but maybe I'm missing a potential problem. Thinking about it, it looks like I'd have to join 4 12AWG wires together with a 12AWG pigtail for each outlet (making 5 wires) inside a wirenut. That might be a problem?
Thanks again...and I appreciate your feedback greatly.
Kevin
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On 18 May 2006 04:15:54 -0700, snipped-for-privacy@blah.com wrote:

That's WRONG. Look, there is no free lunch. Your fixtures and appliances will draw their rated current, and incur the same voltage drop no matter how well balanced the multiwire circuit is.
It is true that, if perfectly balanced, there will be no current in the neutral wire in a multiwire circuit. But in this case, the entire load is borne by the two hot leads. Any imbalance results in a current in the neutral lead equal to the magnitude of the imbalance (assuming pure resistive loads). But at ALL times, the total current is the same.
The multiwire circuit is just a "trick" to enable having two hot lines share a single neutral that is undersized by 50%. It works because the worst-case imbalance, where 100% of the load is on one of the hot leads, can only result in the same current flowing in that neutral as is flowing in the hot wire.

Nope.
Think of it this way. You have a pair of 100W incandescent lamps, one connected to each circuit, at the end of 250 feet of wire. (For this example, I assume these bulbs are ideal constant resistive loads.) A 100W light bulb looks about like a 144 ohm resistor, so it will draw about 0.83A when the voltage across it is 120V. When both lamps are on, that current is flowing through both lamps. No current is flowing in the neutral. The total length of wire feeding this circuit is 500 feet (the two hot leads in series).
Now turn off one of the lamps. What happens? Now the same 0.83A is flowing though the one hot lead, and the neutral lead. The total length of wire feeding this circuit is exactly the same, 500 feet (one hot lead and the neutral lead). So the total voltage drop is the same.
I'll leave the thought experiment for an partial imbalance to you. Try it with one 200W and one 100W lamp.
--
Art


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On Thu, 18 May 2006 04:22:16 GMT, Toller wrote:

Bzzzt! Wrong. That would violate some rather basic physics.
You simply don't understand what happens in a multiwire circuit.
--
Art


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Kevin, stop worrying about the voltage drop in your basement. It won't be a problem with the intermittent loads that you will have on your outlets. You can have 2- 20amp circuits or 4-20 amp circuits. That decision is up to you. Don't double up any hot wires. If you have 12/3 going to each box you can alternate circuits in every other box. For instance box one will be fed from the black wire, box two will be fed from the red wire, box three will be fed from the black wire and so on. That way you will have more even use of the circuits, but balancing them should not be a major concern of yours either.
Now get to work. I want to see pictures of the finished product. :-)
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I don't know how you have done your wiring but I don't think you have any outlets that are really 250' of wire length to the panel. Didn't you go to a certain place and branch out? I think you would calculate the drop of each outlet independently. The furthest one away would be the worst case everything else would be in between. If you are really worried about it then run 10/3 from the panel to the first junction. However I would consider splitting things up and adding at least 2 more breakers.
FYI- I have a 1300 sf basement and have 6 circuits for outlets/lights. 3 are 20A and 3 are 15A. Not including a bathroom and furnace room which also have there own circuits. That is for 5 rooms and a hallway. A couple of the circuits are used for lighting as well as outlets.
I used 12/3 on the 3 rooms that I finished myself. I use the extra wire to provide some switched outlets.
Kevin
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Thanks again to all for your feedback. I'll try not to worry about the voltage drop but I just want to make sure I do this right! :-) I will be living with it for a long time. :-)
See the image below for a rough drawing of my layout. The outlets are spaced so that they meet the spacing required by code with a couple extra thrown in just for convenient locations.
http://img77.imageshack.us/img77/6990/layout27yn.jpg
My current plan is to separate the circuits at the 125ft corner rather than have one big 240v split circuit with alternating outlets that I have to balance. If anybody else has feedback I'd love to hear it. Specifically, is there any way to reduce voltage drop on a long run other than to use super high gage wire that is a PITA to work with?
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You already put in 12-3, you might as well alternate on both circuits. But it really doesn't seem to matter.
Consider: do you have electricity in the far reaches of your first floor? And are these wires not simple 12ga or 10ga runs from the same panel? And are these not even longer runs than your basement run?
-Kevin
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I just thought of that. If I have room in my box for two double pole breakers, I'll try to do alternating circuits for both branches. I think I only have 3 slots left so maybe I'll do one double pole and one single pole.
I do have electrical service in my garage and it's at about 125'. I believe it's a 14GA 15A circuit. I know there's a voltage drop issue there because I can't run my little pancake air compressor there (it will barely kick over and never actually run) but the compressor runs fine when run off an existing outlet right by my circuit breaker (for the sump pump).
I have a bunch of 15A breakers in my box for my house. Is there any way to make more room in my circuit breaker box for additional breakers or is the only solution to upgrade to a bigger box (which I have no plan on doing)?
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I ran out of room, so I got duplex breakers. They combine two 240v circuits in the space of one. Maybe they are available for your brand. They don't give your box any more electrical capacity, just more room; so don't over do it.
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You don't need two separate double breakers. Just use a single double breaker for both runs. The blacks on both runs go to one phase, the reds go to the other phase.
I think you can get rid of some breakers if you like by combining two runs into a single circuit (as long as you are careful about hidden split circuits (where they didn't use a double breaker as they should have). Combining two 15A circuits from one of the phases into a single 15A circuit on the same phase will in most cases be fine. Most circuits in your house will have less than a few amps at all times (unless you have a penchant for plugging in and turning on ALL your appliances and lights at the same time -- and even so... last time I tripped a breaker due to simply too much draw was at least a decade ago).
Check with some others first, and think about the circuits a little before doing anything. Sometimes there are reasons circuits are split: maybe they really do need that much current (space heaters, AC, larger loads, etc.), or are required to have their own circuit for code reasons, etc.
-Kevin
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Toller- I will look into the duplex breaker. Sounds like exactly what I could use.
Kevin- I believe that, according to code, it's against the rules to to have two wires attached to any terminal in the box. So I think what you're saying will technically work but is considered a 'no-no'.
Thanks again for everybody's input!
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My circuit breaker box has Siemens Type QP single pole 15A breakers. Any idea what the compatible duplex version of the breaker is so I can make some more room in my box?
I'm looking on the Siemens website but it's not jumping out at me...
Thanks, Kevin
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Look on the panel cover or inside the panel for the maximum allowed circuit breakers for that panel. It will also tell you what breakers are approved for that panel. I think Murray circuit breakers are compatible and approved for use with Siemens. You can buy Murray at Home Depot.
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Thank you very much. You all have been very helpful and I appreciate your taking the time to give me feedback! Hopefully I can pass on what I've learned to help others like you have helped me.
Thanks again, Kevin
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