Trying to figure this one out

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  #1  
Old 03-29-06, 03:45 PM
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Trying to figure this one out

I have a 35' run of 10-3 from my 200A sub-panel. I am planning on runnibg two 12-2 circuits from the J-box. My question is can I run the #12 65' from the J-box where it connects to the #10? I know it will depend largly on the load. It will be typical house lighting (1 circuit) and the other one will be outlets for a family room. I cant think I could laod it at more than say 6-8 amps at any time. I have the option of using either a 15A or 20A breaker. Any and all thoughts from all the brilliant folks in this forum are welcome.

 
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  #2  
Old 03-29-06, 04:45 PM
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Hooter,

I gather from what you are saying you are going to be attempting a Multi-Wire Branch Circuit which will be sharing the neutral on these (2) 20A circuits you would like to have supplied by it.

It is very important to understand the correct way to do a Multi-Wire Circuit as doing it incorrectly in the panel can lead to a neutral that is undersized.

http://www.electrician.com/vd_calculator.html

Above is a nice online voltage drop calculator to assist you in figuring the overall voltage drop but I will try to break it down in easier terms.

The 65' of # 12AWG should be fine as long as the amps on each line does not exceed 10 amps...then the voltage drop would start to creep to exceed the recommended allowance.

BUT keep in mind that in the panel you MUST protect the smallest wire in this case the # 10AWG would need to be on a 20A breaker....also if you ARE going to do a multi-wire circuit make sure the breakers are on different phases in the panel itself......this requires an approved handle tie breaker as well.

BeCareful as Multi-Wire Circuits in this fashion are not recommended because of again the potential for error and hazards. If done incorrected the neutral carries the unbalanced load of BOTH ungrounded conductors.

Yes, it offers a benefit of running less wires but it is important to know exactly how to do it.
 
  #3  
Old 03-29-06, 04:55 PM
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I think I understand what you are saying. If say both branches of the 10-3 were maxed or shorted, the neutral(white) is gonna be too small. To let you know, I have bonded the ground (bare) to the box per code. Not sure if that is what you meant by improper subpanel. All of the entrance, subpanels and wiring is new and the Main and both subs have passed city inspection. I think I got it. Thanks
 
  #4  
Old 03-29-06, 04:55 PM
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Originally Posted by Hooter
I have a 35' run of 10-3 from my 200A sub-panel. I am planning on runnibg two 12-2 circuits from the J-box. My question is can I run the #12 65' from the J-box where it connects to the #10? I know it will depend largly on the load. It will be typical house lighting (1 circuit) and the other one will be outlets for a family room. I cant think I could laod it at more than say 6-8 amps at any time. I have the option of using either a 15A or 20A breaker. Any and all thoughts from all the brilliant folks in this forum are welcome.

first of all and maybe someone can verify this. I would never change the thickness of the wire on any given run. Also you say you have an option of using a 15-20A breaker. I have never used less than a double pole 30A on 10-3 wire and never less than a 20A on 12g. It doesn't seem quite kosher. If you want to run 2 circuits off one line for that long a run I probably would have run 12-3 to the junction on a double pole 20A then split it with 12-2 from there. Again maybe someone else can tell you exact codes for that.
 
  #5  
Old 03-29-06, 05:11 PM
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BuiLDPro68:

Remember that #10 wire is physically larger than #12 wire.

There is no code prohibition against using a _lower_ amp breaker than a particular conductor can handle. This means that while 30A breakers are very common on #10 conductors, it is entirely allowed to protect that #10 conductor with a 20A or smaller breaker.

There is also no code prohibition against mixing wire sizes in a circuit, as long as the breaker protects the _smallest_ conductor in the circuit. This means that if you have a mix of #10 and #12 in a circuit, the largest breaker that you are permitted to use is a 20A breaker. Using larger wire for the initial part of the run to reduce voltage drop is one of the common reasons for mixing wire sizes.

Hooter:
If you do not understand how multi-wire branch circuits work, then I strongly suggest that you do more reading on the subject. If _properly_ installed, then when both circuits are used at 'max', then not only will the neutral not be overloaded, it won't be carrying any current at all. On the other hand, if _improperly_ installed, the neutral will quite easily overload, even if the circuits are not being fully used. Before you continue, make sure that you understand such circuits.

The suggestion that you use a handle tie breaker is _not_ a code requirement, but it is a _very_ good idea. By using a two pole handle tie breaker you make it easier to be certain of a safely connected circuit.

-Jon
 
  #6  
Old 03-29-06, 05:39 PM
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Winnie..... I am not well versed on multi-wire circuits. In fact until this post, I saw no reason for concern. I have not had any problems with any of my wiring to date, but now I wonder if I have any bad conditions. I do not have any multi wire circuits in my house yet. When you speak of "properly installed", I am not sure what I need to look for beyond solid connections, gound to the metal J-box and whites (neutals) together..... What am I not getting here?? Thanks
 
  #7  
Old 03-29-06, 05:44 PM
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On a multiwire circuit, the voltage between the two hot wores must be 240 volts. If the voltage between the hot wires is instead 0 volts, then you don't have a multiwire circuit, but rather you have a fire waiting to happen.

To get 240 volts, the two hot wires must be connected to alternate sides of your incoming 240 volts.
 

Last edited by racraft; 03-29-06 at 06:03 PM.
  #8  
Old 03-29-06, 05:50 PM
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Bob... I fully understand the concept of the voltage per hot on the 10-3. What I dont understand in your post is the reference to 0 volts. I assume you are reffering to some type of wiring error that would cause that condition. Please clarify

Thanx Doug
 
  #9  
Old 03-29-06, 05:51 PM
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Hooter:

The circuit that you described in the original post, where you use 10/3 with ground and a shared neutral, _is_ a 'multi-wire branch circuit'.

As racraft notes, you should have 240V between the 'hots' of these two circuits, and as ElectricalMan notes a good way to get this is by using a handle-tied 240V double pole breaker. In addition to the above comments, I would suggest searching these forums and a couple of wiring books for 'multiwire branch circuit'.

-Jon
 
  #10  
Old 03-29-06, 06:02 PM
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If you incorrectly wire a multi wire circuit to two hot wires on the same side of the incoming 240 volts, you will not have 240 volts between the wires, you will have zero volts between them, indicating a problem.
 
  #11  
Old 03-29-06, 06:16 PM
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Thanks all......I am shakey in theory, but OK at the mechanics .....After a little more research, I think I am much better served by pulling TWO 10-2, reducing to 12-2 at seperate J-boxes, and using 2 seperate 20A breakers. This will also allow me more flexability when it comes time to "balance" the sub-panels. I understand the need to get power for a MWC from both sides of the Main......
 
  #12  
Old 03-29-06, 06:26 PM
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You don't have to do two runs of 10-2, just use a 240 volt breaker.
 
  #13  
Old 03-29-06, 06:41 PM
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Originally Posted by winnie
BuiLDPro68:

Remember that #10 wire is physically larger than #12 wire.

There is no code prohibition against using a _lower_ amp breaker than a particular conductor can handle. This means that while 30A breakers are very common on #10 conductors, it is entirely allowed to protect that #10 conductor with a 20A or smaller breaker.

There is also no code prohibition against mixing wire sizes in a circuit, as long as the breaker protects the _smallest_ conductor in the circuit. This means that if you have a mix of #10 and #12 in a circuit, the largest breaker that you are permitted to use is a 20A breaker. Using larger wire for the initial part of the run to reduce voltage drop is one of the common reasons for mixing wire sizes.
I just find it unusual to use 10-3 for regular outlets. I have run it only for high draw dedicated circuits like an oven or dryer. I understand that there is no danger being under code just not over. Like running 14-3 on a 20A say.
 
  #14  
Old 03-29-06, 06:52 PM
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BuildPRO..... The reason for the #10 is that my runs are very long and I will suffer a high voltage drop. By coming from the box for 1/3 of the distance, I decrease the drop. I am not using #10 for the outlets, merely "supplying" power to a #12. A house wiring system is a series of different sized wires being "mixed". That all starts with your main entrance. Mine is 4-0. But by the time it gets to the outlets or lites, I am down to #12. I wont use #14 for anything except tie wires!!!! Hope that helps

Doug
 
  #15  
Old 03-29-06, 09:11 PM
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"Tested" Master Electrician.
ARE'NT We all tested? Even After we get licensed?
Is there another way to get one? (license)
 
  #16  
Old 03-29-06, 09:38 PM
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Originally Posted by Hooter
I have a 35' run of 10-3 from my 200A sub-panel.
I plan to run two 12-2 circuits from the J-box.
My question is can I run the #12 65' from the J-box where it connects to the #10?
Yes. (That doesn't make it the greatest option.)


> the other one will be outlets for a family room.
> I can't think I could load it at more than say 6-8 amps at any time.

But it probably will happen eventually.


> I have the option of using either a 15A or 20A breaker.

Yes. I see no reason not to use 20A.


> The reason for the #10 is that my runs are very long
> and I will suffer a <s>high</s> voltage drop.

True if you delete the word "high".
What are you running for which this even matters?


> By coming from the box for 1/3 of the distance, I decrease the drop.

Very true.


> I am not using #10 for the outlets, merely "supplying" power to a #12.

This is fine. Solid #10 is a harder to wire on screws because it is stiff. But with backwired devices (spec grade receptacles, GFCI recepts), it is no problem.

If you use a double-pole breaker, the multi-wire setup actually decreases voltage drop without risking a overloading the neutral.
 

Last edited by bolide; 03-30-06 at 03:24 PM. Reason: with ==> without
  #17  
Old 03-30-06, 08:35 AM
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> I know it will depend largly on the load.
> It will be typical house lighting (1 circuit) and

100' away?

When you factor the distance up to a 12' ceiling, across the trusses, back down to floor , then the "chain" of outlets, it is 108' to the last outlet on my run.[/

True if you delete the word "high".
What are you running for which this even matters?

The word high is a relative word. I caslled it high drop because it was over 5%. The reason that it matters, as I understand, is that as your voltage drops, your amps increase.

If you use a double-pole breaker, the multi-wire setup actually decreases voltage drop with risking a overloading the neutral.

I thought that using the double pole breaker, hence insuring a different circuit on each side of the buss, lowered the risk. What am I not getting here??Thanks all
 
  #18  
Old 03-30-06, 09:20 AM
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Hooter:

You are getting it just fine.

A multiwire circuit properly wired uses less wire, and has lower voltage drop. These are the positive reasons for using a multiwire circuit.

A multiwire circuit is more complex, and has a greater risk of being wired incorrectly. If wired incorrectly, then the voltage drop is worse than for a normal pair of circuits, and the neutral is no longer properly protected from overload. These are the negative reasons for _not_ using a multiwire circuit.

Using a 240V breaker rather than two separate 120V breakers to supply the pair of circuits which comprise the multiwire circuit significantly reduces the risks of miswiring. This is not required by code, but is my strong suggestion for most circumstances.

IMHO you are in a good position to study up on multiwire circuits, and then go ahead and use one in this situation.

-Jon
 
  #19  
Old 03-30-06, 09:58 AM
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"A multiwire circuit is more complex, and has a greater risk of being wired incorrectly. If wired incorrectly, then the voltage drop is worse than for a normal pair of circuits, and the neutral is no longer properly protected from overload. These are the negative reasons for _not_ using a multiwire circuit."

Since the wiring in the J-box should be a "no brainer", black to black, red to other black, whites together, grounds togeter and attached to metal J-box, then the only real danger of mis-wiring is at the subpanel by getting both legs of the 10-3 on the same buss. There, I think I got it.........
 
  #20  
Old 03-30-06, 12:19 PM
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You got it.

I would use two single-pole breakers, and ensure they are on opposite phases. No common handle is required, as long as neither circuit is present on the same device. An example would be if you had two big cord-connected appliances that you wanted to give 20 amps to, on their own circuits.

You can break the tab on a regular duplex receptacle that connects the two gold (hot) screws, and have a multi-wire receptacle that serves the two appliances.

The problem is, if your buddy comes along behind you to replace the receptacle, plugs in a lamp to one half of the duplex, not realizing that the other half of the duplex receptacle is on another circuit, he will shut off only half the duplex, and get shocked when he pulls out the receptacle he thought was dead.

So the code requires us to connect the handles of the two breakers, so that one half of the receptacle can't be shut off without the other. This applies regardless of whether it is a multiwire circuit or not.

My point: There's no reason to connect the handles and force the lighting to be shut off with the receptacles. It would be a superior design (in my opinion) to allow the receptacle's breaker to trip on it's own, without killing the lights in the same area.

If folks are reading this and not fully getting it, then they should proceed with the two-pole solution, because it eliminates one complication of the design, as you physically can't accidentally place both hot conductors on the same phase, creating a fire hazard neutral.
 
  #21  
Old 03-30-06, 03:22 PM
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Originally Posted by Hooter
>it is 108' to the last outlet on my run.
On the second floor?

This would be a case for a subpanel to avoid so many branch circuits travelling across the house.


> I called it high drop because it was over 5%.
I have my doubts. Are you really drawing 20A at the last receptacle?


> as your voltage drops, your amps increase.

Not in a receptacle!

As voltage drops, amps decrease.


> I thought that using the double pole breaker,
> hence insuring a different circuit on each side
> of the buss, lowered the risk.

Correct. I made yet another typo!
 
  #22  
Old 03-30-06, 07:14 PM
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On the second floor?

This would be a case for a subpanel to avoid so many branch circuits travelling across the house.
Not a 2 story just high ceilings. Already planning a 100A subpanel from the same 200A sub-panel. Notsure I could pull another one.

> I called it high drop because it was over 5%.
I have my doubts. Are you really drawing 20A at the last receptacle?
If I use #12 for the 108' run, a 14 amp load at the last recepticle would cause a 5.2% drop. I know a 14 amp load is not likely, but a slumeber party and 2 blow dryers,a popcorn popper all on the same family room circuit....who knows. I wold rather be safe and not risk any overload.

Since we are on the subject, could I pull a second 100A sub from this 200A subpanel?? Would be alot easier than J-boxes.......Not sure if NEC approved......
 
  #23  
Old 03-30-06, 08:56 PM
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> Not a 2 story just high ceilings

Probably would have saved 20' if the wires were run from underneath.


> If I use #12 for the 108' run, a 14 amp load at the last
> receptacle would cause a 5.2% drop.

Not a problem from what you describe. What did you use as ambient temperature?

> I know a 14 amp load is not likely, but a slumber party and
> 2 blow dryers,a popcorn popper all on the same family room
> circuit....who knows.

No problem.


> I would rather be safe and not risk any overload.

The breaker protects the wire. There is no danger to #12 wire with a 20A breaker.


The voltage drop actually decreases the amperage somewhat.
It never increases it.


> Since we are on the subject, could I pull a second 100A sub
> from this 200A subpanel??

Yes.
 
  #24  
Old 03-30-06, 09:10 PM
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Many thanks to all that contributed to my post.......

 
  #25  
Old 03-30-06, 11:11 PM
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volt and amps

I hope this doesn't constitute beating the subject to death...


The reverse of "decreasing voltage decreases amps" also holds:
increasing the voltage increases the amps drawn.


Suppose you have a household light bulb that draws 0.8A at 125V.

Now you take this same bulb and hook it to a car battery that is 12.5V and can supply 600A.

That bulb will now draw 8A (ten times what it did in the house) because of the 90% voltage drop, right?

Not at all!

The bulb will draw perhaps 0.1A and not light up.


Suppose you took a 12V bulb that draws 4A (do not try this at home!) and connected it 120V.
Would it now draw only 0.4A because of the 900% voltage increase?

No! It won't even be happy with just 4A (which would make it its normal brightness).

It will draw close to 40A and burn out practically instantly.


What is true is that a 12V 100W bulb draws ten times the amps that a 120V 100W bulb draws.
This is true only when the bulb stays on the voltage for which it was designed.

A 130V bulb draws less current (fewer amps) at 120V than it does at 130V.
So a voltage drop always means a lower amperage draw.

If the voltage is 125 and drops and drops to 117 at the last outlet, a blow dryer just won't get quite as hot and the popcorn popper will take a little longer.

117V is above the 110V needed to operate appliances.


(None of this is intended to dispute the value and efficiency of heavier conductors and the importance of limiting voltage drop. This is only to address the safety of circuits with voltage drops down to 110V.)
 

Last edited by bolide; 03-30-06 at 11:21 PM.
  #26  
Old 03-31-06, 08:43 AM
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Well stated bolide.... Thank you for clearing up a misunderstanding that I had..... I have always tried to "up-size" my wiring projects....... The wire is cheap when compared to the security and peace of mind.... I have always believed that #14 wire was made soley for the pur[pose of tieing tomato plants to their stacks, not for wiring...... The example of the broken #14 at the pigtail in another post only solidifies my position

Thanks All
 
  #27  
Old 03-31-06, 02:27 PM
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gardening

> I have always tried to "up-size" my wiring projects.
I agree to a point.

> The wire is cheap when compared to the security
> and peace of mind....

My reasons are improved peformance and efficiency.


> I have always believed that #14 wire was made solely
> for the purpose of tying tomato plants to their stacks
smoke detectors and ground pigtails.

May I trust that you use 1/2" EMT to support your tomatoes?


> The example of the broken #14 at the pigtail in
> another post only solidifies my position

I know what you mean.
 
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