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Using multi-wire circuits for lighting


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This topic has gotten quite a bit of discussion in the thread "New Electrical Question" and there seems to be a lot of confusion.

It's always nice to be able to get the most electrical capacity with the fewest wires. One common technique is referred to as a multi-wire circuit.

A multi-wire circuit allows two separate circuits to be fed with a single 3-conductor cable (plus ground), instead of using two separate 2-conductor cables. As a bonus, there will be less voltage drop in a multi-wire circuit compared to two separate circuits, as much as 50% less if the load on both sides of the circuit is equal.

This technique is commonly used where you need to power two high-current loads at the same location. It is used frequently for kitchen appliance circuits. A common application with LOR would be to power the 2 sides of a 16-channel controller separately to get the full 30 or 40 amp capacity.

This approach works because the two hot wires are fed from opposite legs of the electrical service. As a result, the current flowing in the single neutral conductor is actually the difference of the currents flowing in the hot wires. Because of this the neutral wire doesn't need to be any larger than the hot wires.

If the two hot wires were fed from the same side of the electrical service, the neutral current would be the sum of the hot currents, or up to 40 amps if using two 20 amp circuits. This would obviously require a much larger neutral conductor.

To guarantee that the two hot wires are fed from opposite sides, a multi-wire circuit must be fed from a 2-pole breaker. This is a breaker that takes up 2 panel spaces and is basically the equivalent of 2 separate breakers with the handles mechanically tied together. Because electrical panels are internally wired such that adjacent vertical spaces are powered by opposite sides of the service, a 2-pole breaker will always have the poles on opposite sides of the service, regardless of where it is installed in the panel.

From the panel you would then run a 12/3 W/G (3 #12 conductors plus a ground) to your receptacles. Typically you would use 2 GFI receptacles on a multi-wire circuit. The neutral and ground wires connect to the corresponding terminals on both receptacles, while the hot wires each connect to the hot terminal of one receptacle only.

The end result is two 20 amp circuits fed from a single cable.

GFI protection of a multi-wire circuit can also be accomplished by using a 2-pole GFI breaker, although these tend to be quite expensive. If you do use this approach, you can terminate the circuit in a single receptacle by electrically splitting it. All duplex receptacles have tabs that connect the upper and lower hot and neutral screws. To make a split receptacle on a multi-wire circuit, remove the tab between the two hot (brass-colored) screws. The 2 hot wires of the cable then connect to the 2 separated hot screws, while the neutral and ground wires connect in the normal fashion.

Now you have two 20 amp circuits terminating in a single receptacle, and the 2 cords from the LOR controller just plug into the top and bottom.

Remember that if you are planning on using the full 20 amp capacity of a circuit on a single receptacle, that receptacle needs to be rated at 20 amps. And please make sure that you have GFI protection, either from GFI receptacles or a GFI breaker.

-jim-

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After reading some NEC sections referred by tsmith35, I have one correction. It appears that a double-pole breaker (as opposed to 2 single-pole breakers) is only required if you are using a split duplex receptacle. If you use 2 separate receptacles then the use of separate breakers appears to be OK per the NEC, but you *must* insure that those breakers are on opposite legs of the AC service in order to avoid overloading the neutral conductor.

-jim-

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Nice article you wrote up jstjohnz. This is some very good information that a lot of LOR users need. I would like to add if you are using 12/3 power cord with a ground that you should use the GFI Circuit Breakers instead to protect the temporary power cord you ran. This added safety is always nice to have around. I would use 15 Amp breakers on the 12/3 power wire to be on the safe side as you might have power cords longer that 50’ feet. If you absolutely need the full 20 amps per circuit then I would use a 10/3 power cord with ground instead.

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jstjohnz wrote:

After reading some NEC sections referred by tsmith35, I have one correction. It appears that a double-pole breaker (as opposed to 2 single-pole breakers) is only required if you are using a split duplex receptacle. If you use 2 separate receptacles then the use of separate breakers appears to be OK per the NEC, but you *must* insure that those breakers are on opposite legs of the AC service in order to avoid overloading the neutral conductor.

-jim-

Can you point to the section of the NEC you are referencing? I want to make sure I get this right, and would prefer to re-use GFCI outlets from Orbit boxes than purchase special GFCI breakers.

Thanks!

Moving Target in NC
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Just one question. All I've ever seen is 15 amp GFCI outlets, possibly w/ 20A pass-thru.

Do they make 20 amp ones for outdoor use?

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I think they are prohibitively expensive. Anyway, why do we need 20amp gfi recepticals? Each channel is only good for like 8 amps right? We have to have a a gfi receptical for each channel right?

I'm not sure if its possible to seperate the halves of a gfi receptical..I dont think so?


(Or does everyone just wire direct to the LOR and we are just talking about plugging in the two feeds too the LOR?








digital analog wrote:

Just one question. All I've ever seen is 15 amp GFCI outlets, possibly w/ 20A pass-thru.

Do they make 20 amp ones for outdoor use?
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movtarget wrote:

jstjohnz wrote:
After reading some NEC sections referred by tsmith35, I have one correction. It appears that a double-pole breaker (as opposed to 2 single-pole breakers) is only required if you are using a split duplex receptacle. If you use 2 separate receptacles then the use of separate breakers appears to be OK per the NEC, but you *must* insure that those breakers are on opposite legs of the AC service in order to avoid overloading the neutral conductor.

-jim-

Can you point to the section of the NEC you are referencing? I want to make sure I get this right, and would prefer to re-use GFCI outlets from Orbit boxes than purchase special GFCI breakers.

Thanks!

Moving Target in NC



The latest copy I have is 1996, and it's 210.4-b. Basically it says that a multi-wire circuit (more than 1 hot wire sharing a common neutral), that is wired to a split duplex receptacle (not their wording but that's what it means) must be fed by a breaker that disconnects all poles simultaneously.

This is only the case if you use a split receptacle. If you use 2 separate receptacles the 2-pole breaker requirement doesn't apply, although it is still good practice from a safety standpoint.

Regarding GFI: If you use the split receptacle, the only way to get GFI protection is by using a 2-pole GFI breaker, available but expensive.

If you use 2 single pole breakers, they cannot be GFI breakers, since single pole GFI breakers cannot share a neutral connection. With single breakers you must use GFI receptacles (at least 2) to get GFI protection.

Use of the orbit GFI receptacles would be fine, but you obviously need two of them per multi-wire circuit.

-jim-
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kb9nvh wrote:

I think they are prohibitively expensive. Anyway, why do we need 20amp gfi recepticals? Each channel is only good for like 8 amps right? We have to have a a gfi receptical for each channel right?

I'm not sure if its possible to seperate the halves of a gfi receptical..I dont think so?


(Or does everyone just wire direct to the LOR and we are just talking about plugging in the two feeds too the LOR?








digital analog wrote:

Just one question. All I've ever seen is 15 amp GFCI outlets, possibly w/ 20A pass-thru.

Do they make 20 amp ones for outdoor use?

One receptacle would power 8 channels of a controller, and the controller is capable of drawing 20 AMPS maximum per 8 channels, so to get the full 20A capability you would need a 20A GFI receptacle, or use the pass-thru (always rated at 20A) of a 15 amp GFI receptacle to power a seperate 20A non-GFI receptacle.

No it is not possible to split the 2 sides of a duplex GFI receptacle.

One source for 20 amp GFI receptacles is B-B trading (bb-trading.com), $11.50 each or $10 each in lots of 10.



-jim-
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Zane wrote:

Nice article you wrote up jstjohnz. This is some very good information that a lot of LOR users need. I would like to add if you are using 12/3 power cord with a ground that you should use the GFI Circuit Breakers instead to protect the temporary power cord you ran. This added safety is always nice to have around. I would use 15 Amp breakers on the 12/3 power wire to be on the safe side as you might have power cords longer that 50’ feet. If you absolutely need the full 20 amps per circuit then I would use a 10/3 power cord with ground instead.


One thing I failed to mention is that another advantage of multi-wire circuits is reduced voltage drop in the wiring, as much as 50% less if the load on both sides is equal.

This is because the neutral wire only carries the difference current so there is less voltage drop on the neutral wire.

-jim-
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I found some 20A GFCI receptacles for $7.34 each from Hardware World. They're "The Designer's Edge" brand. They're available in White and Ivory. And there's always eBay. :)

Tom

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In reference to another current thread: I joined because you can't see very much unless you do.

This is the kind of stuff that keeps me here!
(I printed this thread out & it's going in my LOR notebook.)

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One thing I failed to mention is that another advantage of multi-wire circuits is reduced voltage drop in the wiring, as much as 50% less if the load on both sides is equal.

This is because the neutral wire only carries the difference current so there is less voltage drop on the neutral wire.


Sweet! :]
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I will be driving a unit this way. Remember you should still need the neutral jumper on the LOR unit.

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I recant my previous statement.

After seeing the url below


It looks like the normal way to do the LOR's is to place them as close to the light strings as possible.



This is true for any wiring "feeding" the LOR. After that we're back to normal return paths for current.. Unfortunatly, this is the majority of the display wiring (outputs from the LOR)


planetchhristmas.mywowbb.com/attachemnt.php?id=3054

jstjohnz wrote:




One thing I failed to mention is that another advantage of multi-wire circuits is reduced voltage drop in the wiring, as much as 50% less if the load on both sides is equal.

This is because the neutral wire only carries the difference current so there is less voltage drop on the neutral wire.

-jim-
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kb9nvh wrote:

I recant my previous statement.
After seeing the url below

It looks like the normal way to do the LOR's is to place them as close to the light strings as possible.

This is true for any wiring "feeding" the LOR. After that we're back to normal return paths for current.. Unfortunatly, this is the majority of the display wiring (outputs from the LOR)

planetchhristmas.mywowbb.com/attachemnt.php?id=3054

jstjohnz wrote:

One thing I failed to mention is that another advantage of multi-wire circuits is reduced voltage drop in the wiring, as much as 50% less if the load on both sides is equal.

This is because the neutral wire only carries the difference current so there is less voltage drop on the neutral wire.

-jim-


I don't think there is any 'normal' way. Some folks like to put the LOR controllers near the display lighting while others like to keep the controllers in a central location.

In any case, the big current draw is in the cables providing power to the controllers. The power from controller to the lights is split over many channels thus many wires, with typically just a few amps per channel.

-jim-
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Jim et al,
Well, I "thought" I knew electrical, but guess not.......

1) If I place GFIs "ahead" of the LOR controller, my assumption is that all 16 of the controlled circuits downstream are protected. But since I'm not running ground conductors to all 16 of those light circuits, then the GFI really is only protecting a ground fault at the controller board correct? Or does the circuit OUT of the LOR board act as a true electrical neutral. Namely, if the dog bites through an individual light circuit, I do not believe it is protected by any GFI located upstream.

2) OK, I understand that I can either a) install a GFI outlet inside the house/garage or at the perimiter of the house; which is now required by NEC or B) I can install the GFI in a box right at the remotely-located LOR controller. It really doesn't matter, but the outlet in the garage or on the outside of the house must be a GFI outlet regardless of it's usage for LOR or anything else (hedge trimmers, etc). namely, it's better to convert the outlets in the garage to GFI and then run a long cord than it is to run a long cord and put a GFI box out at the end by the controller on the lawn....

3) I don't get the use of 12/4 multiconductor cable to feed two sides to a LOR controller? Since you're using a 2-pole breaker (basically 220 across the poles), I would think that a short circuit in either half of the LOR controller (independent 8-channel controllers) would not be enough to cause that 2-pole braker to trip? I was planning on running two separate 15-amp grounded branch circuits to EACH 16-channel LOR controller (which are out in weatherproof boxes, etc)

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DrHitch wrote:

Jim et al,
Well, I "thought" I knew electrical, but guess not.......

1) If I place GFIs "ahead" of the LOR controller, my assumption is that all 16 of the controlled circuits downstream are protected. But since I'm not running ground conductors to all 16 of those light circuits, then the GFI really is only protecting a ground fault at the controller board correct? Or does the circuit OUT of the LOR board act as a true electrical neutral. Namely, if the dog bites through an individual light circuit, I do not believe it is protected by any GFI located upstream.

2) OK, I understand that I can either a) install a GFI outlet inside the house/garage or at the perimiter of the house; which is now required by NEC or B) I can install the GFI in a box right at the remotely-located LOR controller. It really doesn't matter, but the outlet in the garage or on the outside of the house must be a GFI outlet regardless of it's usage for LOR or anything else (hedge trimmers, etc). namely, it's better to convert the outlets in the garage to GFI and then run a long cord than it is to run a long cord and put a GFI box out at the end by the controller on the lawn....

3) I don't get the use of 12/4 multiconductor cable to feed two sides to a LOR controller? Since you're using a 2-pole breaker (basically 220 across the poles), I would think that a short circuit in either half of the LOR controller (independent 8-channel controllers) would not be enough to cause that 2-pole braker to trip? I was planning on running two separate 15-amp grounded branch circuits to EACH 16-channel LOR controller (which are out in weatherproof boxes, etc)


1) The GFI will protect everything downstream from it. Note though that a GFI does not give you any protection from a line-to-neutral fault. If your dog bites through a wire and contacts just the line and neutral wires he's going to get shocked and the GFI won't trip. This is true though for any GFI installation, the assumption is that most accidental shock scenarios involve line-to-ground. A ground wire in the protected circuit isn't necessary for proper GFI operation. A typical GFI trip is caused by leakage current from the hot wire to earth ground.

2) Basically true. Any receptacles on the exterior of your house must be GFI protected. Sometimes lighting displays involve temporary wiring setups and the use of a direct-wired cable feeding GFI receptacles at the display site might be appropriate in some cases.

3) A 2-pole breaker is nothing more than 2 individual breakers that have their handles mechanically connected so that when one trips the other shuts off also. Electrically speaking it is no different than having 2 separate breakers.

-jim-
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