Shared Neutral Cables
This is still a work in progress. Pictures still need to be added. "Please see the discussion page for an explanation"
One way to save money and setup time on your display is to have the neutral line shared among several channels. As jeffmill has shown, however, there are many considerations that come into play to keep your display safe when using them. He also tells about some of his uses of Shared Neutral Cables in his display.
All the standard Disclaimers apply. If you have any questions about the use of electrical wiring and its construction you should consult a qualified electrical professional who is licensed and bonded who is knowledgeable of the electrical codes in your area. The information that follows is not to be considered a guide, but rather a discussion about the safe use of multiple circuit cables for the connection of temporary seasonal lighting. You are solely responsible for any use of this information. DoItYourselfChristmas (Hereafter known as DIYC) and its members take no responsibility for any damage or harm that may occur from using the information found herein.
There is no reason whatsoever to fear using a single neutral along with several strands of a smaller gauge wire as the hots, as long as a few safety precautions are followed.
Using SPT Outdoors
The use of SPT outdoors is a long standing debate with folks setting on both sides of the fence. Regardless of what the concerns are, the only issue here is the lack of UV protection on the wire. If you toss an indoor rated extension cord (zip cord, SPT) out in the sunlight it will eventually break down the insulation and short the conductors. No more, no less. Every one of my cables is wrapped in UV protected 3/4" black electrical tape.
I think using SPT and/or THHN is acceptable and safe to use in these cables. The indoor/outdoor status becomes a non issue when you alleviate the UV problem with UV protection. 'nuff said.
The neutral line should be of sufficient gauge for the total load of every circuit in the cable shared by that neutral. You must know the gauge and capacity of the wire you use for the hots, and multiply that by the number of conductors you will use in your neutral to determine the gauge of it. It is a common practice to design an 80% loading into every aspect of electrical wiring in a home - that gives you a 20% buffer as a safety margin.
Take into consideration any future additions you are planning. It may be less costly to build for a larger capacity, as smaller gauge wire can be more expensive if it is not a commonly stocked item. Also, keep in mind that SPT I or SPT II is not an indication of the capacity of the wire, it only describes the amount of insulation between the wires.
Multi-Conductor Wire Capacity
The NEC provides a reduction in the overall current carrying capacity of conductors in a multi-conductor cable such as this there is a mutual inductance, or interaction between the conductors that reduces their load carrying capacity. I believe that the 80% safety margin is sufficient to allow for this issue. Interpret it this way: If you have a conductor that is rated for 10 amps, design your construction so that it will carry 8 amps maximum.
Ensure quality when constructing the cables. It is my personal opinion that they should be constructed by carefully soldering the connections. I also use heat-shrink tubing for its insulating qualities, along with the neatness of the connection which might otherwise become a big bulky wart in the cable where solder joints are located. I always do the neutral connection first, and then try to offset the position of the hots enough so they don't add to the bulk of the neutral connection. These cables will be used over and over again, year after year. Taking the time to construct them well will pay off in the end.
Ensure that you have enough length to prevent them from being tripped over, or pulled. When I designed the cables for the major items like the mega tree, and the bundled group of 15 mini trees I looked around to see if there was any other possible locations I would want to place them. I made the cable length to fit as much distance as possible within reason. Since I use Molex connectors on the controller end the possibility of adding extensions exists, so the length isn't written in stone for me.
If you don't use GFCI, you will probably lose a fuse or a Triac that thinks it’s a fuse. GFCI is a very simple differential protection scheme. It monitors the flow of current to and from a load (load = lights) If the current flowing to the load does not equal the flow back from the load down to a very minute quantity, it trips open at a very high speed.
Dropping a set of lights into a bucket of water that does not have a path to ground or some other return path to the source will not trip a GFCI. That is because the current it monitors will still be equal, and not considered a differential fault. In a ground fault, some of the current flowing to the load bypasses its normal path back from the load and travels through the ground instead. That results in an unequal measurement in the scheme = TRIP! Design cables so you can keep the connections off the ground, where rain water can flow through them and drip away. This will prevent ground faults that will trip GFCI.
GFCIs do not only trip for ground faults. It can occur when the current leaks over to another circuit and finds a return path that is not its normal course. You can prevent this in a multi-conductor scheme by ensuring quality in your construction, and also by keeping all the conductors in the cable on a single GFCI.
The use of my design of cable is solely for US power systems. I have no experience in foreign power systems. There are other folks that will surely be able to help the 220V users. When powering one of these cables with a common neutral always ensure that you are on a single hot leg of your electrical service in a given cable. This will prevent the possibility of 220Vac if the neutral becomes unbonded anywhere.
Some of the controller manufacturers design their products to be used in Europe, the UK and Australia, with the controller divided down the middle that can have 2 opposing legs of 120Vac powering each side, in effect having 220Vac powering the board. This is not an unsafe practice because the boards were designed for it, and 220Vac is the standard in foreign applications. However, when given the choice, I believe it's always a good idea to choose the safer option and try to minimize the possible dangers associated with with higher voltages by keeping a single leg on a controller. My concern for this is directly related to the construction of these cables. If you were to set up a 16 channel controller with two opposing legs then place 12 of the channels in a single cable as I did in my mega tree design, you have 220Vac in the cable. Squirrel chews the cable, you pick up the cable, the path provided by the squirrel lets 220Vac cross the upper portion of your torso on the way to ground or worse you get 120 in your left hand and opposing 120 in your right... Then just before everything fades to black, you see two squirrels doing that funny hand-slap dance from the car insurance commercial...
This is an example of how shared neutrals can be used in your display.
My first cable is directly based on the design of a control cable used in the electrical industry. I chose the number of slices for my Mega Tree based on the design of this cable. It utilizes three sets of 4 #16 conductors (or two strands of SPT) with a single #12 THHN common neutral for each set. Altogether there are 15 conductors to handle the work of 24. At one end of a 40' cable there are 12 Dollar Store three-outlet extension cords, spray painted with Krylon Fusion to UV protect them. At the other end there is a set of 15 pin Molex connectors with 12 strands of #14 THHN and 3 of #12, approximately 3' long. I connect the 3' piece and female connector to the controllers. It makes for a quick connection to the tree. There are 5 of these cables to light 60 channels. 12 slices each of Red, Green, Yellow, Blue and Clear minis. Each slice of colored lights is 150 count, and the clear are 200 count.
The second cable is a little more complicated. It combines the AC power to remotely located SSRs, along with the wire connected to the output channels to power my rope light candy canes. Once again I used Molex connectors, this time not only for the end but also to connect the SSRs to the circuit. It is a very quick way to snap in/out SSRs from the scheme even though I haven't had any failures that required this.
I calculated the total load of all the rope light combined in my canes and ran a single strand of THHN of sufficient gauge from a terminal strip at the power source. This runs all the way out to the corner and across the front of my yard. Then I marked the strand to evenly space 24 canes which came out to a little over 2' per division.
I cut 24 pieces of heat shrink about 1 1/2" each and slid them onto the wire about a foot down from the end. I stripped the very end off about 3/4" and soldered the 2' neutral for the first cane to it. Heat shrink and move everything over a couple feet. Strip 3/4" off the next mark, solder the next neutral, heat shrink and move on. 24 times and the basic cane locations are in place along with a 2' pigtail for each neutral.
Next I located an SSR just after the fourth cane. Measure out four pieces of wire to power canes 1, 2, 3 and 4. Cut them to length and begin to wrap them with 1/2" electrical tape. Leave enough conductor to tin the leads and stab them into the end of the rope light. At the place where they connect to the SSR they can be landed into the screw terminals, or in my case Molex pins were crimped and soldered on. Add one power wire all the way back to the terminal strip for the SSR and move down 4 to just after the 8th cane.
Repeat the process for the 4 cane hot leads and tap off the power wire to the first SSR. Move down four canes, but this time run a new power wire. To break it up into bundles of eight I cut the power wire and neutral just after the 2nd SSR and added a 2 pin Molex connector. The same thing just after SSR 4 and add a 4 pin Molex. Then cut all 4 just after the last cane and add another 4 pin Molex. One more 4 pin Molex at the terminal strip and power source. The SSRs are placed in a Pod made from PVC pipe and end caps wire tied to a stake. It’s a very neat design that can be easily maintained and repaired. Everything bundled up in a single cable. Now all I have to do is get the other 14 Candy Canes converted over to Ropelight….
The difference in cost to wire my Mega Tree cables was offset by the mere fact that I decided to add Molex connectors to them. I believe I saved at least 50% had I not added all the bells and whistles. I cannot claim a savings on the design of the Candy Canes either as the wire used was scrap that I recovered from a dumpster. However, this method will save you money, and will make the Annual set-up and take-down easier.