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Custom extension cords & C7/C9 Stringers Print E-mail

While it doesn't make much sense for the normal homeowner to make their own extension cords or light strings, large displays - especially computer controlled ones - can benefit from custom wiring harnesses.

A custom wiring for your display, be it a single custom-length cord-set or a multi-channel harness, can have several advantages over pre-made cord-sets:

  1. They can be made to exact dimensions, eliminating 'cord puddles' (wound up piles of extension cords that were too long).
  2. They are typically less bulky than an equal number of pre-made (store bought) cord-sets, making them easier to store.
  3. They can be more inexpensive than pre-made cord-sets.

In this article:

Safety Tips

Generally advanced decorators use 'zip' cord, also known as Lamp cord, for custom harnesses.  While typically used for indoor applications,  zip cord can be used safely outdoors as long as you observe some simple safety precautions - the same precautions you should be using for ALL your cord-sets and light-sets:

  1. Cord-sets should only be used temporarily outdoors.  When the season is over, bring your cord-sets INSIDE.
  2. Every year carefully inspect your cord-sets for cracks or brittle insulation.  If you find any problems, don't use the set until it is repaired.
  3. Don't overload your cord-sets.  Stay well within the amperage limits of the cord-set.  Overloaded cord-sets can melt (or catch fire).  Melted cords can expose live wires posing a shock hazard.   Since these cord-sets are NOT protected by a fuse you MUST exercise caution.  If you are unsure about the load, buy an inexpensive, easy to use 'Kill-A-Watt' device and use it!
  4. Never use ANYTHING electrical (extension cord, light-set, tool, etc) outdoors unless connected to a circuit protected by a GFCI (Ground Fault Circuit Interrupter).  The danger of electric shock is REAL!  Do NOT put your, or your visitor's life on the line by NOT using GFCI protected outlets.

A few notes about wire terminology:

Typically, 'zip cord' is referred to as SPT-1 or SPT-2.  Don't confuse the number shown as the conductor size.  The conductor size is referred to as the wire's 'Gauge'.

AWG, American Wire Gauge, or simply 'gauge' is the size of the actual conductor and the most important factor when it comes to how much current a wire can carry.  Remember that AWG works BACKWARD:  SMALLER gauges mean BIGGER wires that can carry MORE current. 

SPT stands for:
Stranded (vs. Solid core)
Parallel (Flat cable. vs. Twisted)
Thermoplastic (vs. Neoprene)

The number following SPT is the insulation thickness and NOT the conductor thickness:
1 = Light duty
2 = Medium duty
3 = Heavy duty (may be suitable for burial at low voltages)

Notice there has been NO mention of wire thickness. Most people confuse SPT-1 with 18AWG and SPT-2 with 16AWG.  That's simply NOT true. You can get either AWG (and I assume, other gauges) in any insulation type. 

Examples of different zip cords that can be used for holiday displays:

  • 18AWG SPT-1
  • 18AWG SPT-2
  • 16AWG SPT-1
  • 16AWG SPT-2

Are they cheaper than regular cords?

The short answer is possibly.  Typically, mass produced extension cords are cheaper than single cords you could make yourself.  Let's face facts:  Wal-Mart is going to get a MUCH better deal on 10 MILLION feet of extension cords than you ever will buying 500 feet of zip cord.  Most of my display is wired with standard store bought cords. 

Where you really save the money is in 'multiple drop' cords (cords with more than 1 outlet on them) and cord-sets in the 15-30 foot range. 

Case in point is the large 'mega tree' in my display.  The tree requires 32 circuits, and if I were to use 32 fifty foot extension cords at $6 each, my cost is $192.  Instead, I use a custom built harness (Please note that these prices are from 2006.  Costs have gone up substatially since then):

  • 1000 feet of 16AWG SPT2 = $65 (each run needs to be only 30 feet)
  • 32 Plugs = $24
  • 32 Sockets = $24

That is a total of $113, a savings of nearly $90.  There are other benefits as well:  The harness is made to the exact dimensions I need so there is no cord puddle, the sockets and outlets are numbered for proper hookup, and the harness is much lighter and easier to store than the equivalent 32 fifty foot cords.

What gauge & insulation should I buy? 

To decide what kind of wire to get, FIRST figure out how much load that wire will carry:
18AWG - 10 Amps MAX (or 1100 watts at 110 volts - approx 30 100 count minis)
16AWG - 13 Amps MAX (or 1430 watts at 110 volts - approx 39 100 count minis)

You must also factor in the length of the cord-set you are going to make.  As cords get longer, their internal resistance becomes a factor.  Here is a table that I use to determine the max load on a cord-set. 

 Length\Wire Gauge  16 AWG   18 AWG
 1-49 Feet  13 Amps  10 Amps
 50-99 Feet  10 Amps  8 Amps
 100-199 feet  8 Amps  5 Amps
 More than 200 Feet

Do not use

If you need to run more than 200 feet, I would put in a REAL outlet with REAL wire (10AWG or 12AWG) closer to where I need the power.

You also need to take into account if you are bundling multiple cords together into a harness.  While low-amperage draws should be no big deal, if you are running multiple cords together at higher loads make sure you are not generating a lot of heat.  Bundling wires together can cause this heat to build up quickly, insulation to melt, and bad things to happen after that.  If you live in a cold climate where it snows, your cord-sets should NOT be generating enough heat to melt snow! 

What do I use personally? 90% of what I have is 16AWG SPT2. You can never have too much power or too big a conductor!  I originally went with all 16AWG SPT2, figuring it would be a lot easier to expand if my power requirements went up in the future.  However the skyrocketing costs of copper the last few years have forced me into using a limited number of 18AWG runs.  Since the wire is smaller, it's generally less expensive.

As for insulation thickness... I would ALWAYS use SPT-2. Remember you ARE working with wire that SHOULD only be used indoors. For that reason, a little MORE insulation won't hurt (and could help a LOT).

Sockets, Plugs & Light Sockets

Once you have your wire picked out, you'll need sockets and plugs.  The cheapest and easiest plugs/sockets that I have found come from an outfit called 'Action Lighting', and are referred to as 'vampire' sockets (or plugs).  The sockets have 2 small metal teeth that pierce the outer wire insulation to make the connection.  They are very fast to install since there is no need to strip wires.  To find the plugs and outlets, go to the Action Lighting website and search for 'ZIP'.

Additionally, these same sockets can be modified so that they can be used in the middle of a cord.  This allows you to have a cord-set that is the exact length you need, with outlets spaced exactly where you need them.  For example, the arches and candy canes on my front walkway are all tied to 1 circuit.  The arches have 800 lights total, and the candy canes add another 1000 - an equivalent total of eighteen 100 count sets.  Plugging all of these in end-to-end won't work & running a single cord to the middle and branching out from there (900 on each side) is still too much.  Instead, I created a custom cord that has outlets spaced every 6 candy canes.  I say 'every 6 candy canes' instead of 'every x feet' to stress a point - DIY cords can be constructed to EXACTLY the dimensions you need!

When ordering sockets and plugs, be sure you are getting the correct size item for the wire you will be using.  Plugs and sockets are ordered by the size of the INSULATION being used, not the wire gauge.  The difference between plugs/sockets made for SPT1 and SPT2 is the size of the 'tooth'.  Since SPT2 has thicker insulation than SPT1, the teeth must be longer.  Using an SPT2 plug on SPT1 wire means the tooth can go in further than the middle of the conductor and possibly out the other side!  Using an SPT1 plug on SPT2 wire means the teeth are not going in far enough - either the cord-set won't work or it will burn up as there is not enough wire in contact with the teeth.

Always be sure to start and end your cord-sets with a plug and an outlet.  There should NEVER be a bare wire end!

Double check those Sockets/Plugs!

China has really been pumping out these plugs and sockets.  Be sure you take the time to ensure the plugs and sockets you have purchased are manufactured correctly.  When you look at the 'teeth' there should be one on EACH side.  If they are both on the SAME side, you have problems - of the dead short KABOOM nature! 


This is VERY VERY BAD... 

Making a custom cord-set

 Now that you have your wire and plugs/sockets lets make a cord-set! (Click on any image for a larger picture)

Take a look at your wire.  One side will be marked in some way - typically it's ribbed, but it may instead be a different color, have a stripe, a different color conductor (white vs copper), or be marked with production numbers/etc.  This marked wire is the NEUTRAL, and it's very important.  Image
 Image Measure the length of wire you need, and cut it flush. 
At each end, split the 2 conductors by 1/4 of an inch.  Be careful to not cut the actual conductors.  Image
 Image Now take a look at your plug.  On one of the prongs, you will see some sort of designation that will say 'White' or 'Neutral'.  Notice that this prong is WIDER than the other.  The wider prong is ALWAYS the NEUTRAL.  When you flip the plug over, you'll see which vampire tooth this prong is attached to.  The marked side of your wire goes over that tooth.
On the inside, take a look a the small cavity at the head of the plug.  See that divider?  That keeps the wires from touching one another.


 Image Take your wire and insert it into the cavity at the head of the plug, ensuring that the ribbed side of the wire is going to go over the wider 'White' or 'Neutral' vampire tooth.  Be sure you get the wire in deep enough and that each conductor is separated by the plastic divider.
With your fingers, press the wire into the plug.  The wire should be FLAT and not twisted.  Image
 Image Slide the cover over the top of the plug.  Note that there are 2 different ends to the top.  One has a small tab.  The end without the tab goes on first.
If you have weak fingers like I do, you won't be able to slide the cover completely over the wire.  Grab a pair of channel lock pliers and finish the job.  Be careful that you don't damage the wire at the same time!  Image
 Image For the most part, the socket end goes on the exact same way.  The only difference is in the marking of the neutral side of the socket.  If you look at the socket, you'll notice one side that says 'ribbed'.  Naturally, place the wire into the socket so that the ribbed wire is on that side.
Want to add an outlet in the middle of a cord?   No problem --  Remove the cut-out at the other end of the outlet with a pair of wire cutters (small saw, old soldering iron, whatever), and allow the wire to pass straight through.  Be sure to still follow the 'ribbed' marking!  Image

Making a custom C7/C9 string:

You can also create your own custom C7/C9 holiday light strings with parts from Action Lighting.  I personally use several custom C9 strings with light sockets spaced every 3 feet for strobe lights.   Creating your own light strings allows you to have whatever spacing you desire for your lights, but they can get to be quite expensive when compared to factory made sets.  Unless you absolutely need the custom spacing,  buy pre-made sets. 

To purchase the sockets, go to Action Lighting's website and search for either "C7 Socket" or "C9 Socket". 

 Image Create a custom cord-set using the instructions above.  Remember, you need to end the cord with a female plug socket since we don't want any bare wire ends.
Take a look at your light socket to see which vampire tooth is connected to the center of the light socket, and which is connected to the tab on the side of the socket.  Place the light socket on your work surface with the open end down, and the 'vampire' teeth up.     Image
 Image Locate the marked (ribbed) side of your cord-set.  Align the wire so that the marked conductor will be pierced by the vampire tooth that goes to the side of the socket (the unmarked conductor will be pierced by the center tooth) and press it in with your fingers. 
Place the bottom of the light socket over the wire and press it down firmly.  Be sure the 2 snaps have engaged.  If they haven't, use a pair of slip-jaw pliers to seat them.  Add as many more sockets as you need, at the spacing you want.  Remember that these light-sets are not protected with a fuse, and you could overload the cord if you place too many light sockets on your custom set.  Always be sure to know your loads!  Image

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