Vampire plugs and zip cord: building custom-length C9 runs
Vampire plugs are how professional Christmas light installers turn a 1,000 ft. spool of C9 socket wire into runs that fit a roofline to the inch. Instead of hiding extra wire in a gutter or buying pre-made strings in fixed lengths, the installer cuts the socket wire where the run ends and closes a male or female vampire plug onto the cut end. Paired with SPT-1 zip cord for jumpers and leader cords, it's the method behind a clean, measured C9 install with no coiled excess.
What a vampire plug is
A vampire plug (also called a slide-on plug, quick-connect plug or Gilbert-style plug) is a male or female plug that attaches to flat two-conductor wire without stripping it. The wire slides into the plug body, and closing the plug drives metal prongs through the insulation and into each copper conductor. The prongs are the "fangs" that give the plug its name.
Because there's no stripping, soldering or wire nuts, a vampire plug goes on in well under a minute on a ladder or a roof. That's why installers carry a bag of male and female plugs on every job. Christmas Light Supplier sells male and female plugs at $37.99 per 50, which works out to $0.76 per plug, with 1,000-count cases for crews that go through them by the season.
What zip cord is
Zip cord is flat two-conductor lamp cord, the same style of wire that C9 and C7 socket wire is built on, just without sockets. It's called zip cord because the two conductors "zip" apart down the center web. Installers use it to build leader cords (outlet to the first light), jumpers (the end of one roof section to the start of the next) and custom extension cords cut to the exact distance.
Christmas Light Supplier stocks 250 ft. spools of SPT-1 lamp cord (18/2 zip cord) at $78, about $0.31 per foot. It's rated 8 amps / 960 watts for indoor and outdoor use and works with SPT-1 male and female vampire plugs.
Match the plug to the wire
Vampire plugs are sized to the wire. SPT-1 and SPT-2 are both 18 AWG, but SPT-2 has a thicker jacket, and a plug built for one won't seat correctly on the other. An SPT-1 plug on SPT-2 wire may not close. An SPT-2 plug on SPT-1 wire may close with the prongs barely touching the conductors, which shows up later as a flickering or dead run. Check that the plug is made for the gauge of the socket wire or zip cord on the job before you close it.
If the job mixes SPT-1 and SPT-2 (common when a commercial property requires SPT-2 on part of the install), keep the plugs sorted by gauge in the van so the crew doesn't have to guess on the roof.
How to put a vampire plug on C9 socket wire
1. Measure the run and mark the cut point. On socket wire, cut in the middle of the gap between two sockets so the last socket has room and the plug has straight wire to grip.
2. Make a square cut with sharp cutters. An angled or frayed end is the most common reason a plug doesn't make full contact on both conductors.
3. Check polarity. Zip cord and socket wire have one ribbed (or marked) conductor, which is the neutral. On a polarized male plug the wide blade is neutral, so the ribbed side goes to the wide blade. Keeping polarity consistent across every plug on a job avoids reversed runs that behave inconsistently with some controllers.
4. Open the plug, slide the wire in until it hits the end stop, and close the plug firmly until it locks. Don't leave the wire short of the stop, or the prongs will pierce only the jacket.
5. Test the run on the ground or at the outlet before clipping it to the roof. Re-cutting and re-plugging at ground level takes seconds. Finding a bad plug on a two-story ridge after the clips are on doesn't.
Jumpers, leader cords and splits
A jumper is a short length of zip cord with a male plug on one end and a female plug on the other. It carries power across a gap in the roofline, such as between a garage roof and the main house or across a valley, without lighting that gap. Cut it to the exact distance so there's no slack to hide.
A leader cord is a jumper that runs from the outlet or timer to the first light on the run. Keep leader cords short and route them along the fascia or downspout rather than across walkways.
Splitting one feed into two runs is common at a roof peak. The split is still one circuit, though, so the combined load of both runs has to stay under the rating of the cord feeding them. See How many C9 LED lights can run on one circuit for the math.
Load limits on custom runs
Every piece of a custom run has a rating: the socket wire, the zip cord, the plugs and whatever timer they plug into. The lowest rating in the chain is the limit for everything downstream of it. The SPT-1 lamp cord Christmas Light Supplier stocks is rated 8 amps / 960 watts, so a single leader cord made from it shouldn't feed more than 960 watts of lights, however many runs are split off after it.
What it costs to go custom
One custom termination is one vampire plug: $0.76 at Christmas Light Supplier. A typical residential job with six to ten custom ends and two or three jumpers uses under $10 in plugs and 20–40 ft. of zip cord (about $6–$12 at $0.31/ft.). A C9 installer's van usually carries a 50-pack of each gender and a 250 ft. spool of lamp cord.
Every Christmas Light Supplier order ships free the same day it's placed, with no minimum, so a single $37.99 bag of plugs to finish a job ships free, the same as a full season order.
Frequently asked questions
What are vampire plugs for Christmas lights?
Vampire plugs (slide-on plugs) are male or female plugs that attach to flat SPT-1 or SPT-2 wire without stripping. Closing the plug drives prongs through the insulation into the conductors. Installers use them to cut C9 and C7 socket wire to exact length and to build jumpers and leader cords from zip cord.
Do you have to strip the wire to use a vampire plug?
No. The wire slides into the plug uncut and unstripped except for a square cut at the end, and the plug's prongs pierce the jacket when it's closed.
What is zip cord used for in Christmas light installation?
Zip cord is flat two-conductor SPT lamp cord without sockets. Installers cut it to length for leader cords from the outlet to the first light, jumpers between roof sections and custom extension cords. Christmas Light Supplier stocks 250 ft. of SPT-1 18/2 lamp cord, rated 8 amps / 960 watts, for $78.
Which side of zip cord is neutral?
The ribbed or marked conductor is the neutral. On a polarized male plug, the neutral goes to the wide blade.
Can vampire plugs be used on SPT-2 wire?
Yes, as long as the plug is made for SPT-2. SPT-1 and SPT-2 plugs are sized to different jacket thicknesses, so match the plug to the wire gauge.