What this guide covers:
Whether polarity matters for plug application to SPT-2 flat wire (including E12 and E17 socket wire)
How to identify positive vs. neutral on the SPT-2 wire and zip plug
How to orient the wire in the plug and socket
Practical tips to prevent shocks, trips, and poor connections
What are zip plugs, and what is polarity?
A zip plug is a simple plug connector that attaches to SPT-2 flat wire. Polarity refers to the direction electrical current flows—one conductor carries current to the bulb (positive), while the other returns it (neutral).
Does polarity matter when applying a zip plug?
No matter which way you apply the plug, the lights will work and illuminate either way because household electricity uses AC power. However, correct polarity is important for safety. If you aply the plug to the wires in reverse polarity, and touch the metal threading of a light socket with wet fingers while screwing in a bulb, you could receive an electrical shock.
How to correctly apply a zip plug to SPT-2 base wire (E12, E17 socket wire, 100 ft and 15 ft power cords)
Before you start:
Identify positive and neutral
The flat SPT-2 wire used as the base for E12 and E17 socket power cords, and in the 100 ft and 1000 ft power cord rolls, has two sides: one ribbed and one smooth.
The zip plug has “ribbed” imprinted on its side to help you align it with the correct side of the wire.
Smooth side = positive.
Ribbed side = neutral.
The neutral ribbed side must link to the wider blade of the zip plug.
Keep the ribbed side of the wire on the right side of the zip plug.
1. Prepare the wire
Split the wire about 1/4 inch down the center of the two conjoined wires to separate positive and neutral sides.
Pull the casing out slightly so the copper sits buried inside the insulation.
2. Insert and secure the wire
Greased plugs contain dielectric grease. This helps drive out moisture and reduces service calls, especially GFCI trips.
With the ribbed side of the wire on the right side of the plug (matching the side where “ribbed” is embossed), push the wire into the top part of the plug and into the grease, and ensure the teeth pierce the wire cleanly.
Lay the wire into the plug “like rolling out a carpet” so it sits flat on the prongs, with no wrinkles.
Slide the slider (back of the plug) onto the plug to lock it onto the wire.
Water management
Newer zip plugs have a hook on the back of the slider.
Bend the flat SPT-2 wire and loop it through the plug hook, so there’s a small curve at the bottom.
This encourages water to drip away from the plug instead of into the socket.
The dielectric grease also adds protection from water.
Keep conductors separated
Inside the plug is a spacer that keeps positive and neutral from contacting each other.
Ensure the split conductors sit properly on either side of the spacer.
Cold‑weather tip (to avoid bent plug teeth)
Do not pull the wire straight down and force the slider on in cold weather.
This can bend a tooth and prevent contact, making troubleshooting difficult.
Instead, push the wire in and “roll it down like carpet,” then slide the plug on by hand.