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SPT cable can be a 2 or 3 conductor cable that you can buy in bulk. SPT is an acronym for Stranded Parallel Thermoplastic. Generally speaking, SPT cable extension cords are cheaper to make than store bought extension cords. You can cut SPT cable to length and not have extra cable lying around. Generally speaking, you can make a drop cord cheaper that you can buy it. Also, we generally use only 2 wire cables.
There are 3 types of SPT cable, SPT1, SPT2, and SPT3. SPT1 and 2 are commonly used in Christmas displays. The difference between SPT cabling is the thickness of the insulation over the wires not the ampacity of the cabling. Lots of people think the difference between SPT cabling is the current carrying capability (ampacity) which is nonsense. The wire gauge or AWG rating is how much current the cable can carry in amperes. Below is the rule of thumb for SPTcabling. A rule of thumb is just that, a good guess under optimal conditions.
SPT Cable #1 20 AWG is rated for7 Amps (some manufacturers say 2 amps)
SPT Cable #1 18 AWG is rated for 10 Amps.
SPT Cable #2 18 AWG is rated for 10 Amps.
SPT Cable #2 16 AWG is rated for 13 Amps.
One problem with SPT cabling is that the end of the cable is not sealed like an extension cord which makes SPT cable more susceptible to water infiltration. So don’t leave the connectors lying on the ground. Even a store bought extension cord left in a puddle is not good. I cable tie the SPT/extension cords to the side of whatever I am connecting the wire. In other words, I cable tie the connector about 4 inches from the ground on a wire frame. If you get into the habit of not allowing the connectors to come into contact with the ground (water) , you will have far less GFCI nuisance trips.
Here are some common questions that I have heard.
Will a 20AWG STP cable allow 10Amps? Why yes it can for a little while then it will burn your house to the ground. Do not force more current through a wire than its rating. If the AWG rating says 10 amps then 8 amps is fine but 12 amps is a no no.
The NEC book says I can run more current through for a certain wire size than what you listed above, why? Don’t even go there. The National Electric Code book says lots of things with respect to wire. You must have a thorough understanding of electricity and industry practices in order to even interpret what the NEC tables mean. Manufacturers will derate the NEC current carrying capabilities of wire due to many factors– a few would be 1) number of conductors (number of wires in the same cable) 2) Where the wire is located, i.e. extension cord, wall cavity, buried, etc. 3) sheathing. i.e. Teflon, PVC, thermoplastic, neoprene, etc. 4)….
Can I put a SPT cable #1 connector on SPT cable #2 wire? Absolutely, you can put a different connector onto the wire, but the cable won’t work or will work intermittently. But, you can force the wrong connector on to different SPT sizes. If you buy SPT1 wire then SPT1 connectors must be used on it and the same goes for SPT2 cable. SPT1 cable has a thinner covering over the wires than SPT2. If you place a SPT1 wire onto a SPT2 cable, the “vampire terminals” (insulation displacement terminals) will not push through the jacketing and touch the wire. If the terminals are longer, like with SPT2, and you put a SPT2 connector on SPT1 wire, you will cut deeply into the copper of the wire, damage and render the wire capable of carrying less current.
What colors does SPT wire come in? I have seen white, cream, green, black, and brown.
Does it make any difference which way the wires go into the connector? Absolutely. Without going into detail, one lead of the wire is designated a “neutral” and the other will be “hot” lead. If you turn them around you could get a shock. I will not elaborate further since the explanation is complex. Just remember the wire must be placed into the connector in a certain manner. Almost all SPT cabling has a smooth conductor (wire) and a ribbed /square wire. The smooth wire is the “hot” conductor and the ribbed/square is the neutral wire. Some SPT connectors have a wider terminal which is the neutral (sometimes called the white) lead.
If I run 200 feet of SPT cable can I still run the maximum current that you specified above? Sure you can, but the voltage drop due to the wire length will be too great and your electronics will not like it and your lights will be dim. Copper has resistance and increasing the length increases the resistance. You will not hurt the wire, but you will mess up your electronics. If you run extra long lengths or wire (say 100’+) you should only be running about 50-75% of the current through it to keep the voltage drop within acceptable limits or you should go up (actually smaller AWG number) a wire gauge size i.e. 18 AWG to 16 AWG for 10 Amps (amount of current a 18 AWG wire can carry under 100’).
What is the difference in SPT Cable connectors? Some connectors are made of a compound that will flex easily others are made of a substance that will not and can be brittle. Some connectors have a larger or polarized plug that keeps you from nesting the connector incorrectly. Some connectors have a pass through capability so you can knock out a little section of the plug and place the connector midway through a wire. That function is nice if you have several small current drawing things like LEDs that you can place at different points onto a long cable. A manufacturer will often make their connectors a little differently. Some manufacturers make better connectors than others.
Who makes the best SPT cable? That is a good question. I buy connectors from Creative Displays, Inc, (CDI or Paul Sessel). I have bought them from other sources with good and bad results. I like CDI quality in connectors. If you asked 10 people where they buy connectors likely all 10 will give different answers. But all 10 of them will say CDI sells good connectors.
Which is better, SPT1 or SPT2? This question reminds me of a similar question, which is better, Coke or Pepsi? SPT1 wire generally is sold with smaller wire gauges (AWG) which means it will carry less current and is cheaper to buy. If you don’t need the bigger wire then buy a smaller wire. You wouldn’t buy a tractor trailer to pull a lawnmower trailer. However, most of us that do this sort of thing, will buy SPT2 just to make matters simpler. I have a small roll of SPT1 wire and connectors simply to modify inflatables which almost always has SPT1 wire. The remainder of my cabling is SPT2. If you want to save money use SPT1. However, it can be quite confusing to repair cables when you use different sizes of wire extensively.
Can I use low voltage, landscaping wiring that I bought at Lowe’s that looks just like SPT cable? The short answer is no. The wire sheathing is rated for a voltage much lower than 120 volts. People on the forums say they use that type of wire and have no problems. I will tell you don’t do that. The landscaping wire coating may wind up electrocuting you.
Original Author: Al in Raleigh