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I currently use a regular 50 foot power cord rated for 15 amps. I use it only for two laptops and one camera, making a draw of about 5 amps at 120 volts, according to the "input" label on the power supplies.

The 50 foot cord is heavy and I would like to find a lighter replacement. I have seen cords rated for 13 amps that are a bit lighter, but I think I can do better.

If I buy some wire and attach standard power plugs that you can buy at the hardware store, what is the smallest gauge I can safely use, assuming I never try to draw higher than the gear listed above? Would this also be safe for 100 foot power runs, if I connect two 50 foot cords together?

To be clear, I'm in the usa and power would always come from standard plugs.

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  • What is the actual wire size of the "15A" cord? – Harper Oct 13 '18 at 17:40
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    what environment do you intend to use this in? Indoor and outdoor? Are you doing this temporary and thus the cord stays in plain eyesight? Will it receive traffic from feet, animals, etc? – noybman Oct 13 '18 at 17:48
  • @noybman Conference audio/visual, so always indoors, sometimes heavy traffic, but always taped down. – fredsbend Oct 13 '18 at 20:03
  • @Harper I want to say 14 gauge. – fredsbend Oct 13 '18 at 20:04
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    Sorry to annoy you... I am referring to NEC 110.3(b), which says you are required to follow the labeling and instructions (typ.) when applying the product. You said in a comment that safety is your first priority, Code requirements run very close to safety requirements. Don't take my word for it, ask your town's fire marshal. – Harper Oct 13 '18 at 22:08
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I would never use zip cord for a long extension cord if you care about safety. SJOW cord would be the lightest safest option with 18-3 for a 50' run but would move to a larger wire gauge for anything longer. Connecting cords is not a good idea especially outside I have seen many cords that the molded connectors have melted down that were used within there rating. When you are looking at cords. The letters SJ is junior hard service cord (just an S is hard service heavier covering). O is oil resistance, W is wet, damp and sunlight resistant. SJOW is the more common so cheaper, or possibly SJOOW both inner and outer insulation is oil resistant.

  • Yeah -- SPT (zip cord) isn't known for ruggedness -- SJx cordage is going to hold up much better over time. – ThreePhaseEel Oct 13 '18 at 18:15
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Regardless, you must use one of the cable types which is specifically designated as cordage for mains power. You may also find better prices at electrical supply houses rather than big-box stores, online prices will be terrible owing to the cost of shipping.

Common mistakes are to take speaker wire or Romex and slap plugs on the end. Romex is listed for use as wiring inside the walls only, as permanent wiring of a structure. Cordage is special stuff, with special insulation optimized for wear resistance, and highly stranded wires meant to flex often.

It is not meant to be walked on, unless it is. Look for cable designed to go under carpets or other such applications. Watch out for NEC 110.3b (must obey labeling and instructions) and NEC 400.6-8 (allowed and prohibited uses for flexible cords). 99% of the time, Code has a safety-related reason for being that way.

For "belt and suspenders" safety, AFCI and GFCI protection will stop arc faults starting fires and ground faults shocking people.

One of the weird problems with "lighter wire" is that below a certain size, wire actually starts to get more expensive. This is very notable in THHN single strand wire; #14 is the cheapest and then #16 is higher.

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600 W sounds like a lot for those loads, but the labels tend to be overly conservative.

In any case, the lightest wire you can use is 2-wire AWG18 "zip cord" or "lamp cord". You should be able to find pre-made extension cords that use this wire, but probably not in 50-foot lengths.

  • Two wire would mean no ground, correct? Is that bad for electronics? The laptops have a ground pin, but the camera does not. – fredsbend Oct 13 '18 at 15:59
  • If you want ground, then use a 3-wire cable, but it won't be as light. What is your real priority here? – Dave Tweed Oct 13 '18 at 16:03
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    I figure if it has a ground pin, it's meant to be used – fredsbend Oct 13 '18 at 16:15
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    In that case, you should probably stick to manufactured extension cords. You can get 3-wire "zip cord" extensions in short lengths (up to about 12 feet), but you'll probably have to go with the AWG14 units in lengths of 50 or 100 feet. No manufacturer is going to be willing to accept the liability associated with someone overloading a long, lightweight extension cord. – Dave Tweed Oct 13 '18 at 16:18
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    You can buy 18/3 SJ extension cord cable by the foot and make your own cord (use SJW for outdoor applications). Note that a 100' run at 5 amps will cause some voltage drop (~5%). – Jimmy Fix-it Oct 13 '18 at 17:37
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There is a table 400.5(A)(1) in the NEC that shows a number of types of 18 AWG cord allowable for up to 10 amps, well over your load. However, in my view, with an extension cord, it's a little more complicated than that. You really have to make this safe from overload in the event that someone unwittingly plugs something in that will push the load on the cord over the safe 10A.

To this end I'd want to fuse the hot wire somewhere near the male plug end. Although some plugs in the UK allow a fuse in the plug itself, I think the only option in the US would be to wire in an inline fuse in the cord. This can be done a number of ways, the most popular being a big ugly lump of tape after splicing in the inline fuse.

The voltage drop with #18 wire would be about 6 volts, which most laptop and camera power supplies will tolerate, but check the specs.

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