Are there any types of DC low voltage power supplies on the retail market that can accommodate for long wire runs (18 gauge wire). I am worried about voltage drop and power failure; I'm trying to power window shade motors with 18 gauge wire over long distance wire run and I need 18 volt output.

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    \$\begingroup\$ Yes. Are you looking for theory or product recommendations? Because product recommendations are considered off-topic here... \$\endgroup\$ – Daniel Apr 5 '16 at 2:45
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    \$\begingroup\$ 1.How many motors will be ON at the same time? 2.How much current will each one require? 3.How far will each one be from the power supply? \$\endgroup\$ – EM Fields Apr 5 '16 at 3:22

Generally, when you need to tightly regulate the voltage for a variable load at some distance from the power supply, you look for a power supply that has "remote sense" terminals. These allow you to create a 4-wire connection to the load, and this allows the internal regulator to compensate for the voltage drop in the main current-carrying wires.

However, you may be over-thinking the problem. AWG18 wire has a resistance of 20 mΩ/m, so a 25-meter run of 2-conductor cable has a total resistance of about 1 Ω. Even with a motor drawing 1 A, this would only create a voltage drop of 1 V.

If all of your motors have about the same length of cable running to them, it may be sufficient to simply turn the voltage of the power supply up by 1 V to compensate for the cable drop. Most pre-built power supply modules have an adjustment control for just that purpose.

  • \$\begingroup\$ @EMFields Then write your own answer instead of hiding behind sarcasm and comments. \$\endgroup\$ – W5VO Apr 6 '16 at 14:33

That's a worthy concern. You need to google the resistance of 18ga wire and do the Ohm's Law calculation. It's quite straightforward.

Keep in mind that 14 and 12 gauge wire are unnaturally cheap because so much of it is used in architectural wiring. Compare the prices of 12 and 14 gauge NM (Romex) multi-wire cable, and 14 and 12 THHN/THWN single-wire. So in your particular situation, "throw copper at the problem" may be the way to go.

The best answer is do the 120V to 18V conversion as close to the point-of-use as possible. One option is relays. Energy loss is the square of current, so wires that have too much voltage drop for full current can easily handle a small relay-coil current.

  • \$\begingroup\$ The OP is probably hoping to use common thermostat wire, which is two AWG18 conductors in a very thin jacket, and is much easier to handle and much less obtrusive than Romex. \$\endgroup\$ – Dave Tweed Apr 5 '16 at 21:11
  • \$\begingroup\$ And he probably has a whole bunch of circuits too. At least he could run the common in 12ga THHN stranded (which is as flexible as thermostat wire and smaller) and that would eliminate half his voltage drop. \$\endgroup\$ – Harper - Reinstate Monica Apr 6 '16 at 0:45
  • \$\begingroup\$ I don't know where you're getting your information. A single strand of AWG12 stranded wire is neither as narrow nor as flexible as thermostat wire. I have samples of both right in front of me. I also don't know how you imagine the hookup of the motors. I'm guessing that the OP has a control panel/box that contains DPDT switches or relays (he needs to be able to reverse the motors), and he needs a 2-conductor connection from there to each motor. There would be no "long common" connection. \$\endgroup\$ – Dave Tweed Apr 6 '16 at 11:09

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