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I read somewhere that an 18/2 wire could be used to connect a transformer to my window blinds motor (12V 1.5 amps). However, it didn't say anything about the distance. Since I am running it inside the wall, I was looking at getting "UL and cUL Listed SPT-2 Wire" 18/2 electrical wire. Will the distance cause a problem?

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    \$\begingroup\$ It's just "lamp cord," so to speak. I tend to think of it being used for AC appliances. SPT-2 has 50% thicker insulation than SPT-1. But that's the only difference I know about. For AC use, it's specified at up to 50 feet and 10 amps. I think you'll be fine with this for AC/DC use at 1.5 amp and 12V. I admit I'm NOT familiar with any NEC code regarding 12 VDC or 12 VAC power lines in the walls. You might want to look it up and see if there's something on the topic. They may have added code for DC wiring, now that it's a more common situation. But they may be behind the times on that, too. \$\endgroup\$
    – jonk
    Oct 13 '19 at 2:12
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    \$\begingroup\$ DC transformers don't exist. You probably mean a power supply. \$\endgroup\$
    – Hearth
    Oct 13 '19 at 2:14
  • \$\begingroup\$ I'm pretty sure SELV (which this is) is mostly unregulated, if your motor needs 12V 1.5A and your supply produces 12V, it's not going to work.... probably the motor can get by on a little less and some length of extra cable will usable, but without knowing the details noone can answer. \$\endgroup\$
    – Jasen
    Oct 13 '19 at 2:36
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According to this site 18/2 SPT-2 lamp cord uses 18AWG wire, which according to this site has a resistance of 0.006385 Ω per foot. You are running 2 wires 47 feet so the loop length is 94 feet and the loop resistance is 94*0.006385 = 0.6Ω.

Assuming the motor doesn't draw more than 1.5A the maximum voltage drop is 1.5A*0.6Ω = 0.9V and the voltage supplied to the motor at 1.5A is 12V-0.9V = 11.1V. It will probably work fine. The wire is rated for 10A so it should be fine too.

The only thing that might be a problem is if the motor stalls, since there is an extra 0.6Ω in the circuit that could prevent a protective fuse from blowing. Hopefully the 'transformer' has over-current protection or is fused at a current not too far above 1.5A.

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  • \$\begingroup\$ Oops, I didn't know there was a difference between a transformer and power supply (@Hearth). I was looking at getting this Monoprice 106876 8 Channel 12V DC 13 Amps CCTV Camera Power Supply, since there are are 8 windows. It says it has "auto-reset PTC fuses", which hopefully provides the "over-current" protection that @bruce-abbott mentions. So, does this look like a good wire and power supply combination? I'm trying to convert battery operated motors that use 8 AA batteries (in a long tube). \$\endgroup\$
    – spockdude
    Oct 14 '19 at 3:29
  • \$\begingroup\$ Side note... I actually have not been able to get exact specs on the motor, but someone running an unknown brand that also used 8 batteries in a tube measured the max draw on their motors and it was under 1 amp. So, I went with 1.5 amps just to be sure. My main concern was whether the length of the line was going to cause too much voltage drop. \$\endgroup\$
    – spockdude
    Oct 14 '19 at 3:46
  • \$\begingroup\$ AA cells have an internal resistance of about 0.2 Ohms each and are good for about 1A max, so your power supply + wiring will probably perform better than the tube of batteries. The PTC fuse rating of 1.85A sounds fine - at least for the wiring. \$\endgroup\$ Oct 14 '19 at 4:32
  • \$\begingroup\$ So, one more follow up question... would a thinner wire work? I have a wall of 8 windows that I need to run wires to. The 18/2 wire I've referenced above is going to take some major holes to feed through (and sounds like it has well over the needed capacity). Would 22/2 wire also work? For example: amazon.com/dp/B01CSWPB7O/ref=cm_sw_em_r_mt_dp_U_OFrRDbZQVKS6P (Alarm Wire 22 Gauge 100' 2 Conductor Stranded Copper Cable White UL Listed) \$\endgroup\$
    – spockdude
    Oct 21 '19 at 2:31
  • \$\begingroup\$ 2.5 x resistance so ~2.25V drop at 1.5A. Do your motors still work with almost flat AA batteries? If so it should be OK. \$\endgroup\$ Oct 21 '19 at 3:23

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