Reply To: what is RK wire?

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USA* builders: I found what I think will work. I haven’t built it yet but I have printed the board, including calibrating it with the test piece (and I printed <span style=”text-decoration: underline;”>that</span> 5 times to get it right – being careful!), and I’ve test-fitted a bit of the wire I found and it seems to be very, very close.

The answer lies in a piece of simple “zip cord“** or lamp/plug cord I had lying around that I’d cut off some dead appliance. Standard white vinyl coating, 2-conductor, goes to a 2-prong plug for a 110/120v AC wall receptacle. Each conductor has apparently 19 strands (seems like a funny number but I counted two different pieces). Looking at specs for lots of examples of this type of cord, it usually seems to have 41 strands (even stranger – maybe a few extra strands on the ‘hot’ side?). The cord’s usually called SPT-1 or -2 (depending on insulation thickness; there’s otherwise no difference) and also known as 18/2 (for the gauge of the overall wire and number of wires). Home Depot or Lowe’s will sell you a foot (30cm, 5 more than you need) – in my area for 28¢ / 37¢ respectively (wow, big difference!).

Mine measures anywhere from 0.28 to 0.3mm at different points using good calipers and I see it generally spec’d at 0.3, which matches what we want here. (The above guess at 28AWG would be 0.321mm.) I am not sure all such cord has 0.3mm strands, but looking up about 5 mfgs specs online, and all either said 0.3mm explicitly, or had 41 strands and I assume that by sheer math that if the strands add up to 18ga they must be the right Ø individually.

I’ll try to remember to come back and post results once I’ve tried it out, but I can’t think of why it wouldn’t be ideal.

  • I’m guessing Canada too as they use they same type of plugs/receptacles and probably same cord types as it would make simple economic sense to share standards. Possibly same for Mexico/Central America.

** not to be confused with the type of zip cord used for speaker hookup – I tried a bunch of different sizes of that that I also had in the wiring “paw-through” bin and most were far too small – more like 0.17mm.