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I have some non-standard bikes to hook up so rather than ordering the kits with fixed length wires, I want to just order appropriate double strand (or coaxial) wire in bulk, along with the connectors, and get the job done that way. I can't seem to find the standard size used for this job, however. Appears to be about 22 AWG.

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  • As mattnz suggests, the wire size choice is based more on the strength/stiffness of the wire, and on the ease of working with it. Nov 24, 2020 at 23:11
  • I will note that there are several places online that sell the Schmidt coax wire in bulk by the foot; it's not that expensive and it is by far the cleaning looking I've worked with if you need double-strand. Everyone who sells it will also have the Schmidt and B&M type connectors; the B&M 2.8mm ones are the same as many other generator lights take. Nov 24, 2020 at 23:56

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Won't matter much, 22AWG is rated at about 1AMP for power transmission and 7AMP maximum current (i.e. that is around the current insulation starts to melt), is plenty large enough to carry the current from a dynamo (a few hundred milliamp's) and provides a strong enough wire that it won't break easily when in use. A size or two smaller or larger won't hurt.

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    I've used 22AWG stranded ethernet cable as a proof of concept, and a year later its still working fine so haven't replaced it.
    – Criggie
    Nov 25, 2020 at 1:07
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    Probably good to emphasize that stranded wire (not solid) should be used, as the wire may flex and solid wire's metal conductor will eventually break after repeated bending.
    – Armand
    Nov 25, 2020 at 1:12
  • @Craggie - Ethernet cable, the #8 wire of DIY electronics and electrical. :)
    – mattnz
    Nov 25, 2020 at 2:07
  • Not a bad idea. I've got a lot of cat 5e sitting around. I've got some salvaged charger cable that's 22 x 2 strand and nice and sturdy that'll work for some of this too.
    – user36575
    Nov 25, 2020 at 2:37
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The "official" answer is AWG 20.

According to SON (hub dynamo manufacturer), their coaxial cable (and it really is coax, not just 2x conductors inside a round, foam-filled housing) corresponds to:

2 x 0.5 mm^2 conductor wires.

That corresponds, according to conversion tables like this one, to an AWG 20.

enter image description here

As noted above, 22 should already be overkill for the current carried, but in terms of durability of the wire and maybe the crimping at the spade connectors, 20 may be the better bet.

I'm not finding a bulk SON coax seller in the US for < $1 / foot, which isn't terrible, unless you want to do 4 long bikes (as I do) and need about 50 feet, given that bulk auto / electronics 2-strand can be had for a fraction of that price, though it may be a lot uglier.

One more tip if you're going this route, there are spade connector + crimper kits available online (here's an example on Amazon) that give you dozens of the 2.8mm and 4.8mm connectors you need, plus housings for them, as well as a crimper, all for <$30.

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@user36575. For clarity and interest - I hope! (even at this late stage): that linked table converts the 0.5mm^2 XSA of round conductor wire to 20 AWG by assuming it is a single disc of conductor (with a diameter of 0.80mm). Electrically for dynamos this doesn't matter, it's the same amount of copper.

I realised that I can't find the diameter of the bundle of stranded conductors from this, there are gaps between the conductors. Because the AWG is actually a diameter measurement that might confuse folks (well, me anyway!).

I guess industry uses a pre-calculated (possibly empirical) packing factor to get the right figure. The maths is involved, e.g. Ref 1, and I need to know the number of strands.

Ref 2 is a 3 Amp (AC) flexible cable that is 0.5mm^2, 20 AWG, 16 strand i.e. the conductor core is likely similar to the inner SON stranded wire (but not the external one, the SON wire is coaxial). Ref 3 (via Ref 1) shows 16 strand cable has a density of 0.75 (a solid single cable is 1). So the XSA including gaps would be 0.5mm^2/0.75=0.67mm^2 and the diameter would be 0.92mm cf. 0.80mm single conductor. So it is 15% larger diameter than a solid wire.

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    Welcome to Bicycles SE. I am having a tough time understanding how your answer answers the original question. It may be good information but the original question was interested in the gauge needed to replace his specific application. Can you edit it to answer their question?
    – Ted Hohl
    Apr 4 at 14:13
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    @TedHohl Hi, OK. I explain that the gauge of a stranded cable (as opposed to a solid cable) does not tell you the diameter even though gauge is usually a measurement of diameter. That to me is so counterintuitive I thought it worthy of note, and I then show how to estimate the diameter. It might have a bearing on practically installing wiring e.g. size of open or closed barrel crimp connections on it. It'd be better as a comment to the OP's own answer on Nov 25, 2020 at 12:48, but I'm a new user, so wasn't allowed to comment. If it's problematic I can just delete it, no worries.
    – Lank
    Apr 5 at 15:22
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In this case, the main concerns are resistive loss, power transfering capacity and durability of the wire.

You need 80cm of wire: 40cm from dynamo to light and 40cm back from light to dynamo. The wire needs to withstand 0.5 amperes of current. 30AWG is enough for 0.5 amperes, and has 0.34 ohms of resistance per meter or 0.27 ohms of resistance total. At 0.5 amperes, it creates 0.135 volts of loss at working voltage of 6 volts. So 2.25% of your energy is wasted as heat in the wiring. I'd say this is on the upper boundary of acceptability. Much more would be unacceptable.

However, 30AWG is pretty thin, and could break easily. A thicker wire might be more durable, but however, if a stick is caught in the spokes, a thicker wire will break too.

I think 20AWG would be far easier to find and more durable than 30AWG.

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