Second try for this question...

Steve Madden's very useful page shows two sizes of cable (4mm and 5mm OD), but could someone tell me if the 12A cable I've bought (info. below) is okay to use for all the wiring on my WM20?

Vintage Motorcycle Black Rubber Insulated Cable
Overall diameter of Cable 4mm
Wire size 55 strands of 0.15mm tinned copper wire
Nominal current rating 12A.

Scorp.

email (option): wm20@scorpionvideo.net

From a quick forum search for "wiring":

John Harris
Sep 15, 2013 - 9:25PM
Quote Reply Re: wiring 1940 M20
"According to maintenance manual (Air Pub. 1657B) the maximum voltage at the regulator can vary between 7.7 and 8.3 volts. Of course, the nominal system voltage is 6v, so the variation is quite large. Limiting voltage drop to 0.1 volts seems 'way too restrictive.

Maximum output of the long dynamo is 60 watts, i.e., 10 amps at 6 volts (or less at higher voltage). AWG 18 or metric no. 1 wire (1 mm.) should be plenty and, since it is easier to source no. 2 or 2.5 wire, the most common wire gauge would be plenty even for the wires carrying the heaviest loads.

BTW, in a direct current system, like a vehicle, the only function of stranded wire is that it is easier to bend and follow curves than solid wire. In alternating current systems, especially of higher frequency, stranded wire carries higher amperage than solid wire of the same cross-section because AC is carried close to the surface of the strand and the higher the frequency the more closely to the surface is the current "squeezed", so at higher frequencies you want to have more strands even if they add up to the same aggregate cross-section"



It would seem that your larger-diameter wire can be used for all wires on the machine as far as the electrical system function is concerned; the question is: are you going for "correct" wiring, or effective wiring? 4mm outer diameter wire seems excessive and might not look the part, JDE

email (option): teladelujo@ msn.com

As stated, it should work. You might also consider adding two fuses to your system: a 10 amp for the regular harness and a 1 amp for your tail light. Those can be the mini plug in style, and there are waterproof holders that are small enough to be hidden in a battery box.

Thanks for your replies, I'm pleased to know that the 12a cable is suitable, and I can now start work on my loom.

I must admit that, I thought that the 4mm, black rubber cable, I've bought would be 'reasonably' correct and not 'excessive', given that, according to Steve Madden, the original cables were 4mm and 5mm OD. Anyway, at least it's not going to be a a multi-coloured, off the peg, loom.
Scorp.

email (option): wm20@scorpionvideo.net

[quote
I must admit that, I thought that the 4mm, black rubber cable, I've bought would be 'reasonably' correct and not 'excessive', given that, according to Steve Madden, the original cables were 4mm and 5mm OD. Anyway, at least it's not going to be a a multi-coloured, off the peg, loom.
Scorp.
[/quote]

I will certainly defer to Mr. Madden assessment on wire size. The 4 to 5mm outer diameter is correct, just sounds large. Some of the remaining original wiring on my bike (leading from the battery to the headlamp shell) was this size, but most had been replaced over time with newer wire with thinner insulation.

email (option): teladelujo@ msn.com

I copied the original loom on my 16H. As John says, heavier cable is needed now to replicate the OD given by the old thicker rubber insulation. I used a combination of 28/030 and 44/030 cable.

The thicker runs were from the dynamo, the horn and the ammeter, and rather oddly the rear light. Bearing in mind that it was only a 5w single filament lamp, this probably had more to do with making the cable physically robust enough to be clipped along the mudguard.

You might consider using some colored heat shrink in place of the original rubber bands at the ends to mark the wire colors. I found a place that had heat shrink colors for $1.50 US for a 4 foot piece. I cut sections and put them at each end just like the original markings.

Kevin

Yep, I've already got a pack of heat shrink in nine different colours.
Scorp.

email (option): wm20@scorpionvideo.net

Sounds like there may be some confusion. The outside diameter of the wire (copper + insulation) is completely irrelevant to the question of how much current (how many amps) it can carry. Only the copper part matters for this question. A thin copper wire with thick insulation does not carry the same amount of current (amps) as a thick copper wire with thin insulation, even if their respective total cross-sections are the same. The size of the wire is the size of the copper, not copper+insulation. A metric no. 1 wire has copper whose cross-section is 1 mm in area. A 2.5 has a cross-section of copper (only) of 2.5 mm in area. The current-carrying capacity is determined by this cross-section. The plastic insulation is only there to insulate, not to carry current. The thickness and type of this insulation determines how many volts (not amps) it can insulate and, in applications such as household/commercial/industrial, how quickly it can dissipate the heat genarated by a high-current high-voltage load. In view of the state of materials science in the 1940s there may have been materials which could not insulate 6 or 12 volts. Today any insulation, in practically any thickness, would insulate that kind of voltage, so you need not concern yourself with the insulation part of the wire (except for looks and authenticity purposes). Think of the inside of the dynamo: the wires are wrapped tightly around with just the shellac-type insulation on them, not any plastic insulation, yet each turn of wire is insulated from each other.

But you don't need any of this for the original question. Bottom line is that if the wire is rated for 12A, it's plenty for any application on an M20 where the highest working amperage would be 10 amps. Of course, there could be higher amperage if there is a short or other malfunction but it is not the function of the wire to prevent those. That's why they make fuses.

email (option): jonny.rudge@verizon.net