Electrics - wire thickness

[Mike40M]

A good voltage supplied to the headlamp is vital to a safe night ride. There is a voltage drop both in the cable from the battery to the headlamp and on the return from headlamp to battery. Add the voltage losses in battery connections, fuse, light switch, dimmer switch and other connections. You could end up with a quite low voltage on the headlamp. Checking headlamp voltage compared to battery voltage is a good thing.
Apart from keeping all those parts mentioned in good order, thicker wires is also a good idea.
I think heavier gauge cables withstand the vibrations on my bikes better.
I have a huge supply of 1.5 and 2.5 sq.mm. wire since my work on industrial control systems, so I mainly uses those.
Completed rewiring my WD B40 a couple of weeks ago. H4 headlamp, Boyer, Rectifier/Regulator from Paul Goff. To my surprise, it is charging with headlamp on when idling.

[Phil C]

I'm reassured by what AWJDThumper says, and am not worried about my wiring. But I find it rather frustrating not to fully understand. James Smith, in his "Classic Motorcycle Electrics Manual", says " The current ratings quoted by manufacturers are typically based upon the maximum current that can be conducted without exceeding the temperature limits of the insulation." The figures given for mamufacturers' max current ratings for 1sq mm wire are: standard pvc wiring 8.75A;  thin-wall 16.5A. Practical current ratings for classic motorcycle applications are given as: standard pvc 4.9A, thin-wall 7A. So this suggests I'm exceeding the limit. See what I mean?  Phil

[AWJDThumper]

With 6 V electrics its all about voltage drop. In my example, loosing 0.3 V when you only start with 6 V is getting to be a problem. Throw in a few bad connections and you can easily end up with only 4 V at your bulb or horn.  This means that you will normally never get close to the current carrying capacity of a 1 mm sq cable. In my example, even at twice the rated current (2 x 8.75 A), you will still only dissipate 12 W in the cable which is nothing from a thermal point of view!

[Rog1]

Wires also have to be able to stand up to general mechanical wear and tear, especially the ones that run around the headstock that are subject to flexing. This means that wires are often vastly over sized for the current they have to carry, just to be strong enough to survive for a reasonable time.

One aspect that doesn't affect most BSAs, especially older ones, is that cables in big bundles have to be derated because they can't get rid of the heat so easily. There's a nasty little cycle where a wire gets warm so the resistance goes up, which makes it warmer which makes the resistance go up, and so on. It isn't often a problem at 6 or 12 volts, but I've seen quite a few mains voltage sockets with heat damage. In those cases, it's a bad connection somewhere.

As the biggest load on the electrical system, if nowhere else on the bike I'd make sure that the wires to the headlight are well up to the current it draws. Looking to the future, when switches and connectors start to show their age it's as well that the voltage lost over the length of the supply wires is as low as possible.

[Phil C]

Thanks Rog. But oh dear, how frustrating this is. My headlight, at 30w, should in theory draw (30 divided by 6 volts = ) 5 amps? As I said previously, James Smith, in his book, gives the practical current rating for classic bikes of 1sq mm wire as 4.9A for standard pvc, 7A for modern thin-wall. So from what you say, if the wire on my bike is old-style standard pvc wiring, doubt creeps in again. I'm not really worried about all this, but it is frustrating when I am assured (and believe) it is safe, but then figures are produced to put that in slight doubt. It all seems contradictory and vague, and I don't really get it. Phil

[Phil C]

Actually, I think lots of replies I get are maybe focusing on whether the headlight will be bright, whereas I am thinking about whether enough heat will be produced in the wire to be a safety issue, ie get hot. I guess what I should remember is what AWJDThumper said, which was the amount of heat produced in my wiring will be insignificant.  Phil

[AWJDThumper]

You only need to worry about the current carrying capacity of your 1 mm sq wiring harness if a part of it gets shorted across the battery with no fuse fitted. Been there and had to put out the flames on a C15 - unfortunately, I was parked in Fleet Street in London at the time!!

[Phil C]

I've fitted a 10amp fuse to each of the two wires coming from the negative terminal of the battery (earth is positive.) So hopefully I'm not going to be consumed by flames. Phil

[Phil C]

Just further to make my point, one of the two wires coming from the battery feeds the headlight (5A), tail light (1A), Speedo (0.4A), and coil (2.5A.) So that's about 9 A in a wire which has a practical current rating of about 5A.   Phil

[Blackthou]

Hi Phil,

An intersting topic, well worth resurrecting as I wasn't on the forum last year and had missed it. I do understand your dilemma and the want to understand. I don't like mysteries and also like to understand.

I've only had my C15 5 months and it's been in bits most of that time while I go over it from one end to the other. When I first got it, the headlamp looked as if it was powered by a drained 1.5v AA battery it was so dim and golden.

One of my first areas of investigation was inside the headlamp. It was fully of years of improvisations and later type connectors. I cut the ends off them all, bought some proper bullets and a crimp and redid the lot. Next time I tried the headlamp it was if an LED unit had been fitted. It was all doown to bad connections and voltage drop.

Rather than worry, I'd just make sure (and I'm sure you will) that everything is sound. I've recently bought a regulator from Paul Goff which I've yet to fit to help protect my system. i've also bought a fuse as there wasn't one at all.

[Rog1]

Thanks Rog. But oh dear, how frustrating this is. My headlight, at 30w, should in theory draw (30 divided by 6 volts = ) 5 amps? As I said previously, James Smith, in his book, gives the practical current rating for classic bikes of 1sq mm wire as 4.9A for standard pvc, 7A for modern thin-wall. So from what you say, if the wire on my bike is old-style standard pvc wiring, doubt creeps in again. I'm not really worried about all this, but it is frustrating when I am assured (and believe) it is safe, but then figures are produced to put that in slight doubt. It all seems contradictory and vague, and I don't really get it. Phil

Phil, 4.9A seems rather low. To confirm, I looked at two companies just now and they both say that 1sq mm wire can take 8.75 amps. That's the same figure I've used for years. I'm pretty sure that my other bike's headlight is wired in 1sq mm, and with a 9w LED spot alongside the 60w headlight, that's 5.75 amps. It's fine. I know the size of the wire because I rewired the lighting circuit myself.

James Smith also reckons (you said) that the maximum current capacity was reached when the insulation had reached it's melting or burning temperature. That's 60 C for normal PVC insulation; almost too hot to touch! That would mean that a huge amount of power was being used to make the wire hot, so you'd be lucky to get more than a volt or two at the end furthest from the battery. Sorry, but I'm not convinced he's right.

[Phil C]

It hadn't occurred to me that he might be wrong. He says the suggested practical ratings allow a safety margin to the manufacturer's maximum rating. As I say, for standard pvc wiring he says 4.9 and 8.75, and for thin-wall pvc 7 and 16.5. A possible explanation that occurred to me is that there could be a difference between capacity and rating.  Phil

[Phil C]

He goes on to suggest, under " General recommendations", after talking about 12-volt systems, "For 6-volt motorcycles where voltage drops are more of a concern it is advisable to use 44/0.3mm (3 sq mm) wiring for the main battery and charging connections. The remaining connections can be made with 28/0.3mm (2 sq mm) wiring, but any thinner cables are best avoided because of the significant voltage losses they will introduce." ( By the way, where I say sq mm he uses mm with a small 2, but I think it's the same thing.) So no mention of heat there, the focus being on voltage drop. But the wiring he recommends is thicker than the stuff we seem to be using.   Phil

[Bess]

Hi Phil,
     
Maybe worth a read:

https://www.12voltplanet.co.uk/cable-sizing-selection.html

You mentioned you don't have a soldering iron. If you are going to rewire its better to crimp anyway, soldering can damage the insulation and causes a stress point between the connector and unsoldered wire.

Best wishes...

[JulianS]

Being practical how does James Smith suggest using 3mm cable for the charging system?

Where would you connect it given that with the Lucas stators the leads are built in and with the later ones encapsulated in resin.

For me thinwall 1mm has proved perfectly servicable over a number of years on my A10. I ride with headlight on and for a while used a coil ignition conversion. No melted cables and no heat  bubbled insulation.

I think it easy to overthink this issue.