Good evening everyone. If you are easily bored, you might want to move on at this stage. But if you are reasonably good at electrical theory (I'm not), and are a particularly patient kind of person, then you might be able to explain something to me. I'm not planning to rewire my bike, but was reading Rupert Ratio and am curious about something.
RR vol 2, ch 17 (p.282), "Rewiring Your Unit Single", says "Cables should be 14 strands each of 0.3mm with a total of 1 sq mm rated at 8.75 amps minimum. With a 12v supply this is capable of carrying 105 watts."
That makes sense, even to me, given that watts = volts x amps, so 105w = 12v x 8.75amps.
But what if your unit single has a 6v battery? Is it not then the case that that gauge of wire will only carry (minimum) half of 105 = 52.5 watts? (52.5w = 6v x 8.75amps.) Otherwise there would be too many amps flowing and the wire would get too hot? Am I getting this all wrong?
If I add up my bike's wattages for headlight, tail light, speedo and coil, I already slightly exceed 52.5w. Adding stop light and horn would take me well over (I know the horn would only be used momentarily but the stop light could be held on for a while, eg at traffic lights.) The stop light would add about 18 (or 21?) watts to the 52.5, making 70.5 (or 73.5?)watts. Are you still awake?
I expect RR is of course right, but I'm struggling to understand it. Is it because the coil current is on and off all the time, and even with the stop light held on for a few minutes the wire wouldn't get hot enough for long enough to cause problems?
Phil.
Phil C, you are right, and I'm still awake! :)
A I sq. mm wire will carry 52.5 watts. The limit is know as volt drop; the (very low) resistance of the wire means there's a very slightly lower voltage at the far end of the wire than at the battery. The volt drop is directly proportional to the current, so the headlight, often the thing using the highest current on the bike, is most affected. It's also most affected by bad connections and manky switch contacts. And because 6 volts means twice the current of a 12 volt system, 6v vehicles are more affected than 12v ones.
But not every wire is carrying the full electrical load, so only the ones that are carrying more than 4.38 amps (in this case) need to be bigger than 1 sq. mm. This includes any earth wires. I'd allow for the horn; you wouldn't want (for example) the brake light to dim every time you sound the horn. The current to the coil being intermittent makes no real difference, you still need minimal volt drop when it is drawing current.
The wires do heat up a tiny, tiny amount, but you'd never detect it normally. It's only a gross overload that makes wires get too hot. If you've ever seen a bike that's had a dead short circuit and a fuse didn't stop it happening, you'll know what I mean.
Worth looking at this thin wall cable which has a higher load. The 1mm size having a 16.5 amp capacity. I have used it for a number of years on bikes.
http://www.vehicle-wiring-products.eu/section.php/198/1/single-core-pvc-thin-wall-cable
Good morning all,
Thanks very much Rog1 and Julian.
Rog says the wires which carry more than 4.38 amps need to be bigger than 1 sq mm. (I assume Rog is talking about standard, not modern thin wall stuff.)
I think on my bike (1965 B40F) that would be the headlight, the horn, and the wires to/from the battery. My next questions are (I hear groans):
1. What would happen if I just left them as they are (I think they are standard 1 sq mm)? Is there an overheating/safety issue?
2. In terms of practicalities of actually making the changes, I guess it would mean replacing the following wires:
a) battery to ammeter
b) ammeter to light switch
c) light switch to headlight
d) battery to horn
e) horn to horn button
f) horn button to its earth (if there is such a wire)
g) battery to earth on the bike's frame (appears to be same as the rectifier's earth point.)
Is the above correct? (turns out to be quite a lot, doesn't it?)
3. Is changing the horn wiring necessary?
4. Is all the above easy? For example, no soldering involved? (I don't have any soldering kit.)
5. What gauge should the alternator wires be?
If you could advise on the above five points, I'd be very grateful.
I notice, by the way, that in the book "Classic Motorcycle Electrics Manual", by James Smith, he gives manufacturers' max current ratings and also "practical current ratings" which are much lower. Eg for 1 sq mm standard PVC wiring he gives manufacturers' max rating as 8.75 amps but he also he gives a figure of 4.9 amps as a practical current rating.
Re Julian's suggestion of thin wall cable, I used 28/0.3 (2 sq mm) thin wall stuff for the short lengths of wire to and from a fuse box which I installed near the live battery terminal recently.
Thanks very much again. Phil.
Thin wall cable is fine for cars. However it doesn't seem to be as flexible as the older type and I therefore do not use it between the frame and the fork areas where it is subject to repeated flexing.
1. Unless you are having electrical problems on your bike, very little will happen. If you are having problems, it may just be bad connections and manky switch contacts, not the size of the wiring. If it's original wiring, it should be the right size. But how do you know the bike is wired in metric size cables? It may well be wired in the original SWG sizes which in your case are slightly bigger, and take more amps. The whole loom may be using, and radiating, all of 2 watts. It will not even get warm.
2. Seems about right, though I don't know how many amps the horn takes.
3. See above.
4. I think it's easy but I'm a sparkie who has worked on industrial control systems as old or older than most of our bikes, and rather more complex. If you have to ask, then it probably won't be easy; there's a bit more to it than people first realise.
5. Depends on the output. If rewiring, I'd go for 2 sq. mm.
Are you having electrical problems? If not, then no worries.
Thanks, Rog1.
No, I'm not having any problems, so maybe I should just leave well alone.
I'm afraid I don't get the bit about 2 watts. Which is maybe another reason I should leave it alone.
Phil.
Phil,
2 watts was just a rough guess. What I meant was that the whole loom produces very little heat over its whole length; you'd never notice it. Glad to hear you are having no electrical problems.
Roger
Morning all,
And yet, and yet.
As I'm having no problems (although I never turn on the headlight except for a few seconds just to check it works), I guess I'll leave well alone.
But it's a bit frustrating not understanding this: wires are given a max current rating. Rog1 you say wires carrying more than 4.38 amps need to be bigger than 1 sq mm. My wires appear to be 1 sq mm, and several might be required to carry more than 4.38 amps (see figures below.) But when I ask what would happen if I just leave them at 1 sq mm, you say very little. So - here is my question - if very little would happen, why do they have to be bigger, and why do wires have a max current rating?
I think my bike's wattages/amps are approx as follows. I've divided watts by 6 to get amps, but maybe I should have divided by 6.3?
Headlight 30w 5 amps
Tail light 6w 1 amp
Speedo 2.2w 0.4 amps
Coil 15w 2.5 amps
Stop light 18w or 21w 3 or 3.5 amps
Horn 51w 8.5 amps
There are two wires coming from the battery -ve (live) terminal: one for the stop light and horn, one for the others.
Phil.
If everything works, and as you are not too familiar with things electrical, I would say just leave it alone but make sure all connections are sound and no wires are chafing against sharp edges of metal that could cause a short circuit.
You can buy new looms if you really want to go back to the start!
When I first got my C15 I couldn't make sense of the wiring until I discovered a previous owner had replaced the loom with one from a later model which had some fundamental differences...after finding that out and also finding the correct wiring diagram for it, it all became clear! Well, sort of.
Phil, as has been mentioned earlier, there are two reasons to select a specific cable size; firstly it has to cope with the current being drawn (ignore the horn, it's not on for long enough to cause an issue), but secondly it is about reducing voltage drop. Voltage drop can also manifest itself where the frame is the sole conductor as an earth; any bolted joints will add to the drop).
Cables on a motorcycle benefit from being exposed to airflow and hence are cooled (although you won't detect any generates heat as it's so minute).
I know this doesn't answer your question; personally i just don't over-think the poor design of Brit. electrics. Ride and enjoy (but carry a fire extinguisher ;D ;D)
James Smith, in his book about classic bike electrics, says (p.86) "The maximum current capacity of a wire is reached when the temperature of its insulation rises to its melting (or burning) temperature." He goes on to explain that the degree of heating depends on the wire's resistance and the amount of current flowing through it.
My amps appear to exceed the max current rating (as discussed above.) So I'm sure you can understand why I am interested in whether, and why, my wiring is safe.
(I've fitted 10 amp fuses, by the way.)
I'm not planning to rewire it, but it would be nice to understand.
Phil.
Further to my most recent post: just thinking, I guess there's a difference between capacity and rating. Phil.
Given there is a degree of turnover of people on the forum, I just thought I might resurrect this topic and see if anyone has any thoughts/advice/opinions about safe wire thicknesses. Phil
Just to put your concerns into perspective:
By way of an example, connect a 2 m long length of 1 mm sq 14/0.3 cable from the battery to a headlight bulb drawing 8.75 A - that is taking 52.5 W at 6 V. This cable is rated at 8.75 A based on voltage drop. For a 2 m length of this cable carrying 8.75 A, the voltage drop will be 0.33 V - in other words, the headlight will only see 5.67 V and not the 6 V generated by the battery.
The current flowing through the 2 m length of cable will heat it up. At 8.75 A, the amount of heat generated uniformly along its 2 m length will be 2.9 W. This amount of heat is not going to raise the temperature of the wiring harness by very much and will therefore not cause any problems.
The total steady state current draw for you bike is a little over 52.5 W and so 1 mm sq is more than adequate. The extra power drawn by the horn and stop light does not really need to be considered because their effective average on time is so low and they will therefore produce minimal extra heating of the wiring harness.
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.
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
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!
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.
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
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
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!!
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
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
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.
Quote from: Phil C on 06 Jul, 2020, 18:44
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.
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
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
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...
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.
Quote from: Phil C on 07 Jul, 2020, 08:40
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
If I was building a new 6 V wiring harness for a classic BSA bike that I intended to ride regularly then I would probably over engineer it in the way that Dr James Smith recommends. A 0.3 V voltage drop experienced in a 2 m total length of wire back to the battery (or rectifier) is something that I would definitely avoid - this is what you would get from 1 sq mm, 14/0.3 cable. Upgrading to 2 sq mm 28/0.3 cable would reduce the voltage drop to < 0.1 V.
However, BSA took the view that the voltage drop indicated would be tolerable and at the same time 1 sq mm cable would save them 40% of the cost of 2 sq mm cable. If you want to avoid a voltage drop of 0.3 V then all you need to do is upgrade the wiring to the headlight from its sources of power (battery or rectifier). However, it is also important to address the earth return path as well especially if this is through the frame since this path can be the sources of even larger voltage drops. I personally, always upgrade wiring harnesses to avoid return paths through the frame or the rear mudguard.