Electrics - wire thickness

[Phil C]

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.

[Rog1]

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.

[JulianS]

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

[Phil C]

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.

[Richard Mills]

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.

[Rog1]

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.

[Phil C]

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.

[Rog1]

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

[Phil C]

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.

[johnan]

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.

[Mark N]

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  )

[Phil C]

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.

[Phil C]

Further to my most recent post: just thinking, I guess there's a difference between capacity and rating.  Phil.

[Phil C]

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

[AWJDThumper]

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.