Author Topic: Wiring Loom Rating  (Read 1105 times)

Offline bikerbob

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Wiring Loom Rating
« on: 08.12. 2014 19:34 »
I have just bought a new wiring loom for my 1956 A7 and while it comes complete with all wires and correctly labelled, it says that the wire used is rated at 16.5 amps thin wall. Looking at the overall diameter of the wire it is only about half of what the original wire is on the bike which I believe is the original BSA loom my question is will the new wire be good enough for the bike.

Offline Johnny J

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Re: Wiring Loom Rating
« Reply #1 on: 08.12. 2014 22:36 »
Have a look here: http://www.engineeringtoolbox.com/wire-gauges-d_419.html
I will do some rewiring on my bike and will use 1.5mm2 wire, will handle about 10A without energy loss (heat), should be enough since the distances are quite short.
   Gothenburg, Sweden

Offline bikerbob

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Re: Wiring Loom Rating
« Reply #2 on: 09.12. 2014 11:33 »
I have been checking the old wire  as against the new. The old wire is 16 strands .0095 dia which I believe equates to 34swg the new wire is 32 strands .0075dia which equates to 36swg. So am I right in thinking that the actual core wire is okay and will the thinner insulation be okay.

Offline Johnny J

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Re: Wiring Loom Rating
« Reply #3 on: 09.12. 2014 20:02 »
I am not sure how you calculate this..
To be really sure you could put some work into testing the wire by connecting two 12V 55 or 60W car light bulbs in parallell and use a 12V car battery as a power source.
Some thicker wire between one pole of the battery and one pole on the lamps and the wire to be tested between the other poles, if the wire doesn't get warm/hot within a minute or two it can take 10A, =60W on 6V.
   Gothenburg, Sweden

Offline sparx

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Re: Wiring Loom Rating
« Reply #4 on: 09.12. 2014 22:43 »
 One of the factors involved in selecting cable sizes is the amount of heat the cable can dissipate without it's temperature rising above a safe level. Modern thinwall cable can dissipate heat a lot easier than the older cable because, quite simply, there is less insulation around the conductor. This means that the current carrying capacity (CCC) is often at least and sometimes more than twice that of the older cable.
  One caveat is that the current rating is given as if the cable was a single strand in free air. Bundling the cable into a harness means the CCC has to be reduced often by as much as half.
  Another thing to be aware of on older 'bikes is that a 6v system will need twice as much current flowing as a 12v system for the same wattage of work (W/V=I) so a 40 watt headlamp bulb on a 12v 'bike will draw about 3.3 amps which rises to about 6.6 amps on a 6v 'bike.
 
You threw me a bit by giving the imperial cable size until I realised it's 32/0.20 cable we're talking about, with a CSA (cross sectional area) of 1mm sq.

  Perfectly fine for all the bits and bobs on a 'bike, but if I was designing/making the harness for a 6v 'bike I'd specify 21/0.30 (CSA 1.5mm) for the headlamp circuit and 28/0.30 (CSA 2.0mm) for the charging circuit. It wouldn't hurt on a 12v bike either.
 I'd say for these two circuits 32/0.20 is marginal.
 
Dave
Peterborough (UK)

Offline Johnny J

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Re: Wiring Loom Rating
« Reply #5 on: 09.12. 2014 23:47 »
If a cable gets warm it's too thin and energy is wasted.
Could also be dangerous and cause a fire.
Cables are not constructed for heat dissipation.
There is no advantage with thin walled cables, we might have better materials nowadays requiring less material for the same protection but normally thicker insulation means better protection from mechanical wear.
   Gothenburg, Sweden

Offline muskrat

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Re: Wiring Loom Rating
« Reply #6 on: 10.12. 2014 07:26 »
Being the dunce in the electrickery class I have to ask. I was told once the sparky stuff travels along the outer surface of a wire and lots of thin wire is better than a few thick ones. Can I take my pointy hat off now and sit back at my desk?
Cheers
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Online RichardL

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Re: Wiring Loom Rating
« Reply #7 on: 10.12. 2014 08:23 »
Muskrat,

Skin Effect is not really an issue at DC and, really, only significant at high frequencies in AC.

About thin-wall insulation, there are a variety of very robust materials used for this which can easily be stronger than thick, soft thermoplastic. About heat getting through the insulation, that's better than fusing the wire, assuming you don't melt the insulation.

Some good info in Sparx's post.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline Topdad

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Re: Wiring Loom Rating
« Reply #8 on: 10.12. 2014 11:16 »
Musky, move over at the desk ! I'll get this ...one day ,Bob
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Offline bikerbob

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Re: Wiring Loom Rating
« Reply #9 on: 10.12. 2014 15:10 »
Thanks for your reply sparx, but I am still a bit confused. The old original wire 16 strands say .009 if you multiply that you get 0.144, if you do the same calculation for the new wire 32strands at .007 you get 0.224. So you have more copper in the new cable which I presume is why it is rated at 16.5amps. Now also why would I need thicker wire for the headlight cable when the headlight is 30w which is only 5amps and the charging circuit thicker again when the dynamo is rated at 8.5amps but I believe I read somewhere that the max rating is 10amps. Surely 16.5 amps would be adequate or am I missing something here I must admit that I am more of a mechanical person  than an electrical  person and in the past when I have fitted anything electrical to my bikes or any new wiring I have usually went to a dealer told him what I was going to do and just bought whatever wire they recommended and it was only when this new harness came and I saw how thin the actual wire looked that I began to have some doubts. The old original harness the ends of the wires when exposed the copper is very discoloured not bright copper like the new cable.

Online bsa-bill

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Re: Wiring Loom Rating
« Reply #10 on: 10.12. 2014 16:36 »
Don't want to muddy the waters too much here as my electricity knowledge is 60 years old now and consisted of school and an apprenticeship that expired after six months due to a location shift ( I also perhaps was a little underwhelmed at my boss's £1 a week value of my good self  *sad2*)
Anyhow what I wanted to contribute was the quality of Copper, the stuff in those original looms was probably of a quality unobtainable these days, my plumber has offerd to get me a stainless steel immersion heater element as I'm on my third copper one now, as he say's "damn stuffs been through the scrappy's to many times".
I can still get my head around the volts - amps - watts stuff tho (it's not entirely the same as gearing - unless you add torque maybe)
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline bikerbob

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Re: Wiring Loom Rating
« Reply #11 on: 10.12. 2014 16:51 »
Just to confuse me  further I went to a vehicle wiring website to look at wire and the wire that sparx recommends for the charging circuit 28/0.30 2mm is rated on that website as 17.5 amps that is only 1amp above what the  new harness consists of.

Offline Johnny J

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Re: Wiring Loom Rating
« Reply #12 on: 10.12. 2014 19:17 »
Just to (hopefully) clearify things a little bit.

There are 4 parameters to consider when chosing a wire for an application.
1. Current (A)
2. Distance
3. Cross area of the leading metal, usually pure copper
4. Insulation type

1-3 are correlated in this way: the higher the current (A) and the longer the distance the wire should cover, the larger the area is required.
The smaller the area, the higher the resistance, =heat/Voltage drop=energy loss. (think lightbulb..)
The longer the wire, the higher the resistance =Voltage drop=energy loss.
4. Insulation type is determined by 3 main factors, Voltage (no concern of ours), surrounding temperatures and mechanical wear (vibrations yes?)

Actually high resistance/heat is something we absolutely don't want in our bikes, that causes voltage drop and dimmer lights.
That's why I will choose 1.5mm2 area wire which will take up to 10A without considerable losses.
1.5mm2 is actually quite a standard size in most households in Europe, but the stiffer type with one or a few strands.

Have built 2 electric cars and been doing some fiddeling with wires. ;-)
   Gothenburg, Sweden

Offline sparx

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Re: Wiring Loom Rating
« Reply #13 on: 10.12. 2014 22:39 »
Bikerbob:-
     That 17.5 amp cable is no doubt the older stuff with a thicker pvc sheath. As a general rule "in the field" we used to halve the number of conductors in 0.30 cable to give the max amperage, so we'd consider 28/0.30 as ok up to about 14 amps. That gave us some leeway to use a rule of thumb guesstimate on the length of run we could use it over. Thinwall 2mm cable is usually rated at about 25 amps, but 2mm thinwall, despite having a higher CCC rating, can still only be used on a run with the same max length as the older cable due to voltage drop considerations. Don't forget about reducing the max current rating when the cable is bundled into a harness.
Johnny J:-
     Couldn't have put it better. I was thinking about voltage drop when I suggested I'd be inclined to use heavier cable for the headlight and charging circuit. Granted we're not talking about a very long run on a bike, but my experience as an auto-electrician these last forty odd years tell me anything smaller would be just plain wrong, even if it is theoretically ok.
   We don't get a lot of choice regarding the sheathing type. It's either pvc or neoprene. PVC cables are not expensive and readily available in a multitude of colours. Runs likely to sustain physical damage are usually sheathed in "Kopex" (trade name) these days, an option not available in the 50's.
 A phrase I've found myself often saying is "if it can't move it can't chaff".
  Neoprene sheathing is expensive and usually obtainable just in black, which requires the cable number to be printed on each cable when used in a harness, but it is less prone to physical damage.
     
     I design and manufacture wiring harnesses for commercial vehicles.
     
Dave
Peterborough (UK)

Offline bikerbob

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Re: Wiring Loom Rating
« Reply #14 on: 11.12. 2014 13:28 »
Thankyou sparx and Johnny j for going into the detail and I will change the wires on the  charging circuit to a higher grade but I think I will leave the headlight circuit as is because it would mean completely stripping the new harness and the headlight is only 5amps.