7 replies to this topic

[minialf]

im stuck on repairing my loom from the alternator. i have a 998 and i think i hav a standard alternator which would be 45 or 55 amp alternator but i looked at the cable i need to repair which is two browns and a brown/yellow these thicknesses are roughly 2,3 and 4 mm well im not sure what cable thickness to buy becuase i have looked for 4 mm cable but it states it only holds 27.5 amps (standard PVC) and 33 amps (thin wall) i even counted the strands on the wiring and the 2 mm has 19 strands, the 3 mm has 21 strands and the 4 mm had 44 strands?? could any one guide me in the right direction of cable?? i think i am supposed to be using thin wall but im not 100% its a 1987 998. thanx alot.

[Dan]

Cable sizes - for all cable that matters - are not rated in on the diameter of the cable becuase there are so many variables in play that the diameter is largely irrelevant. Cable sizes are based on the total cross sectional area of the copper in the cable because that's what's important. 44 strand is normally 3 sq mm and I would suggest you may have miscounted the others as 19 and 21 strand cable would both be very unusual. 3 sq mm of copper may only be rated to 27.5 amps in regular insulation, but bear in mind that there are two main feed cables at the alternator running in parallel so that's a total of 55 amps capacity. The brown/yellow is not a load carrying cable, it's the signal cable for the alernator warning and allows the alternator to detect battery voltage. 1 sq mm cable would be fine for the brown/yellow. A 1987 998 would not be wired in thinwall as standard, it didn't appear in the harness until the redesign of the wiring halfway through the SPi period. Will you be sticking with a standard alternator?

[bigmatt4]

Cable sizes - for all cable that matters - are not rated in on the diameter of the cable becuase there are so many variables in play that the diameter is largely irrelevant. Cable sizes are based on the total cross sectional area of the copper in the cable because that's what's important. 44 strand is normally 3 sq mm and I would suggest you may have miscounted the others as 19 and 21 strand cable would both be very unusual. 3 sq mm of copper may only be rated to 27.5 amps in regular insulation, but bear in mind that there are two main feed cables at the alternator running in parallel so that's a total of 55 amps capacity. The brown/yellow is not a load carrying cable, it's the signal cable for the alernator warning and allows the alternator to detect battery voltage. 1 sq mm cable would be fine for the brown/yellow. A 1987 998 would not be wired in thinwall as standard, it didn't appear in the harness until the redesign of the wiring halfway through the SPi period. Will you be sticking with a standard alternator?

also to add to that the alternator wil be very rarely putting out full amperage max amps under normal operating conditions will be about 20amps unless your battery is so dead dead flat you jumpstart it then turn on your lights heater fan demister wipers will it pull max amperage so no concerns normally cable is rated continuious rate so will handle 27.5amps forever but will handle higher loads for short time

[minialf]

excellent thanx to the both of you for helping me with this really appreciate the help. will double check strand wires... prob hav counted wrong lol. thanx again for the reply i did find a calcualtion to work out cross section which was...

say that each wire was 0.2mm

0.2 x 0.2 = 0.04
0.04 x 3.14 (pi) = 0.1256
0.1256 x 4 = 0.5024
then if it has 41 strands for example
0.5024 x 41 = 20.598mm2

lol i think this one is a bad example as i have got it wrong but just to show the calcualtion i have found...
if someone does know where im coming from it would help in working out the sizes lol + i dont know why you have to multiple it by 4??

[Dan]

You cannot calculate the cross section of copper from the outer diameter, as I said there are a huge number of variables involved that stop that working. If you can accurately measure the diameter of the strands and find an average, then multiply that by the number of them in the cable you can work it out but you will need a very precise micrometer to measure them. Cables are sold based on their cross section, you don't need to calculate the size if you are buying new cable. Just buy cable of the size you want.

[cooperrodeo]

In your example you need to divide by 4, not multiply. The reason is that you need to square the radius to get cross sectional area, but if you have squared the diameter (as you appear to have done) then you must divide by 4.

[minialf]

yes, this has helped alot lol and now i am getting the right calculations. i triple checked the 19 strand cable and it does have 19 strands then 3mm has 28 and 4 has 44 when i did calcualte the 19 strand wire thou it did come out correct as the end result was 0.5966 so i will use 0.65mm2 cable for that. thanx again for your help on this. now i can actually get the right cable lol.

[Ethel]

The easiest/safest route to sizing the cable would be to calculate the load on it then double it for safety. You won't need to be that precise as you only need to pick the cable size from limited choices

E.g. a light switch supply would be 65w headlight + 5w side + 5w tail + 5w numberplate = 80w x 2 sides = 160 x 2 for safety = 320w

320/12 volts = 26-27amps which will likely be 2 or 3 sq mm cable depending its rating.

I've missed out the dash illum, but it's unlikely it would make a difference @ about 1w/bulb.