Technical Electrical wire thickness

[TpT]

Hi all,

I have a '67 500f with alternator. The project didn't have any electric wires to begin with, and I bought a new wiring loom for it. The wire going from the regulator (in original generator setup) to fuse box, and from there on to ignition lock seem thin to me. How thick should these wires be? Preferably in mm².

Thanks in advance!

 

[fiat500]

Hi all,

I have a '67 500f with alternator. The project didn't have any electric wires to begin with, and I bought a new wiring loom for it. The wire going from the regulator (in original generator setup) to fuse box, and from there on to ignition lock seem thin to me. How thick should these wires be? Preferably in mm².

Thanks in advance!

I expect that although no-one has answered this query so far, my previously proven lack of technical training in this precision subject will prompt someone to reply.

What I can give you is this link to a previous discussion started by me in which the consensus seemed to settle at 5 or 6mm2 cable being adequate. Bear in mind that once the output from the alternator reaches the post on the starter it will use the existing cable back to the main post on the battery....no worries about under-sizing there!

https://www.fiatforum.com/500-class...round-engine-cowling.html?446087=#post4248637

 

[Vitesse]

It doesn’t need to be too large as it’s effectively a spur of 12v from the main battery feed from terminal 30 on the voltage regulator. Which in turn supplies 12v to the main fuse box terminal 30. From the same terminal on the fuse box, not via a fuse, it goes to the ignition switch as the 12v supply. When the key is turned it supplies 12v back down the blue wire to the + terminal on the coil. The primary coil resistance is in the region of 3Ω, so the current draw is going to be about 4 amps when the points close.

Therefore the wire only needs to be large enough to handle 4 amps. Ideally you would want 1mm squared, which has a diameter of 2.6mm, this will handle about 8 amps.

 

[fiat500]

It doesn’t need to be too large as it’s effectively a spur of 12v from the main battery feed from terminal 30 on the voltage regulator. Which in turn supplies 12v to the main fuse box terminal 30. From the same terminal on the fuse box, not via a fuse, it goes to the ignition switch as the 12v supply. When the key is turned it supplies 12v back down the blue wire to the + terminal on the coil. The primary coil resistance is in the region of 3Ω, so the current draw is going to be about 4 amps when the points close.

Therefore the wire only needs to be large enough to handle 4 amps. Ideally you would want 1mm squared, which has a diameter of 2.6mm, this will handle about 8 amps.

....as my teachers used to say to me, "Read the question first Peter!" [emoji3]
I know where TpT is coming from though. I think that I studied an early wiring diagram where that main fusebox feed came directly from the battery across the front of the bulkhead.. The situation where a relatively small gauge wire travels from the back to the front of the car and at times carries the load for every lamp in the car seems a bit of dodgy design to a layman like me.
I don't worry now that I've modified it so the heaviest use (to the headlights) comes more directly from the battery via relays.
What would be the total possible amperage of the lights and wipers and hence the cross-sectional area of wire that would need?

 

[TpT]

....as my teachers used to say to me, "Read the question first Peter!" [emoji3]
I know where @TpT is coming from though. I think that I studied an early wiring diagram where that main fusebox feed came directly from the battery across the front of the bulkhead.. The situation where a relatively small gauge wire travels from the back to the front of the car and at times carries the load for every lamp in the car seems a bit of dodgy design to a layman like me.
I don't worry now that I've modified it so the heaviest use (to the headlights) comes more directly from the battery via relays.
What would be the total possible amperage of the lights and wipers and hence the cross-sectional area of wire that would need?

Thank you both for your answers.
As you mentioned Peter, the wire from back to front, between terminals 30 in regulator and fuse box, is the only wire that is feeding fusebox, so all the load (except starting and loading the battery) goes through that wire. Or am I missing something?
None of wiring diagrams that I have seems to have direct feed from battery to fuse box.

 

[Vitesse]

I stand corrected. I have just looked at the wiring diagram again for the F and it is indeed the single feed for the fuse box. There are subtle differences between the different models diagrams. The only one I know of that is specifically for the F is on the Axel Gerstl website but it’s in German.

When you look at a 55W headlight lamp that is going to be drawing 4.5 amps, so x 2 = 9 amps.

You have to bear in mind that not everything is going to be on all the time. Indicators only two at a time for short periods, brake lights only when braking, horn as and when required, wipers potentially for longer periods of time, engine obviously running.

Looking at the red wire on my car as per the photo below it is slightly larger gauge than the rest, so it is likely to be 2 or 3mm squared? 3mm squaredhas a diameter of 3.7mm which can handle 27.5 amps or it could possibly be 2mm squared, diameter 3.3mm which can handle 17.5 amps.

 

[fiat500]

I stand corrected. I have just looked at the wiring diagram again for the F and it is indeed the single feed for the fuse box. There are subtle differences between the different models diagrams. The only one I know of that is specifically for the F is on the Axel Gerstl website but it’s in German.

When you look at a 55W headlight lamp that is going to be drawing 4.5 amps, so x 2 = 9 amps.

You have to bear in mind that not everything is going to be on all the time. Indicators only two at a time for short periods, brake lights only when braking, horn as and when required, wipers potentially for longer periods of time, engine obviously running.

Looking at the red wire on my car as per the photo below it is slightly larger gauge than the rest, so it is likely to be 2 or 3mm squared? 3mm squaredhas a diameter of 3.7mm which can handle 27.5 amps or it could possibly be 2mm squared, diameter 3.3mm which can handle 17.5 amps.

No correction intended or implied by me... I'm the least bright spark round here.[emoji3]
The attached image is from my 1964 edition of Glenn's, which is for the US version of the 500 and the primitive one at that! But it shows a (presumably) thicker direct, red feed from battery to terminal 30 on the fusebox. My replacement positive battery cable came equipped with an integral, spaded terminal on a short length of cable which looks sufficiently heavy to serve that purpose.
...just spotted, a diagram which I have for the Giardiniera shows it wired that way too. I think it's identical to the one pictured.

 

[Vitesse]

No correction intended or implied by me... I'm the least bright spark round here.[emoji3]

I realise that That could be the answer to either of those replies.:devil:

I can just about see that diagram and can see that wire direct from the battery to the fuse box. It is odd that they decided to go via the voltage regulator and then onto the fuse box.

You’ve got me started now! When you look at the individual fuses, all 8 amp.

Fuse 1 has main beam right 45W, left front parking light with indicator 25W, right rear parking light 5W. Total max current draw is 6.25 amps

Fuse 2 has the same but opposite sides. Same max current draw as above.

Fuse 3 & 4 are headlamp low beam 40W. One for each side. Max current draw is 3.33 amps each side.

Fuse 5 is remaining indicators with pilot lights, instrument lights, stop lights.

Fuse 6 is horn, wipers, rear view mirror interior lights.

Fuse 1 and 2 are obviously allocated these different loads, as the main beam isnt expected to be on for long periods of time, therefore they have stuck some additional loads on.

3 & 4 are higher loads but they are expected to be on for longer periods of time. Therefore they have each been allocated a single fuse each.

5 & 6 are again things that aren’t particularly high loads or on for prolonged periods of time.

Unprotected circuits are charging, ignition and starter.

If the ignition circuit is drawing 4 amps, there is potential if you have both dipped beams onto draw an additional 6.6 amps, parking lights another 1.66 amps maybe wipers on, I don’t know the power rating of the wiper motor but I doubt whether it draws more than 4 amps max. When those are all added up it comes to 16.2 amps. Everything else is used occasionally. Seeing as the voltage regulator also regulates the current at 16 amps and also the dynamo max continuous output is 16 amps. I would guess that the average current draw is below 16 amps. So 2 or 3mm squared wire would seem correct.

It’s taken me ages for me to work this out, so if it doesn’t make sense then I apologies.

 

[fiat500]

That's similar to the logic I took in sizing the output cable from the alternator. It's rated at a maximum of 33A and my cable provider (Halfords! . ) biggest size is 27A. That's a pretty chunky cable and I'm unlikely ever to have the full current demand, so that's what I used.