Car battery cable size
Discussion
What cable size would you recommend? I'm going to move the battery from the front back to the boot some 3 - 4 metres away, but unsure what thickness or rating I need. Car has a 2 litre engine and I think maybe 170A 25mmsq 10.5 OD maybe? Other options are:
345A, 50mmsq, 13.5mm OD or
415A 60mmsq, 14.3 OD or even bigger..
345A, 50mmsq, 13.5mm OD or
415A 60mmsq, 14.3 OD or even bigger..
Useful calculator for voltage drop here: https://www.rapidtables.com/calc/wire/voltage-drop...
Krikkit said:
Useful calculator for voltage drop here: https://www.rapidtables.com/calc/wire/voltage-drop...
NiceHere's another with good info that gives different results https://www.12voltplanet.co.uk/cable-sizing-select...
Evoluzione said:
What cable size would you recommend? I'm going to move the battery from the front back to the boot some 3 - 4 metres away, but unsure what thickness or rating I need. Car has a 2 litre engine and I think maybe 170A 25mmsq 10.5 OD maybe?
I'd suggest you go a lot bigger. IMO 100mmsq would be reasonable for that length. Within any practical limits there's no great disadvantage to going too big, but go too small and all your starting problems will be aggravated by the extra voltage drop.
Good luck with using 95-100mm sq cable, that'll be about 20mm diameter, massively oversized for the job....... imho.
Moved the battery to the boot on my car, TVR S3 with 2.9 V6, like yours somewhere between 3 or 4 metre run. Used some 35mm cable I had available and works fine.
If I'd had to buy some cable would maybe have gone as big as 50mm for "belts and braces" job
Thanks for all the replies. Yes indeed there does seem to be a huge discrepancy between what people say I should use and what people actually use, 95 mmsq is getting on for the size of a babies arm and there is a downside to it - buying it, routing it and it weighing the car down.
Look for instance here:
http://www.autospeed.com/cms/article.html?&tit...
Scroll to the bottom for the bits we want. The chap that writes that is usually very thorough and he uses 7mm, I was thinking of using 10.5.
The question is this, if I doubled it up and ran 2 x 10.5mm would it act like 21mm? (or 2 x 25mmsq acting like 50mmsq) ?
Look for instance here:
http://www.autospeed.com/cms/article.html?&tit...
Scroll to the bottom for the bits we want. The chap that writes that is usually very thorough and he uses 7mm, I was thinking of using 10.5.
The question is this, if I doubled it up and ran 2 x 10.5mm would it act like 21mm? (or 2 x 25mmsq acting like 50mmsq) ?
Evoluzione said:
Thanks for all the replies. Yes indeed there does seem to be a huge discrepancy between what people say I should use and what people actually use, 95 mmsq is getting on for the size of a babies arm and there is a downside to it - buying it, routing it and it weighing the car down.
Look for instance here:
http://www.autospeed.com/cms/article.html?&tit...
Scroll to the bottom for the bits we want. The chap that writes that is usually very thorough and he uses 7mm, I was thinking of using 10.5.
The question is this, if I doubled it up and ran 2 x 10.5mm would it act like 21mm? (or 2 x 25mmsq acting like 50mmsq) ?
That installation seems to have been designed for a peak load of about 100A and the design doesn't address voltage drop at all. All the references to welding cable current capacity also ignore voltage drop and are only concerned with overheating the cable itself. All the numbers you're suggesting look very small to me. To answer your specific question about 7mm versus 10mm or 20mm, you're ignoring the fact that cross section area is proportional to diameter squared. You need four 5mm dia conductors to have the same area as a 10mm dia conductors, and four 10mm dia conductors to have the same area as a 20mm dia conductors.Look for instance here:
http://www.autospeed.com/cms/article.html?&tit...
Scroll to the bottom for the bits we want. The chap that writes that is usually very thorough and he uses 7mm, I was thinking of using 10.5.
The question is this, if I doubled it up and ran 2 x 10.5mm would it act like 21mm? (or 2 x 25mmsq acting like 50mmsq) ?
GreenV8S said:
That installation seems to have been designed for a peak load of about 100A and the design doesn't address voltage drop at all. All the references to welding cable current capacity also ignore voltage drop and are only concerned with overheating the cable itself. All the numbers you're suggesting look very small to me. To answer your specific question about 7mm versus 10mm or 20mm, you're ignoring the fact that cross section area is proportional to diameter squared. You need four 5mm dia conductors to have the same area as a 10mm dia conductors, and four 10mm dia conductors to have the same area as a 20mm dia conductors.
You best read it again.....The f
king idiot fitted a 75 Amp circuit breaker by the battery???????????????????????????????The mind bogglesphillpot said:
Good luck with using 95-100mm sq cable, that'll be about 20mm diameter, massively oversized for the job....... imho.
It's pretty close to what I use on mine. The conductor diameter works out to be about 12mm with an overall cable diameter around 15mm. Not massively more than you'll see on typical installations with the battery up front. Not especially big or expensive or difficult to work with. The cable I used was nominally rated for 500A, which seems like massive overkill, but the overall resistance is what matters and this was right for me based on the expected peak current draw and cable length.
Going over the top is relatively harmless - even quite a big increase in csa only increases the cable dia slightly. For reference the 20mm dia you mention would give you about 300mm2 csa. Yes, I agree that's overkill.
100mm2!!! I have done quite a few conversions over the years which have involved relocating batteries to the boot area. Never have I ever had a problem using cable sizes between 40 and 60mm2, granted these are all no bigger than 2 litre engines some turbo others high compression 16v engines. Take a look at cable sizes on some jap cars, you will be surprised how small they are. A lot of standard cables are normally no bigger than 25 to 35mm2 on runs with battery in the engine bay.
You talk about voltage drop, you could probably lose more voltage by using cheap crap terminals on the end of the cable than the size of the cable itself.
You talk about voltage drop, you could probably lose more voltage by using cheap crap terminals on the end of the cable than the size of the cable itself.
Not measured any voltages as in the installations over the length, a 0.2 to 0.4v drop was deemed to not be an issue when a rested battery is showing 12.5v +. So even worst case you still have 12v + going to rest of electronics.
Just out of curiosity I have checked on my C Class 3.0 diesel that has the battery in the boot. Cable diameter was measured at 17.5mm so even if it had 1mm insulation that's a 15.5mm conductor, this was protected by a 350A fuse at the front end
DC clamp meter showed a peak current of 360A starting from cold. That's a big engine to turn and it's currently about 6 degrees up here.
My logic in sizing the cables I have used previously voltage drop never factored, but doubling the CSA of the original cables certainly hasn't caused any harm and looking at the other replies most others seem to use a similar theory. As I have said, shody terminals probably cause more issues with voltage drop than the CSA.
Just out of curiosity I have checked on my C Class 3.0 diesel that has the battery in the boot. Cable diameter was measured at 17.5mm so even if it had 1mm insulation that's a 15.5mm conductor, this was protected by a 350A fuse at the front end
DC clamp meter showed a peak current of 360A starting from cold. That's a big engine to turn and it's currently about 6 degrees up here.
My logic in sizing the cables I have used previously voltage drop never factored, but doubling the CSA of the original cables certainly hasn't caused any harm and looking at the other replies most others seem to use a similar theory. As I have said, shody terminals probably cause more issues with voltage drop than the CSA.
When you pick up 6m of 100mm2 cable and buckle under the weight you might change your mind; the mind boggles at where you'd run a cable that size, it will be hugely stiff and not want to go round corners. I've moved a few batteries to the rear of cars using 35mm2 welding cable as it was the cheapest available option. Starting was never a problem.
I think we can conclude that anything sized smaller than ship rope is going to cause your car to burst into a ball of flames and won't start, funny how the majority of people who have actually done it (I will say I think 35mm2 is insufficient, but that depends on the engine) have had no issues 
This is a 95mm crimp on lug near as damn it life size (hole is 10mm dia.), it's about 60 mm long!
Have fun getting that onto your starter solenoid...............
ETA... life size on my computer screen, possibly a little under sized (like the cable on my car that gives me no issues) if viewed on a phone
Have fun getting that onto your starter solenoid...............
ETA... life size on my computer screen, possibly a little under sized (like the cable on my car that gives me no issues) if viewed on a phone
Edited by phillpot on Sunday 28th October 17:00
phillpot said:
This is a 95mm crimp on lug near as damn it life size (hole is 10mm dia.), it's about 60 mm long!
Have fun getting that onto your starter solenoid...............
ETA... life size on my computer screen, possibly a little under sized (like the cable on my car that gives me no issues) if viewed on a phone
Despite the size of the cable you are unlikely to use it direct to starter motor, I would imagine it would go to a junction box and split to smaller cables for various uses around the system.Have fun getting that onto your starter solenoid...............
ETA... life size on my computer screen, possibly a little under sized (like the cable on my car that gives me no issues) if viewed on a phone
Edited by phillpot on Sunday 28th October 17:00
Looking around the interweb, there appears to be a lot of people not using 100mm2 cable for their boot battery conversions. This may be due to weight, cost, ease of use, or simply not required. I will say however you should always upsize and never extend at the same CSA and if your original cable was rated at 200A consider something that will take twice that.
At the end of the day we are assuming (and we should never assume as you can make an ass out of u and me) that the OP's application requires maximum currents and minimum voltage drop. Those that have responded with experience of smaller cable diameters are being called dodgy and are of shody workmanship without looking at the requirements of each application. Something running carbs and dizzy will worry less about voltage drop than something running a high tech ecu and ignition system.
I have never had a problem with my conversions and it seems very few have had issues using their choice of cable. Thats not to say no one has and I can imagine those in the business of fixing others issues probably see more shody workmanship than the likes of me and you.
I got better things to do than argue on the internet so I suggest if the OP is unsure how to tackle it he goes to someone more experienced in the conversion and pay them to make the decisions.
GreenV8S said:
Evoluzione said:
What cable size would you recommend? I'm going to move the battery from the front back to the boot some 3 - 4 metres away, but unsure what thickness or rating I need. Car has a 2 litre engine and I think maybe 170A 25mmsq 10.5 OD maybe?
I'd suggest you go a lot bigger. IMO 100mmsq would be reasonable for that length. Within any practical limits there's no great disadvantage to going too big, but go too small and all your starting problems will be aggravated by the extra voltage drop.
I had a "racing" harness initially on the mojo. It worked but the drain on the battery was considerable...In the end I was able to get cables from a 48kVA GPU made off and fitted. Completely eliminated that issue. And
on to the next one...Edited by Kccv23highliftcam on Sunday 28th October 20:58
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff