Thread: 16mm square twin cable?

Originally Posted by drivesafe

Hi bacicat, the 6mm auto cable I supply is 65/.30, 4.59mm2, 75 degree premium AUSTRALIAN MADE automotive cable.

As to the number of strands, this means absolutely nothing as far as current carrying capacity and heat handling goes. The square rating of cable is based on the total amount of copper in a cross sectional area on the cable.

So whether you have a single 4.59mm2 rod bar of copper, or high grade 4.59mm2 audio cable with a few hundred thin strands, the current carrying capacity is the same.

The advantage of more but thinner strands just means the cable is more flexible.

Furthermore, there is no way you could get 32 amps down 5mm Auto cable without having a huge voltage drop and regardless of the tolerable operating temperature, 75 verses 110, the cable will still heat to the same temperature, the higher rating just means it will need to get to a higher temperature before it start to melt.

Your 110 degree cable will be just as hot to handle at 75 degrees as the 75 degree cable will be.

Hi Drivesafe - apologies for my comments if they caused offence, that was not my intention. Your points are correct and valid, but I stand by what I said about putting 50 amps down a 4.5mm2 piece of cable - it should not be an issue - at 12 volts or 240 volts. Yes the voltage drop will be higher at 12 volts, but not 2 to 3 volts per metre. About the tempurature rating, it will get to a similar tempurature, but will live with this constantly & happily throughout its life, and not turn your car into a BBQ...

Also, Pando's comments on flexible 240mm2 cable are not quite correct either, we use 240mm cable in huge quantities at work, and we buy the flexible cable, because as per AS3008, it does have a higher legal current carring capacity than XLPE, added to that, use the 110 degree insulation, and the legal rating goes higher again. Both these together give about a 20% better legal current carrying capacity that standard XLPE.

Just talking current legal ratings here, obviously at 415 volts, the voltage drop issues are completely different.

Cheers - Gav.

Originally Posted by bacicat2000

Hi Drivesafe - apologies for my comments if they caused offence, that was not my intention. Your points are correct and valid, but I stand by what I said about putting 50 amps down a 4.5mm2 piece of cable - it should not be an issue - at 12 volts or 240 volts. Yes the voltage drop will be higher at 12 volts, but not 2 to 3 volts per metre. About the tempurature rating, it will get to a similar tempurature, but will live with this constantly & happily throughout its life, and not turn your car into a BBQ...

Also, Pando's comments on flexible 240mm2 cable are not quite correct either, we use 240mm cable in huge quantities at work, and we buy the flexible cable, because as per AS3008, it does have a higher legal current carring capacity than XLPE, added to that, use the 110 degree insulation, and the legal rating goes higher again. Both these together give about a 20% better legal current carrying capacity that standard XLPE.

Just talking current legal ratings here, obviously at 415 volts, the voltage drop issues are completely different.

Cheers - Gav.

You make no mention of length of the cable.

Legal ratings may be important for domestic and commercial (AC) installations, but don't have much relevance for an auto system, where the runs of cable are short, cable is cheap, and you usually want to minimise voltage drop.

e.g. - 50A has been mentioned a lot.
So 50A at 13.5V - if 10% voltage drop is acceptable on a 4m run of cable (which the OP has) - then you need ~10mm2 cable.

Originally Posted by grover7488

Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable CROSS SECTION in mm.sq.

example:-
6m x 35A x 0.017 / 10mm2 = 0.36v drop

Note: These are calculated using a common ground - chassis

Love this thread, so many opinions! Who is the most right?

According to this formula using 4 metres of Narva `6mm twin core, made in Australia (4.58mm2) cable at 50 amps will have a voltage drop of 1.5 volts. At 10 amps, which is a reasonable load to expect in fridge/lights load then the voltage drop would be 0.3 volts. Run bigger cable by all means but it's not a winch you are running back there.

Now for a real world test, I have about 25 metres of this twin flex left on a roll, so running 7 amps down one strand and back up the other gives a voltage drop of 1.25V in total. So using 4 metres of twin cable at 50 amps would give 4/25 x 50/7 x 1.25 = 1.42V which is close to the figure given by the formula. The TOTAL power loss in the 4 metre run would be 70Watts (P = VI = I2R) so I wouldn't recommend coiling the cable up. It will get warm.

At 10 amps the drop would be around 0.3V as previously noted. Power loss 3 watts, so stone cold.

To go back to the original post, how much continuous current is expected to flow in this cable? You might need that 16mm stuff after all....

Hi all and yes 2 to 3 volts drop was an exaggeration on my part, it was intended to show the problem of going on the going by the current rate marked on the cable and what is REALLY working useful current rate.

I’ll have some time tonight or in the morning and I’ll do a 50 amp load test on some 6mm auto cable.

bee utey, I reckon the voltage drop will be greater than 1.42v but I won’t know for sure till I have done the load test.

Originally Posted by bacicat2000

Hi Drivesafe - apologies for my comments if they caused offence, that was not my intention. Your points are correct and valid, but I stand by what I said about putting 50 amps down a 4.5mm2 piece of cable - it should not be an issue - at 12 volts or 240 volts. Yes the voltage drop will be higher at 12 volts, but not 2 to 3 volts per metre. About the tempurature rating, it will get to a similar tempurature, but will live with this constantly & happily throughout its life, and not turn your car into a BBQ...

Also, Pando's comments on flexible 240mm2 cable are not quite correct either, we use 240mm cable in huge quantities at work, and we buy the flexible cable, because as per AS3008, it does have a higher legal current carring capacity than XLPE, added to that, use the 110 degree insulation, and the legal rating goes higher again. Both these together give about a 20% better legal current carrying capacity that standard XLPE.

Just talking current legal ratings here, obviously at 415 volts, the voltage drop issues are completely different.

Cheers - Gav.

G'day Gav,

Like I said, from memory regarding AS3008, there is a point where flexies cannot carry as much as stranded, thought it was 240's and up but must be in the bigger sizes, sounds like I need a bit of a refresher........

Cheers

Chris

As Bee Utey says - a very interesting thead with lots of lively discussion. I like the idea of some practical testing, and I think I might do some experiments to see if all the theory that is floating around - including mine, is anywhere close to reality. I will try and get some results up this weekend.

Cheers - Gav.

Hi Folks and it got the better of me so I did some testing.

Using 5m of 6mm Auto Twin Sheathed cable running at 14v at the source, at 50 amps there was a 2.82v drop at the load.

Note this cable was in an “open air” environment yet after just 10 minutes the cable temperature was a whooping 60.5 degrees.

Not only would the temp have been much higher had the cable been in an enclosed environment, but because of the higher cable temperature, the resistance would have also increase resulting in an even greater voltage drop.

While I was at it, I tested the voltage drop at 25 amps, again at 14v and the voltage drop was still an unacceptable 1.31v.

I also tested the voltage drop at 10 amps but this time I reduced the source voltage to 13v to simulate more closely to what the voltage would be from a battery rather than for a running alternator.

The voltage drop was exactly 0.5v and this would only be acceptable when running of an auxiliary battery and having the fridge setting at 10.5v

Quite frankly I agree that the 6mm cable 50A rating is optimistic. When I install this stuff I consider a 30 amp fuse is high enough. At 30A the 4m cable would radiate 25 watts instead of 70 odd. The power of squares!

I still stand by my original recommendation for a 10 amp maximum load. I have an old 6 amp Engel and a couple of lights which should be quite happy on this cable. Any more load just super-size it.