what size core for 21 amps?

[macafee2]

Originally Posted by Avulon

Yes the calculated figure is a peak figure - but isn't far off if you reverse at night with the indicator on - what's missing? - one indicator? By all means fuse at 15A and you'll likely be fine but make sure the cable is capable of handling more than the fuse - a 15A fuse should probably be cabled for 20A minimum. All the other calculation re: voltage drop and cable power loss still stand.

thank you, your previous reply was a bit above me

macafee2

[Avulon]

Originally Posted by Avulon

The insulation has nothing to do with the current capacity of the cable. Unless the cable is heating up in use.


supply Voltage doesn't matter when it comes to current capacity. Only to the insulation rating.


Good calculation - however remember that that current figure is for the current once each bulb is lit and warm. They'll be higher current involved as lamps are turned on, turned off, turned on (indicators for one).


Another thing to consider is voltage drop on the wire - this depends on three things. The supply voltage (nominally 14.2v with engine running), The load (I.e. your calculation above with some allowance for switch on surge), and the resistance of the wire itself. (distance * ohms/metre). The ohms/metre figure will depend on the cross section area of the cable and cable material. Here's a handy voltage drop calculator - I'd aim to have at least 12.6v (from 14.2v) at the end of the cable run - It's still got to go through the trailer wiring!


For the calculator you could use your calculated standing current of 14.0A - once recalculated using the final voltage at the cable end as the voltage source (12.94). Remember that those wattage figures are the 12v wattage - you'll have to first calculate the resisitance of each bulb at those wattages and then recalculate the current using that - should be close enough.


And when complete that calculation comes to - 15.09 A.


Calculating voltage drop over 3m : 16AWG = 8.9% (14.2v give 12.94) Clearly (18AWG) 1.00mm2 is borderline too thin (you'd be losing 25Watts in the cable). Next up is the 2.00mm2 thinwall which will exceed 16AWG so be fine (still losing 16Watts to the cable though). 14AWG? - loses 10.5Watts to the cable. 14AWG = 2.5mm2 cable.

So you'd get away with 16AWG - but 14AWG would be a marked improvement but getting quite thick and difficult to manage. The choice is yours.






PS> using the thicker cable with give a slightly higher voltage at the relay end as well (13.5 from 14.2 at the battery) - better power for your trailer = better rear lighting.

Originally Posted by macafee2

thank you, your previous reply was a bit above me

macafee2

In Short:
1.00mm2 is too thin and will not only waste a lot of power in the cable but result in a low voltage at the relay.
2.00mm2 Is OK
2.5mm2 is Better. - no worthwhile improvement going larger than this.


N.B. Fuse rating and cable rating are two different considerations.

[RoverP480]

Originally Posted by Avulon

The insulation has nothing to do with the current capacity of the cable. Unless the cable is heating up in use.
.

I understand that's exactly the reason and the harder plastic has a higher abrasion resistance . Any resistance will produce heat to a greater or lesser extent. Nearly all automotive harnesses now employ the thin wall cable. I always go for increased cross section cables for any installation of higher current applications.

[macafee2]

Originally Posted by Avulon

In Short:
1.00mm2 is too thin and will not only waste a lot of power in the cable but result in a low voltage at the relay.
2.00mm2 Is OK
2.5mm2 is Better. - no worthwhile improvement going larger than this.


N.B. Fuse rating and cable rating are two different considerations.

not disagreeing with you on size but 2.5 seems really big considering all the other cables in the car.

I thought this was going to be easy, am I just making it complicated?

I appreciate the advice on cable size, whilst I don't think I will exceed
the cable rating I want the cable rating to be able to run all the lights at once which I think is sensible. Speaking with towsure they disagree.

macafee2

[Avulon]

Originally Posted by macafee2

not disagreeing with you on size but 2.5 seems really big considering all the other cables in the car.

I thought this was going to be easy, am I just making it complicated?

I appreciate the advice on cable size, whilst I don't think I will exceed
the cable rating I want the cable rating to be able to run all the lights at once which I think is sensible. Speaking with towsure they disagree.

macafee2

It is simple - once you've calculated your requirements. Remember that 2mm2 is the cross section area of the cable - the diameter of 2mm2 cable is around 1.5mm. A 7 strand cable of this size is quite manageable. the 2.5mm2 cable isn't much thicker.




Your choice....

[stevestrat]

Originally Posted by AndyN01

Try here:

http://www.vehicle-wiring-products.e...hp/48/1/cables

I've used them quite a bit and found them to be a very helpful company.

Good luck.

Andy.

Another vote for Vehicle Wiring Products. I've used them for years since my working on motorbikes days.

[COLVERT]

Originally Posted by macafee2

not disagreeing with you on size but 2.5 seems really big considering all the other cables in the car.

I thought this was going to be easy, am I just making it complicated ?

I appreciate the advice on cable size, whilst I don't think I will exceed
the cable rating I want the cable rating to be able to run all the lights at once which I think is sensible. Speaking with towsure they disagree.

macafee2

YES.-----Fit the 2.5 and stop worrying.---

[stevestrat]

http://www.vehicle-wiring-products.e...2mm-sq-28-0-30