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Old 10th December 2018, 19:41   #1
macafee2
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Default what size core for 21 amps?

Putting in a power supply for a TEB7AS relay for towing electrics, just lights.
Instructions say the feed to the relay should be rated at 21 amps.

What size core do I need for 12v 21 amps?

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Old 10th December 2018, 19:56   #2
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This might help: https://mz147-258aa0.pages.infusionsoft.net/
So, it's saying 2.0 - 2.5mm, which sounds good to me.


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Last edited by T-Cut; 10th December 2018 at 20:02..
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Old 10th December 2018, 21:13   #3
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28/0.30 2mm thinwall hard PVC will take 25 amps , but only 17.5 amps if soft PVC.
https://www.autoelectricsupplies.co....0/category/114
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Old 10th December 2018, 22:17   #4
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Quote:
Originally Posted by RoverP480 View Post
28/0.30 2mm thinwall hard PVC will take 25 amps , but only 17.5 amps if soft PVC.
https://www.autoelectricsupplies.co....0/category/114
I could not work out if soft or hard wall was on offer

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Old 10th December 2018, 22:19   #5
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Quote:
Originally Posted by RoverP480 View Post
28/0.30 2mm thinwall hard PVC will take 25 amps , but only 17.5 amps if soft PVC.
https://www.autoelectricsupplies.co....0/category/114
i'm not sure. the cable that came with a realy a friend gave me is quite thin but also that link is for 240 v, 12 v is different I think as the amperage increases.
To be honest i'm confused

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Old 11th December 2018, 07:43   #6
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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.

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Old 11th December 2018, 08:40   #7
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watts / volts = amps

3 x 21 watt stop lights
4 x 5 watt side lights
2 x 18 watt indicators
1 x 21 watt? reversing light
1 x 21 watt? for light

total 161 watt / 12 volts = 14 amp, well 13 point something
It could be possible for all of those to be on at the same time so I'll go for 17 amp cable.



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Old 11th December 2018, 13:44   #8
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Quote:
Originally Posted by RoverP480 View Post
28/0.30 2mm thinwall hard PVC will take 25 amps , but only 17.5 amps if soft PVC.
https://www.autoelectricsupplies.co....0/category/114



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




Quote:
Originally Posted by macafee2 View Post
i'm not sure. the cable that came with a realy a friend gave me is quite thin but also that link is for 240 v, 12 v is different I think as the amperage increases.
To be honest i'm confused

macafee2



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




Quote:
Originally Posted by macafee2 View Post
watts / volts = amps

3 x 21 watt stop lights
4 x 5 watt side lights
2 x 18 watt indicators
1 x 21 watt? reversing light
1 x 21 watt? for light

total 161 watt / 12 volts = 14 amp, well 13 point something
It could be possible for all of those to be on at the same time so I'll go for 17 amp cable.



macafee2

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.
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Last edited by Avulon; 11th December 2018 at 13:49..
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Old 11th December 2018, 14:14   #9
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spoke to towsure as they recommend fusing the relay and thus trailer lights at 15 amp.
Their argument was no-one will sit there with all the lights on that I mention. Probably unlikely but it could happen.

The relay is years old and has probably sold in it's millions so why am I querying the fuse rating?

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Old 11th December 2018, 14:52   #10
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Quote:
Originally Posted by macafee2 View Post
spoke to towsure as they recommend fusing the relay and thus trailer lights at 15 amp.
Their argument was no-one will sit there with all the lights on that I mention. Probably unlikely but it could happen.

The relay is years old and has probably sold in it's millions so why am I querying the fuse rating?

macafee2

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.
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Need a T4 ?: T4 Owners Map thanks to Stevestrat ( use at your own risk)

Where?:
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Mods/Retrofits:

PCV vortex 'filter'; bluetooth; inline thermostat; reversing sensors; plenum spyhole ; headlamp washers ; Diy mp3 player replacing CD multichanger; FBH with remote; Headlamp washers; black/chrome front grille, rear blind; Xenon projectors
To do:
puddle lights; 2 Din cd/nav to fit; boot release button
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