Guide to Electrical Wire Thickness





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  1. #1
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    Default Guide to Electrical Wire Thickness

    We all know that certain applications draw a lot of current and thicker wire should be used than other applications

    Can some of the electricians amont us (Albert - asb ) give us a guide to what wire thickness should be used for various current? Also, how much should you increase your thickness with for long wires

    Sal 10A application - i.e. single 100W spotlight - x mm wire if less than 1m, y mm wire if less than 2m, z mm wire if less than 5m

    Some sort of guide like this will be very usefull for guys fitting spotlights, dual battery systems, powerpoints int he rear of the vehicle, etc.
    Christo Davids
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    If I remember correctly, there is a formula in Andrew St. Pierre White's 4x4 book (Complete Guide to 4 wheel drive in Southern Africa) that gives wire size based on current drawn, length and maximum allowable voltage drop.

    If no reply, I can have a look and post here tonight.

    Regards
    Hein

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    Interessante fred Christo sal die ene dophou om te sien wat hier is. Ek vat so met drade soos oa 2 de battery wat is rerig geskik en wat nie!
    "Ability is what gives you the opportunity belief is what gets you there"

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    SWAMBO
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    A very rough guide will be 10amp for every 1 square mm. Of cause the lenth of the wire has a big influance on the wire size. If the wire lenth is less than 2 meters the above guide can be used, but remember it is only a rough guide. To get the correct wire size you will have to take into account the lenth, the current and the voltage. The higher the voltage the less voltage drop you will experiense for the same current, same wire thickness and lenth.
    For a second battery in the back of a double cab you will have to use at least a 16 suqare mm wire preferbly a 25square mm. For spot lights (100w 12v) you can use 1.5mm wire. but you must run it via a relay to avoid damage to your switch. In all cases make sure that the wire you install are protected with a suitable fuse. NEVER INSTALL A WIRE FROM THE BATTERY WITHOUT A FUSE!!!!!
    Andre van Rensburg<br>Swambos baby V6 Pajero shorty<br>My overland vehicle "Datsun" 3.3l v6 hardbody d/cab.<br>Play vehicle Sizuki SJ413. Mods: Toyota 2c Diesel engin and 5 speed gearbox, 50mm lift<br>Love the bush, overlanding and crazy about animals.

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    I have looked at the reference mentioned in my previous post, with no success. However, online I found the following calculator:

    http://www.kilowatts.com.au/calculator-voltage-drop.php

    The maximum allowable drop is expressed as a percentage (i.e. 1% on a 12V system will be 0.12V). For example, if 10A will be drawn over 4m cable length with a maximum drop of 0.12V (or 1%), you will need a 16mmsq cable area (minimum).

    Regards
    Hein

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    Playing with 220v is one thing but the lower voltage is more susceptible to voltage drop and increased amperage draw to compensate for that. Just remember watts = volt X amp if you drop voltage the appliance would compensate by drawing more amps resulting in a further drop in voltage .........
    If you calculate that you need a 1.5mm cable rather go one bigger and avoid trouble. 12v is not forgiving in that sense.
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    Christo,
    Die maklikste om te dit te bereken is soos hbannink bo gese het.

    Watt = Volt x Amp

    Dus:

    Amp = Watt / Volt

    In jou geval... 2 spotlights van 100Watt elk sal wees...

    200W / 12V = 16,67 Volt

    Op koper kabel kan jy die volgende gebruik:

    1.5mm2 kabel - Max 15 Amp
    2.5mm2 kabel - Max 25 Amp
    4.0mm2 kabel - Max 30 Amp
    6.0mm2 kabel - Max 40 Amp
    10mm2 kabel - Max 60 Amp
    16mm2 kabel - Max 80Amp

    So vir daai 2 ligte sal ek 2.5mm kabel gebruik met 'n 20 Amp fuse.
    Albert Gravett
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    Ek sien die nuwe spots is nou 130w!!
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    How can I identify the thickness of wire already in use on a piece of kit? My tyre compressor is mounted in the engine bay, but up to now I've always kept it disconnected and just connected it with its own wire and crocodile clips to the battery when I needed to use it. I would now like to wire it directly to the battery with a relay and switch, but I'm not sure if it's original wire would be OK for that purpose. There is already an in-line fuse in the wire iirc.

    I've seen images of some kind of disc with holes for measuring the various wire thicknesses, which I suppose is available at electric hardware shops, but would it be accurate enough?

  11. #11
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    Default Wire

    Hi Lunar,

    The table although correct, may be even a little optomistic.

    It is fine for 220V where a volt drop of say 1V will give you 219V.
    But at 12V the same current drawn will give you 11V - a much bigger difference percentage wise.

    Assuming the following.

    12 Volts

    100W bulb

    2 meters of 2.5 mm2 cable

    Then

    Bulb resistance will be 1.44 ohms
    Current drawn will be 8.333 amps

    Add 0.13 ohms for the cable and you get a new load resistance of 1.57 ohms

    Working backwards this means that your 100W bulb will give you 91 Watts of illumination. This is due to the 0.99V drop and 9 Watts lost in the cable.

    If the same cable feeds both lamps then you will get 84 Watts from each lamp.

    2.5mm2 will work but I would go for 4mm2

    With 4mm2 you will get 95 Watts in the above example.

    Other conciderations - you get wire and you get wire. A good quality 2.5mm2 wire will be 2.5mm2. El cheepo can anything.

    Cheers
    Fluffy

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    To add a bit more fuel to the this fire ....

    ZS6HDV is SPOT ON with his comments regarding VOLTAGE DROP.

    The cable to the second battery is the most critical in this hole setup ! The switch gear, connectors AND cable should have as little voltage drop as possible. This relates to the high voltage required at the second battery to ensure MAXIMUM CHARGING. IF the voltage drop is such that the second battery only see 12,8V instead of the typical 13,2 V required .... well the battery will not charge properly.


    For the rest of the items, voltage drop may impact on the performance, but mostly the cable needs to cope with the current.

    Lastly, the duration of the current draw should also be considered. The voltage drop in the cable causes the cable to heat up. In a well selected cable this is negligible, but a borderline cable operating a full tilt for hours can very easily build up heat causing more and more losses ......

    So different cable sizes for different purposes - in theory.

    In practice it seams the 16mm2 cable is very popular.

    Would like to hear from those have practical experience with different setups.

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    The table given by lunar is correct as far as carrying capacity is concerned but that is under ideal circumstances. The voltage drop depends on the type of wire as well as the length of the wire. A solid strand of say 1mm has less voltage drop than a fine stranded wire of the same size. the physical size will be different with the stranded wire being much thicker but the overall conductor size will be the same. Wire used in the automotive industry is of a very fine stranded type to accommodate the vibration that the wire will be subjected to but the drawback of this is a higher voltage drop than a thicker stranded wire.

    Christo's spots will require 16a to run and in theory a 2.5 should carry this without problem as long as the run is not too long. If it were my spots I would feed each spot with its own line from the relay and not loop from one to the next as I consider this a high current drain.

    In the high voltage side (220v) I am allowed a drop of 5% that gives me 11v, if you apply this same limit to the 12v side you are allowed only 0,6 v drop, this is a slightly different picture. I have never heard of problems arising from using too thick wires but have sen cars burnt out due to undersized wiring.

    For the slight extra cost involved rather opt for oversized cables

    Henk
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    Quote Originally Posted by stadleroux View Post
    How can I identify the thickness of wire already in use on a piece of kit? My tyre compressor is mounted in the engine bay, but up to now I've always kept it disconnected and just connected it with its own wire and crocodile clips to the battery when I needed to use it. I would now like to wire it directly to the battery with a relay and switch, but I'm not sure if it's original wire would be OK for that purpose. There is already an in-line fuse in the wire iirc.

    I've seen images of some kind of disc with holes for measuring the various wire thicknesses, which I suppose is available at electric hardware shops, but would it be accurate enough?
    Hello Stadler, with some compressors there is a heat issue with it installed in the engine bay because of the extreme heat generated by the engine, the overload heat generated overload kicks in to soon and you have to wait for the thing to cool down.

    I installed mine in the back of my D/C bak and ran a 2x10mm wires direct from the battery to one of those fancy grey plugs. I fitted a fuse in the wire near the battery. I shortened the leads of the compressor and fitted the other end of this grey type plug. I use the compressor when the engine is running as i get more power out of it. It runs more efficiently now. I disconnect the plug on the compressor when not in use.

    I know it might be different in the Jeep at the back for a compressor to be installed, but i know you will come up with something wise

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  15. #15
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    Ian, yes, I was also a little concerned about the heat in the engine bay, but the compressor is supposed to be suitable for being mounted in the engine bay, according to the manufacturer. The maximum operating temperature is 60 C. I'm sure that's not very far from the usual temperature under the closed bonnet when the engine has been running for a while, but with the Wrangler's engine sitting relatively high between the fenders the worst heat is ventilated away pretty quickly when the bonnet is left open, especially with the viscous fan working most of the time.

    I forgot to stop my in-car video camera when coming out of Atlantis and stopping to reinflate in the parking lot once, so I now know that it takes me roughly 12 minutes, give or take half a minute or so, to reinflate my tyres. The maximum duty cycle of the compressor is 40 minutes, but yes, that's at 24 C ...

    Your method of connecting/disconnecting your compressor to the battery has set me thinking though, and I'll investigate those plugs. Thanks!

  16. #16
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    This is dangerous but here goes, Christo use with caution and rather err on the bigger size. The size might not be the exact size available at the local midas but it is as close as damnit to swearing to correct as per formula

    Amps
    @ 12 Volts LENGTH OF WIRE
    Cross Sectional Area
    (mm2)
    1m 1.5m 2m 3m 4.5m 7m 7.5m
    0 to 1 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    1.5 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    2 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    3 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    4 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    5 0.82 0.82 0.82 0.82 0.82 0.82 0.82
    6 0.82 0.82 0.82 0.82 0.82 0.82 1.31
    7 0.82 0.82 0.82 0.82 0.82 0.82 1.31
    8 0.82 0.82 0.82 0.82 0.82 1.31 1.31
    10 0.82 0.82 0.82 0.82 1.31 1.31 2.08
    11 0.82 0.82 0.82 0.82 1.31 1.31 2.08
    12 0.82 0.82 0.82 0.82 1.31 1.31 2.08
    15 0.82 0.82 0.82 0.82 2.08 2.08 3.31
    18 0.82 0.82 1.31 1.31 2.08 2.08 3.31
    20 0.82 0.82 1.31 1.31 3.31 3.31 5.26
    22 0.82 0.82 1.31 2.08 3.31 3.31 5.26
    24 0.82 0.82 1.31 2.08 3.31 3.31 5.26
    30 0.82 2.08 2.08 3.31 5.26 5.26 5.26
    36 1.31 2.08 3.31 3.31 5.26 5.26 5.26
    40 1.31 2.08 3.31 3.31 5.26 5.26 8.36
    50 1.31 2.08 3.31 5.26 5.26 5.26 8.36
    100 3.31 3.31 5.26 5.26 13.29 13.29 21.14
    150 5.26 5.26 8.36 8.36 21.14 21.14 33.61
    200 5.26 8.36 8.36 13.29 21.14 21.14 33.61
    Attached Images Attached Images
    Last edited by hbannink; 2009/08/28 at 01:19 AM.
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