7 replies to this topic

[KernowCooper]

When Making any modifications on the Minis Wiring there are important steps to take into consideration, before you start wiring anything up.

You need to find the amps the new component will draw which will determine the correct gauge of wire to be used in any one application and also determine where its feed is taken from.

For example you wish to fit a pair of additional driving lamps, each lamp has a 55w bulb = 110w the pair the formula for finding the amps is

Watts divided Volts = Amps

So in our example Watts = 110w and the volts is a 12v System 110dev12= 9.16amps

Now the cars wiring loom and the headlight circuit won't like another 9.16amps put through it so you run the lights through a relay and the switching is done by the cars headlight circuit in this example.

Some common rules : the smaller the diameter of the wire the higher the resistance and thus lower current capability over a given
length. It is almost always OK to use a larger gauge wire. When in doubt about the load go up a gauge. Other factors can affect the
capacity of wire besides the length, and this includes if it is in a hot environment, duration of load, stranded vs. solid wire, wire plating.

The voltage drop value varies depending on what your application. From what it looks like in many automotive applications a 5% or more voltage drop is more common and acceptable.

As there are AWG (American Wire gauge) And SWG (Standard Wire Gauge) I have listed the conversion to mm below.


For Safety And best results
When the circuit is longer than 10 feet thats the feed into the component and the wire out to earth, add the 2 lengths together to get the total circuit lenght, always use the next larger gauge size, which is the next lower even number.

Length of the Circuit also designates the gauge of wire, the longer the circuit the more the voltage drop is on the wire
Use the following table to check the circuit length across the top range in feet and the amps of the circuit down the left side to select the best gauge wire

Attached Files

•  wire sizes.png   17.61K   328 downloads
•  SWG.png   12.33K   271 downloads
•  awg.gif   12.12K   203 downloads

Edited by mra-minis.co.uk, 01 April 2013 - 12:15 AM.
After discussion with Dave (KernowCooper) I have deleted one chart and added the correct chart for wire size / current capacity

[tiger99]

Beware of using "gauge" as it could be AWG (American Wire Gauge) or SWG (Standard Wire Gauge) or something else entirely. AWG and SWG are not the same, or even close.

It is safest to specify by wire cross-section e.g. 1 mm².

Edit: Thanks, KernowCooper, for taking that observation on board. Your tables are now much more useful.

Edited by tiger99, 28 March 2013 - 09:20 AM.

[KernowCooper]

I forgot were a International Forum - Good Point

[Cooperman]

Well done, Dave. That is just so useful. There are so many threads on here that ask about electrical cable sizing. This needs to be a 'Sticky' methinks.

[MRA]

The CSA (Cross Sectional Area) of round wires is simple to calculate..

Area of each strand multiplied by the number of strands, is the total CSA

(pi*D²)/4 = Area

Work out the answer for those numbers contained within the brackets first.

Pi = 3.142
D = diameter of each strand in (mm)
D²= diameter squared, or simply multiplied by itself.

An example would be

3.142 * 0.26 * 0.26) = 0.2124 (for all you budding mathematicians the units for this part of the answer are also mm², however that is because Pi and the "4" do not have any units)

Divided by 4 = 0.0531mm² (area of one strand)

Now a common wire size has 19 strands, 19*0.0531 = 1mm² total wire CSA.

Edited by mra-minis.co.uk, 27 March 2013 - 11:13 PM.
updated the superscript (2)

[KernowCooper]

Right now say that again Martin, like going back to Tech there mate I have to reread it a few times before I can even understand it let alone comment.

[KernowCooper]

I have updated the post with AWG and SWG conversion to MM to avoid any confusion as lot of wire reels are in MM

[MRA]

To calculate wire size from existing wire or were the information has been lost.

Example a wire's size is designated as 19/0.26 this means it has 19 strands with each strand being 0.26mm in diameter.

Area of a circle is pi*D²/4

Pi = 3.142 and is a constant, it will NOT change ever.
D²= is diameter squared, ie 0.26² is 0.26*0.26 = 0.0676

Pi*D² = 0.2124

0.2124/4 (Pi*D²) divided by 4 = 0.0531

Therefore the area of 1 strand is 0.0531mm²

The total wire (not strand) CSA (Cross Sectional Area) is 0.0531*19 = 1.001mm²

All you have to do is use the equations above and substitute your wire spec i.e. strand count (19 in this example) and strand diameter (0.26mm in this example)

Edited by mra-minis.co.uk, 27 March 2013 - 11:14 PM.