# Forum Home Renovation Electrical  how do you measure wire thickness? e.g. 2.5mm etc..

## wozzzzza

you walk into an electrical store and ask for 2.5mm twin and earth wire, or 8mm building wire etc..., how do you actually workout what you are given if its correct? how is this wire thickness figure determined??

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## thatirwinfella

rather then being designated 2.5mm or 6mm etc, it's actually 2.5mm^2 or 6mm^2. It's the cross sectional area of the conductor.

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## Vernonv

As thatirwinfella said, the 2.5 is the cross sectional area. So to work out the diameter: 
Diam = (Sqrt (CSA / Pi)) x 2. 
Note that the diameter of stranded wire will be somewhat larger than that (and is more difficult to measure accurately). 
If you know the manufacturer of the wire you could go to their web site as often they have external diameters/sizes (i.e. the outside of the cable including insulation) specified.

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## Claw Hama

It's the square of the diameter or radius I can't remember which one too long since tafe.

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## Claw Hama

:Wat they said:

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## nev25

Experience

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## familyguy

At the risk of getting flamed; a good ballpark figure for sizing the wire is 5amps per square mm, so 6mm cable is good for about 30 amps and 2.5mm cable is good for about 12.5 amps. You may need to increase wire size for long runs as voltage drop will come into it. Voltage drop in a cable will increase as the current through the cable increases, if you're running 5 amps through a cable and the load increases to 10 amps the voltage drop across the cable will increase.

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## nev25

> At the risk of getting flamed; a good ballpark figure for sizing the wire is 5amps per square mm, so 6mm cable is good for about 30 amps and 2.5mm cable is good for about 12.5 amps. .

  Way off

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## Claw Hama

There was a thread the other day about why you should use licenced trades people!!

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## familyguy

Just as predicted... perhaps you'd care to elaborate.

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## Pulse

Or just check AS3008.11 Electrical Installations - Selection of Cabling 
My copy has 101 pages on finding the current carrying capacity of different wires... 
Cheers Pulse

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## nev25

Pulse is right for working out Current carrying capacity 
But the original question was   

> you walk into an electrical store and ask for 2.5mm twin and earth wire, or 8mm building wire etc..., how do you actually workout what you are given if its correct? how is this wire thickness figure determined??

  The answer is EXPERIENCE or its written on the cable drum.
There is one manufacturer that prints the size on the Sheath but not sure what one.

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## familyguy

I was flamed very quickly, but.... I'm still waiting for someone to elaborate. The replies so far amount to something akin to 'i know a secret but i won't tell'. I may be just a dumb electronic technician and may have erred on the safe side but I do know my theory.

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## nev25

> I was flamed very quickly, but.... I'm still waiting for someone to elaborate. .

  
Whats to elaborate read the above responses

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## Ausyuppy

> At the risk of getting flamed; a good ballpark figure for sizing the wire is 5amps per square mm, so 6mm cable is good for about 30 amps and 2.5mm cable is good for about 12.5 amps. You may need to increase wire size for long runs as voltage drop will come into it. Voltage drop in a cable will increase as the current through the cable increases, if you're running 5 amps through a cable and the load increases to 10 amps the voltage drop across the cable will increase.

  Family guy,
As mentioned above, AS3008 will give you the current carrying capacities of all sorts of conducters in various conditions (direct buried, in open air etc) 
The current standard (AS3000) allows unlimited GPO's on a 20A CB using 2.5mm cable, and lighting ccts run with 16A breakers with 1.5mm cable. So without opening AS3008 the current carrying capacity would be greater, but not by a hell of a lot. 
Wozza, 
If your not sure about whether you were given 2.5mm you only have to look at 1.5mm to see the difference. 4mm cable tends to get quite stiff. 
Cheers
Steve

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## elkangorito

> you walk into an electrical store and ask for 2.5mm twin and earth wire, or 8mm building wire etc..., how do you actually workout what you are given if its correct? how is this wire thickness figure determined??

  If you're interested;
Pi x radius squared = 2.5mm squared.
therefore;
radius squared = 2.5 divided by Pi
= 2.5 divided by 3.142
= 0.7956715 
radius = square root of 0.7956715
= 0.892
Diameter = 0.892 x 2
= 1.784mm 
Take a rule & a calculator with you. 
Other than this, the cable size is usually written somewhere on the cable sheath. Alternatively, it is written on the cable drum so you may wish to witness from which drum the cable is removed.

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## wozzzzza

with all this talk about the AS3008, can it be downloaded from somewhere??  i cant find it.

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## nev25

> with all this talk about the AS3008, can it be downloaded from somewhere?? i cant find it.

  Yep Here http://www.saiglobal.com/shop/script...=stds000017409
At a cost 
But it wont answer your original question 
Its only about current carrying capacity's of cables in different environments.

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## taswood

Leave the work to trained and experienced electricians. I have seen lighting circuits done in bell wire and power circuits added onto in a domestic laundry done in figure eight, no earth!. "Accident " waiting to happen.

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