Calculating the thickness and length of wire

[Wet5park]

What are the rules for calculating the thickness of wire.

All DIY books state 1.5mm for 6amp light circuits and 2.5mm for 32amp ring main.

But based on info gained from other topics - What thickness of wire would you use for a 10amp Light circuit?

I assume the thickness of wire is based on the power? but how is this calculated? power = volts x amp, but what about thickness?

also i assume that the max length of the wire is also important...how is this calculated?

Don't worry i'm not trying to push any limits or take risks, i am just interested in the theory/ understanding

 

[ban-all-sheds]

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• http://www.kevinboone.com/cableselection_web.html
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Technical/Lighting/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• //www.diynot.com/forums/viewtopic.php?p=81696#81696[/list]
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[ericmark]

There are a number of factors which control thickness of wire.
1) Min is 1mm from table 52.3
2) Using table 4D5 we can see the volt drop per meter per amp and we have to ensure it does not exceed 3%
3) tables 41 give us the max earth loop impedance and again we need to calculate to ensure we are within limits.

The list goes on. But most electricians have a pretty good idea what will pass and don't sit down with a slide rule before starting but look to see if they are likely to be near limits. We use "Rules of thumb" etc.

We also use a PDA with excel when we are near limits. This will give you some info

As to 32A on 2.5mm cable you will see it is more like 21A and 32A only permitted when used in a ring main.

If you want I can go further but this is really why DIY electric is not recommended as with most engineering it does contain a fair bit of maths.

 

[rebuke]

Also note that 1.5mm and 2.5mm aren't measurements of the thickness, they are in fact measurements of the CSA (Cross Sectional Area) of the copper conductor (so strictly speaking should be written as 1.5mm^2 and 2.5mm^2).

 

[ban-all-sheds]

slide rule

Showing your age there, Eric....

 

[ricicle]

(so strictly speaking should be written as 1.5mm^2 and 2.5mm^2).

Hold down Alt and type 0178 to get ²

 

[Wet5park]

Thanks BAS for the links.

I have found the Kevinboone link very useful, although i haven't read the others in detail.

And its also saved me from starting another topic in regards to whether 1.5mm wire is suitable for a small the bathroom circuit including the 4 IP65 (max 50 watt) downlights as well a Fan/ shaver socket/ towel radiator...based on Kevinboone i think i've sorted it.

I have been advised to have all the bathroom appliances including the bathroom lights on one circuit, however i wasn't sure whether i needed to have a 2.5mm wire as there were other appliances than lights...and DIY books say 1.5mm for lights and 2.5mm for sockets (shaver)

Just working out the total power used in the bathroom and will then calculate the wire thickness as to whether 1.5mm will be sufficient with the correct MCB.

...but before anyone worries...everything is being checked by a qualified electrician (friend), i'm just keen to learn for myself by working it out and then have my answers checked.

 

[kirkgas]

(so strictly speaking should be written as 1.5mm^2 and 2.5mm^2).

Hold down Alt and type 0178 to get ²

2.5mm tried alt and 0178 and it didnt work

 

[RF Lighting]

What sort of keyboard do you have?

It works on mine if I use the number keypad to the right of the letter keypad, but not if I use the numbers in a line above the letters.

Failing that, you can go into character map and copy and paste it from there.

 

[ricicle]

2.5mm²²²²²²²²

Are using the number pad on the right of your keyboard ?