domestic

[c.randall7]

Hope every one had agood new year
If you meter tails of 25mm goes into a CB and comes out in 16mm to the CU can the 16mm earth dropped down to 10mm at this point or should it stay at 16mm. (this is a PME system). i'm looking in the regs but isn't clear to me

 

[ezzzekiel]

whats the cb? remove it and leave as 25/16, but either way if it stands at 16mm why would you want to reduce it?

 

[Sparky Ninja]

The size of the protective conductor can be calculated via the adiabatic (543.1.3) or selected according to table 54.7 (543.1.4)

 

[c.randall7]

it's for cu change. that is how it is now just wondering if i should uprate the earth or leave it.

 

[c.randall7]

the cb is to protect the tails as the cu is 6m away

 

[Sparky Ninja]

The size of the protective conductor can be calculated via the adiabatic (543.1.3) or selected according to table 54.7 (543.1.4)

Use your regs book.

 

[ezzzekiel]

ah i see - what is the rating of cb, also you say its already a 16mm earth so no need to upgrade.

 

[c.randall7]

I've looking at that table and what i understand it as is if the line is 16mm the earth must also be 16mm?

 

[c.randall7]

63a but the earth just goes into a conector at 16mm but comes out at 10mm then goes to the cu

 

[ezzzekiel]

if its rated at 63a then 16mm tails 10mm earth and 6mm bonds would suffice although for the costs that would be incurred i would replace with 25, 16, 10 respectively.

I've looking at that table and what i understand it as is if the line is 16mm the earth must also be 16mm?

as widdler stated if you use the adiabatic then you will find the earth size will come out considerably smaller

fault current= uo/ze
s= rootsquare isuared x t / k

 

[Sparky Ninja]

I've looking at that table and what i understand it as is if the line is 16mm the earth must also be 16mm?

If you choose the selection method, then yes.

 

[c.randall7]

cheers chaps. many thanks

 

[ezzzekiel]

the main point to note on these often asked questions is that a lot of books provide examples where main incoming fuse is rated at 100a which in majority of cases it wont be in which case you now have to look at wether costing or safety is the issue to the client, if you can size your cables and protection and back it up with calculation then your signature on the Eic proves you are competent in the case of an inquiry.