Tails for flat

[mnmwill]

Got to replace a CU in a flat this week, question is do I need to upgrade the tails?

TNC-S, supply runs from meter in common cupboard on GF, into 60a fused isolator, then from there in 16mm T&E.

I am going to upgrade the main earth and bonding to services to 16mm and 10mm respectively but unsure what I am required to do qith the tails on this one. Distance from Isolator to CU is prob about 12m.

Thanks in advance

 

[Guest123]

IMO as the 16mm sub is protected by a 60A switchfuse at the origin it's fine, no need to upgrade. Unless theres going to be an increase in load in the flat or something requiring the 60A fuse to be upgraded.

 

[mnmwill]

Cheers Lenny, would you upgrade earth from switchfuse to CU then or leave as is and just upgrade from Earth block to switchfuse??

 

[Guest123]

What size is it at the moment???

 

[mnmwill]

I'm thinking its 6mm, its the earth from the twin, so I will upgrade it to 16mm, bigger the better as they say.

Thanks again mate

 

[Guest123]

An adiabatic may prove the 6mm to be sufficient.......up to you though.

If you are going down the new earth road though, a 16mm will be more than adequate.

 

[mnmwill]

So for a 60a bs 3036

I = 800

sq root of I sq x t

sq root of 800x800x0.20 = 358

358/115 = 3.11

Is this correct???

 

[Guest123]

Are you sure you have a 60A re-wireable???

For a correct calc you would need the measured Zs, You can use a best case scenario by using the max Zs for the pretective device though.

 

[mnmwill]

Yep, 60a re-wireable (ceramic) at switchfuse. Max zs 1.12?

Heads fried!!!!!!!!!!

 

[Guest123]

Lets give it a bash then.

Obviously as you are using the max Zs, the fault current will be at it's lowest yes....


So, 230V/1.12 = 205.35A, which gives a "T" of 5s

205.35 x 205.35 = 42168.6

42168.6 x 5 = 210843.

Sqrt of 210843 = 459.17.

459.17 / 115 = 3.99mm.


As I said, if the measured Zs is lower.....which it should be, the fault current will be higher.

 

[mnmwill]

Cheers Lenny, I am on my way there now so will measure the zs and re do.

Again, thanks m8