Earth Requirement

[Michaelwgroves]

I need to run several new supplies for studios. 16mm T&E is fine for the supplies. But I'm worried the earth may not be big enough. Longest run is 12M from Meter. I could just run cable then if it fails test run 16mm earth. But would be interested to learn the calculation so I can be sure from the outset.

Thanks
Michael

 

[telectrix]

adiabatic. but easiest way is to use 16mm 3 core SWA. that also removes the possible need for RCD protection.

 

[deano116]

S=(sq root) of I2xt/K

 

[telectrix]

√(I² x t)/k

 

[onions1066]

What's the earthing arrangement ?

 

[Michaelwgroves]

SWA adds about £150, although could use 2 core and armour as earth for only small increase in cost. Had not thought about RCD for supply cable. I thought this was only a requirement for sockets. What would make this install require RCD on supply cable?

Earth is TN-S

Need to get my head round adiabatic

 

[onions1066]

It's all relates to the installation method of the supply cable to weather it requires rcd protection

 

[Michaelwgroves]

Just a thought, if I only need 60A to each studio I can use 16mm T&E. Similarily I could run 6mm 2 core SWA. Assuming minimum 10mm earth required, how big would armour work out to be on 6mm SWA ?
Would also need to look at voltage drop, but thinking out loud.

 

[onions1066]

24mm I think

 

[Michaelwgroves]

It's all relates to the installation method of the supply cable to weather it requires rcd protection

I don't recall ever seeing this in 17th edition. I know the regs for sockets, where are the regs for supply ?

 

[Sintra]

Is the supply cable buried in a wall or partition at a distance of less than 50mm. If it is then it needs additional protection.

 

[Michaelwgroves]

Is the supply cable buried in a wall or partition at a distance of less than 50mm. If it is then it needs additional protection.

But isn't that just for sockets, you don't need this for lights, so why for supply?

 

[Sintra]

But isn't that just for sockets, you don't need this for lights, so why for supply?

Think you need to have a look at the regs as to when additional protection is required.

 

[telectrix]

that's why i suggested SWA.

 

[Tim]

But isn't that just for sockets, you don't need this for lights, so why for supply?

The regs in this case apply to all power cables, submain, lighting switch drops, sockets, cooker. All cables require rcd protection if buried in a wall less than 50mm deep, unless they are suitably protected (metal conduit, armoured cable etc)

 

[Michaelwgroves]

That changes things. I've not run cable yet, so may or may not be burried less than 50mm. Glad you mentioned that, thanks.

 

[Michaelwgroves]

Okay, looking at everything. It just makes sense to use 16mm 2 core SWA. So does anyone know how big the armour would calculate to. I assume bigger than 16mm ?

 

[Sintra]

20.3mm copper equivilant for BS 6346 70 degree SWA and ONLY 13.5mm for BS 5467 & 6724 XLPE 90 degree.

 

[Michaelwgroves]

So which one do I use, or just go with adiabatic ?

 

[telectrix]

use 3 core then you know it's sufficient. and as for the buried in walls<50mm deep, read 522.6.101/102. any cable