Earthing a secondary CCU

[george765]

I am running 16mm T+E from my house CU (50A mcb) to a second CU in garage extension.
I've been advised that I could have extended the house wiring as the garage is an extension to the house, but I thought, for convenience I would put in a second CU.
Will the earth in the 16mm T+E be enough for the second CCU or do I need a seperate earth cabe ?

 

[flameport]

Will the earth in the 16mm T+E be enough for the second CCU or do I need a seperate earth cabe ?

It might be big enough, depending on various other factors.
In any case, it's a poor choice of cable for a submain.

 

[george765]

In any case, it's a poor choice of cable for a submain.[/quote]

Why ?

 

[Chri5]

In any case, it's a poor choice of cable for a submain.

Why ?[/quote]

Zero mechanical protection and earth not suitable since its a 4mm. TNS / TNCS requires 16mm.

So if you'd used 3 core 16mm SWA you wouldn't have to be running a seperate earth cable.

 

[mfarrow]

TNS / TNCS requires 16mm.

Where does it say that?

 

[securespark]

Can you quote which reg relates to that?

 

[ColJack]

doesn't need mechanical protection, the rest of the T+E in the house is ok so why not one more circuit?

it's not like it's running outside, it's a garage extenstion..

 

[Chri5]

Chri5 wrote:
TNS / TNCS requires 16mm.


Where does it say that?

Table 54-7 minimum cross sectional area. A 16-35mm line conductor requires a 16mm protective conductor




Accepted comment of SWA, but it would have saved running 2 x cables (1 x earth + the TE)

 

[mfarrow]

Only if you can't fulfill the obligation of 543.1.3, which in most situations* you can.

If we always had to adhere to Table 54.7, T&E would be long gone by now.

* you may struggle if you're right next to a substation and have a very low Ze value, for example. Even so, the limiting let-through of the DNO's 100A cut-out should be no problem for a 16mm cable. Might get a tad warm though

 

[ColJack]

that's the cookie cutter advice..
not relavent to T+E which is / was designed to comply with the adiabatic..

 

[george765]

Only if you can't fulfill the obligation of 543.1.3, which in most situations* you can.

Could you explain this please ?

 

[mfarrow]

Assuming we have nothing more than a 100A BS1361 cutout, the I²t value total at 240V is 57,300 A²s (Busman data). So even if we have a fault close to the supply origin, the minimum size of protective conductor we require shall be

S = sqrt (57,300) / 100 = 0.75mm²

So we're compliant. Note this does not take into account thermal effects, but if I had a short circuit supplying that much energy at the origin of my installation, that's the last thing I'd be worrying about.

 

[GaryMo]

OOI, where did you get the k rating of 100 from?

 

[george765]

So, just to confirm, my 4mm earth conductor in the T+E is suffient ?

 

[GaryMo]

So, just to confirm, my 4mm earth conductor in the T+E is suffient ?

It's a 6mm2 CPC in 16mm2 t&e.