Garage Supply

[eckersecker]

I 've run a 6mm 3 core SWA from a new consumer unit in a garage to a IP rated enclosure on the external wall of the house.
From this enclosure I am going to use 6mm twin and earth into the main consumer unit.
It struck me that the 6mm twin and earth only has 2.5 mm CPC whereas the SWA has a 6 mm earth ( using the black core as earth). Is this method OK ?
Thanks.

 

[johnboy6083]

i dont see any reason why not to, as long as you check the calcs. i would note it on the EIC.

 

[malcolmsanford]

There is a very good chance that it will be fine. What I would do just to confirm that is was is the adiabatic equation from section 543.1.3.

So you will need to decide what your protection device will be. Once you know that you will then need to

S = sqrt(I^2 x t) / k

where

S = Min CSA of CPC required in mm^2 to satisfy thermal constraints
I = Uo / Zs in Amps
t = Operating time of the protective device in seconds
k = a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material


The Zs will be the distribution complete circuit from which will be the T+E and the SWA from CU to garage CU.


t will be the tripping time, chances are if your going to use a MCB BS 60898 or an RCBO 61009-1 that will be 0.1, if your using a fuse then you will need to check the tripping times in the fig in appendix 3.


k will be most likely 115 taken from table 54.3 as your using a conductor within the SWA.

This will then confirm if the CPC within the 6mm T + E will be ok.

Is there a need for bonding in the garage ?

 

[eckersecker]

No, bonding not required. Thanks.

 

[pushrod]

t will be the tripping time, chances are if your going to use a MCB BS 60898 or an RCBO 61009-1 that will be 0.1,

Hi Malc ( and eckersecker) Agree with above but as it is a distribution circuit think 5 seconds (411.3.2.3) can be used for the disconnection time with other PDs. This will work in your favour if you are using a fuse of some description for the PD to that circuit - what do you think Malcolm?

I = Uo / Zs in Amps

Malc isn't this the absolute max value for I, if Zs is known. If it is an mcb as the PD you can use the values as shown in the tables in appendix 3???

What earthing arrangement is it?
What is the length of the respective runs of t&e and swa, and Ze if you know it and what PD is going to be used for the distribution circuit as well - then it is possible for one of us (with too much time on our hands) to calculate it.

 

[eckersecker]

The main CU is tncs. There will be a total of 3 metres of 6mm twin and earth, and 3.5 m of 6mm 3 core swa. The breaker at supply will be type 2 and 32A.

 

[pushrod]

Just quickly put the figures in working out R1+R2 for the cables and assuming a max of 0.35Ω for Ze so get an estimated Zs of 0.405 Ω and get an answer of 1.56mm² (or 1.3mm² with a conductor temp multiplier of 1.2) for the CSA. So plenty of surplus capacity in the 2.5mm² cpc of the T&E.

 

[Deleted member 26818]

If you're running part of the circuit in T&E, there may be a requirement to RCD protect the cable, if it is concealled in walls.
You should also ensure that the armour of the SWA is earthed.

 

[eckersecker]

RCD protected at main CU. Armour is earthed via banjo. Thanks all.