couple of circuits sharing same earth

[amlu]

Hi
Im finishing rewiring at the moment, place has concrete ground floor, all cables going under the floorboards on first floor or in the loft. There is couple of steel conduits embedded in the building from where consumer unit goes to floor void above. They end up nicely in a metal box where connections can be made (singles in the conduit to twin and earth) Added couple of new circuits and not sure if all will fit in the conduits.
Got 3x 4mm radial, 1x 2.5mm radial and 10mm bonding for water service to squeeze in, to save on space can I run one earth and split it into separate circuits, wonder if thats any good ?

 

[Richard Burns]

It is allowable, It might make testing a problem, but is the conduit not usable as the earth?

 

[amlu]

Conduits may be usable for earth but they are old and rusty, metal enclosure on the other end does not have any terminals, its just a box, plan is to drop another run of 10mm earth single and terminate it with an earth block...

 

[Marvo]

Isn't there a limit in the British regs that limits the percentage you can fill a conduit. Current carrying capacities are usually given for cables in conduits assuming a certain airflow and cooling effect, if the conduit is filled tight with wiring you'd usually need to derate accordingly. This is the case with my local SA regs and withsome of the ISO regs.

 

[High Tower]

Run a 16mm earth in and fix a EMT to the back of the box adaptable box.

 

[Deleted member 26818]

Isn't there a limit in the British regs that limits the percentage you can fill a conduit. Current carrying capacities are usually given for cables in conduits assuming a certain airflow and cooling effect, if the conduit is filled tight with wiring you'd usually need to derate accordingly. This is the case with my local SA regs and withsome of the ISO regs.

Yes there is, but it's rarely complied with.