mixed cable types in single circuit

[gmdaly]

good evening all,

If I have an existing 10mm T+E from the CU to the rear of the house external ending in a junction box on the wall already in place (total length 9M) and I want to connect this to an armoured cable 2 core + CPC from the exterior junction box to a rear garden new future sauna (13M) buried in ground 600mm. The new total length will be 22M. The internal T+E is ref method B and the buried SWA will be ref method D.

Just considering the current carrying capacity and voltage drop only and no other factors. should I consider the total length for each cable type over the entire distance and assume the lowest capacity and highest voltage drop or is there another way of considering this where it factors both cable types for the distance they each run only? In other words the worst case or something smarter?

Thanks for your help.

g

 

[pc1966]

Correct approach is to compute for each and add the VD terms, and take the lowest CCC and that is going to set the over-current limit (probably, as unlikely to be a fixed load here).

Also you ought to consider the fault clearing Zs at the end of cable, and as the first run is in T&E it probably needs RCD protection if at any point hidden but less than 50mm from the surface.

What sort of a load are you actually needing? That might decide what you do. e.g. if all you need is 5-15A then 2.5mm SWA is fine and you can largely ignore the 10mm T&E contribution.

 

[pc1966]

OK, just noticed "sauna" so I guess quite a high load.

Still, start with that and compute the existing 10mm VD and then what is left from 5% is what you need to meet for that.

From your MCB to match load, check minimum Zs. You might find you need 3C SWA to keep CPC down. You can disconnect on the RCD at higher Zs but it is (in my view) poor practice to depend on that unless no other sane option.

Finally will there be any "extraneous conductive parts" to be bonded like steel framework or metallic water pipes, etc? If there is and you are on a TN-C-S supply then you have additional limits on the CPC size and would not meet it on 10mm T&E so you might have to make the load a TT setup (so local earth rod, etc, lot of hassle).

 

[gmdaly]

Correct approach is to compute for each and add the VD terms, and take the lowest CCC and that is going to set the over-current limit (probably, as unlikely to be a fixed load here).

Also you ought to consider the fault clearing Zs at the end of cable, and as the first run is in T&E it probably needs RCD protection if at any point hidden but less than 50mm from the surface.

What sort of a load are you actually needing? That might decide what you do. e.g. if all you need is 5-15A then 2.5mm SWA is fine and you can largely ignore the 10mm T&E contribution.

Thanks for reply mate, Specifically for this scenario for VD and CCC, is there anything specific in the regs or OSG?

 

[pc1966]

Thanks for reply mate, Specifically for this scenario for VD and CCC, is there anything specific in the regs or OSG?

The VD requirement is per circuit, so no matter how it is constructed it is the end-end effect at the working load (either fixed, or at MCB limit if variable due to sockets, etc)

For the CCC you have to consider the whole run as it might vary even with the same cable type (e.g. as the "method" goes from clipped direct to under thermal insulation in a loft, etc). There are rules about the location of the OCPD but if the supply limit matches the lowest CCC then it is fine. You could have more complex arrangements with branches, etc, where you might have a local OCPD (like the fuse in a FCU) that protects a sub-section of the circuit if the supply/CU does not.

 

[gmdaly]

The VD requirement is per circuit, so no matter how it is constructed it is the end-end effect at the working load (either fixed, or at MCB limit if variable due to sockets, etc)

For the CCC you have to consider the whole run as it might vary even with the same cable type (e.g. as the "method" goes from clipped direct to under thermal insulation in a loft, etc). There are rules about the location of the OCPD but if the supply limit matches the lowest CCC then it is fine. You could have more complex arrangements with branches, etc, where you might have a local OCPD (like the fuse in a FCU) that protects a sub-section of the circuit if the supply/CU does not.

got it, thanks again