long ring main

[drgnb]

My understanding is a 2.5mm ring main with a 32A mcb and RCD protection can supply 100m2 floor area, and is limited to 66 m cable to comply with the 4% voltage drop rule.

I need to supply a wing of a new house which is too far away to manage only 66m cable. I could increase the cable to 4mm which would reduce resistance, hence increase length allowed. However as the room is quite small I was thinking of reducing the MCB to 20A (which would allow 106m cable with a 4% voltage drop). Is this allowed, and if so what is the maximum floor area I can supply (?is it still 100m2 or less)

 

[ban-all-sheds]

My understanding is a 2.5mm ring main with a 32A mcb and RCD protection can supply 100m2 floor area, and is limited to 66 m cable to comply with the 4% voltage drop rule.

It's actually just under 64m, (230V remember, not 240), but that's a minor point. I don't think, though, that you need, or even should, assume a load of 32A when designing a ring. The OSG quotes a length of 84m, which implies an Ib of about 24A.

I need to supply a wing of a new house which is too far away to manage only 66m cable.

How far away is it??

 

[plugwash]

is your 64m figure based on all load at the furthest point or evenly spread load?

 

[drgnb]

The distance to the entrance to the wing is 27m. The rest of the house is reasonably centred round where the consumer unit will be. The room is about 6m by 4m and is a breakfast room (unlikely to thave many high current items attached).

Thanks for the help

 

[ban-all-sheds]

Run a 20A 4mm² radial.