Maximum circuit length

[sparks1234]

Looked in the onsite guide today as we thought we may be close to the limit of a ringmain circuit length, the guide gives out a maximum length of just over a hundred mtrs for a 2-5 / 2-5 / 1-5 ring main. Now as we are looking at a trunking system which is to be wired in singles but all at 2-5mm, does that allow extra length?

 

[Sintra]

A ring final circuit must not serve a floor area greater than 100m2. p362
It doesn't matter what size the conductors are 2.5mm with a 1.5mm cpc or 2.5mm with a 2.5mm cpc it is the floor area that matters not cable length.

 

[sparks1234]

A ring final circuit must not serve a floor area greater than 100m2. p362
It doesn't matter what size the conductors are 2.5mm with a 1.5mm cpc or 2.5mm with a 2.5mm cpc it is the floor area that matters not cable length.

Thanks for reply but doesnt help too much, we have a longish thinnish office which we are running trunking up both sides, so floor area is not a problem, so this is commercial not domestic.
Its easier for us to run 2 circuits, there is no problem with overload as there are only about 8 dsso on each circuit feeding 8 desks with a computer and screen on each desk.

If it didn't matter about circuit length they wouldn't be published as a guide in the on site guide

 

[Guest123]

Hi.

The way I calculate the maximum circuit length for a ring is this, 4 x 11.5 x 1000 / 32 / 18 = 79.9M. This assumes an Ib of 32A, but...... the current in a ring circuit will not be the same all around the ring so the way to do it is to assume 20A at the furthest point plus an additional 12A evenly distributed.

The Ib of 32 then becomes (32 + 20)/2 = 26A

So the calc is.....4 x 11.5 x 1000 / 26 / 18 = 98.3M

Where 11.5 is the max VD permitted, 26 is Ib and 18 is the mV/A/m value.

This is assuming though that Ct is 1.

So the max circuit length is 98M contained within a maximum floorspace of 100M squared.


Hope this helps.