Current Carrying Capacity question...

[Mikey_A]

Just wondering - 2.5/1.5 T & E has a Current Carrying Capacity of 27A when clipped direct (method 1) and is recognised as the standard for ring final circuits, as per OSG.

With minimal correction factors this can easily be increased to above 34A. Straight away we have to increase the cable size.

For example, lets say the ambient temp is 25 degrees and the cables are grouped single layer with spaces. the calc is as follows:

It = (In / (Cg x Ca)) therefore:

It = (32 / (0.90 x 1.03)) = 34.52A

Surely these are minimal correction factors for any ring circuit. My calc doesn't even include thermal effects. Therefore, shouldn't the minimum cable size for ring circuits be 4mm in all cases?

 

[davy_owen_88]

I'm not quite following you.

As long as the CCC of the cable used for each leg is atleast 20A it is fine. Remember that a ring circuit isn't going to be continually loaded and that any load is going to be shared between 2 conductors (albeit not equally).

For a ring final circuit protected by a 30 A or 32 A protective device complying with BS 88, BS 1361, BS 3036, BS EN 60898, BS EN 60947-2 or BS EN 61009-1 (RCBO) and supplying accessories to BS 1363 and wired with copper conductors, the minimum cross-sectional area of both phase and neutral conductors is 2.5 mm2 except for two-core mineral insulated cables to BS 6207 for which the minimum is 1.5 mm2. Such ring final circuits are deemed to meet the requirements of Regulation 433-02-01 if the current-carrying capacity (Iz) of the cable is not less than 20 A, and if, under the intended conditions of use, the load current in any part of the ring is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable.

 

[Mikey_A]

I'm still a little confused. The regs state that during design stage the selection of cable for any circuit must be determined by calculating the CCC, which is dividing the protective device rating into the applicable correction factors. Which, as far as i can see is what i've done. The CCC has worked out at above 34A and according to table 4D5A is too high for 2.5mm T & E

Am i getting this totally wrong, probably!

 

[ColJack]

that's fine for single cables, the calculation you made shown that for a 32A breaker, you need a cable that can take 34.52A ( which is why 32A radials are 4mm² or more )

if you divide this by 2 for the legs of a ring then you get 17.26A per leg ( assuming equal distribution of current down each leg.. )

reference method 1: clipped direct, this is 1.5mm² ( 20A )

to allow for uneven distribution of current we use 2.5mm² rated at 27A per leg..

 

[Mikey_A]

The penny has finally dropped. Thanks for your replies Davy/Coljack.

 

[ColJack]

not a bad reply.. i was actually typing that in response to your other post.. just as well it was bang on for answering your next question..

and no I have no idea what next weeks lottery numbers will be...