Current Capacity of a Ring Main

[self builder 123]

Good Afternoon

I am sure that there is an obvious answer to this question ?


With reference to the IEE Regulations when a 2.5 sq.mm ( typically used for a ring main ) is enclosed in wall ( and before applying other derating factors e.g. ambient temperature ) the current capacity of the cable is reduced to 18.5A.

Why is a ring main then typically protected with a 32A MCB ?

Many thanks.

 

[davy_owen_88]

When a 2.5mm² t&e cable is buried directly in the wall its CCC is 27A, even in conduit within an insulated wall the rating is 20A so I have no idea where you got 18.5A from.

A 32A ring main must be made up from cables with a CCC of atleast 20A each. So under most circumstances 2.5mm² is fine for this purpose.

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.

 

[Adam_151]

The current carrying capacity of a cable on a rfc is only required to be 20A due to the ring nature of the circuit* (you do though need to try and keep it in a a reasonable state of balence - so no kitchen at one end of a rfc that serves all downstairs please, etc)

not sure what install method you are selecting to get 18.5A, but plastered into a chase is method one.


*The exact reg is somewhere near the end of section 43 if memory serves

 

[self builder 123]

Gents

Thanks for your replies.

I took the current capacity for 2.5 sq.mm cable from the cable selection part of the www.kevinboone.com website.

This web site also lists the following CCCs based on the cable being enclosed in a wall:

1.5 sq.mm 14A

6 sq.mm 32A

10 sq.mm 43A


Would you agree with these ratings ?


So how does a 32A MCD protect a cable with a CCC of 20A ?

Cheers

 

[JohnD]

Because in a ring final circuit, there are two cables which are joined together to form a ring.

Notionally, you have a socket half way round the ring, drawing 13Amps. Half the current travels clockwise round the ring, and half travels anticlockwise.

So each leg carries only 6.5Amps.

The whole circuit is capable of carrying up to 32A protected by the MCB, and half of it goes round each cable.

In fact, of course, it is not as simple as that. but you get the idea.

Many unfortunate foreigners do not have the benefit of our wonderful Ring, so they have circuits which are not capable of carrying so much to so many outlets, and do not use our fused plugs which enable you to safely and flexibly protect e.g. a table lamp with a 3A fuse even though you are supplying it from a 32A circuit using a socket which is equally capable of running a 13A heater, and can be economically installed safely using low-cost 2.5mm cable.

p.s. I like rings

 

[davy_owen_88]

Those figures are taken from table 4D2A, the correct table to use for flat t&e is 4D5A and the figures are generally a bit higher. Those figures are also for cables installed within conduit in a thermally insulating wall, not buried in plaster/clipped direct.

In a ring circuit there are 2 cables and the current is shared between them depending on the placement of the load on that ring. When installing/designing a ring circuit you must take into account the chances of a single leg being overloaded and measures should be taken to avoid such an occurrence. As long as each leg can carry 20A it is deemed safe as small overloads for short duration are not a problem.

 

[self builder 123]

Those figures are taken from table 4D2A, the correct table to use for flat t&e is 4D5A and the figures are generally a bit higher. Those figures are also for cables installed within conduit in a thermally insulating wall, not buried in plaster/clipped direct.

Could you please tell me what the ratings for 1.5, 2.5, 6 and 10 sq.mm are from the table 4D5A for cables clipped to a timber frame and fully insulated all around.

Cheers.

 

[empip]

Look here Click
Gets you this :- http://www.iee.org/Publish/WireRegs/2001_Amd_1.pdf
-

 

[securespark]

*The exact reg is somewhere near the end of section 43 if memory serves

In my yellow book (Huge sharp intake of breath, I know, but my brown copy's in the van & if you must know I'm surfing nude (TMI, I know) & can't be bothered to get dressed to go get it!!) it says:

Such rfc's are deemed to meet the requirements of 433-02-01 if Iz is not less than 0.67 times the In of the CPD.

 

[davy_owen_88]

That was changed in the 2002 amendment where the requirement became 20A.

 

[ricicle]

That was changed in the 2002 amendment where the requirement became 20A.

Correct Davy

 

[JohnD]

mmmm, 0.67 x 30A as was (or 32A more recently).

Nowhere near the old 20A then