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Hope someone can help. When finding the cabling current capacity for a ring (wired in 2.5), would you use the data for 4mm in either/or table D5 of BYB or table 7.1 (11) in the OSG ? As the ring circuit is in effect 5mm !

Looking at the OSG ref methods; a 2.5 with ref method 101 says 17A, with 4mm its 22A. Atm the ring (run through loft covered in insulation) is on a 32A so needs swapping out. Can anyone advise
 
You use the ratings for 2.5mm. The total rating of the ring is then doubled.

So for ref method 101, it's 17A giving a total maximum current carrying capacity of the ring of 34A.
 
Each part of the ring must be able to carry a minimum of 20 amps to comply with regulation 433.1.1 for overload.
See regulation 433.1.204
 
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You use the ratings for 2.5mm. The total rating of the ring is then doubled.

So for ref method 101, it's 17A giving a total maximum current carrying capacity of the ring of 34A.
You use the ratings for 2.5mm. The total rating of the ring is then doubled.

So for ref method 101, it's 17A giving a total maximum current carrying capacity of the ring of 34A.

That cannot be correct, a 6mm only has a ccc of 27A in install method 101; a 10mm just beats your 34 by 2A (36a)!!
 
That cannot be correct, a 6mm only has a ccc of 27A in install method 101; a 10mm just beats your 34 by 2A (36a)!!
Your 17amps reference method 101 will not comply with regulation 433.1.1 /433.1.201 as the ccc is only 17amps for that part of the ring.
4mm is 22 amps ref method 101 unless you get the 2.5mm above the insulation , 2.5 will carry 21 amps reference method 100 not exceeding 100mm thickness of insulation
 
Your 17amps reference method 101 will not comply with regulation 433.1.1 /433.1.201 as the ccc is only 17amps for that part of the ring.
4mm is 22 amps ref method 101 unless you get the 2.5mm above the insulation , 2.5 will carry 21 amps reference method 100 not exceeding 100mm thickness of insulation

Theres something more toit than that, a 2.5 clipped direct is 27A. There are alot of 32a ring mains installed to reference methods higher than clipped direct operating at 32a. Which is why I enquired do you use the ccc for a 4mm ?
 
As I said read regulation 433.1.204 it states 2.5 mm as a minimum and can be supplied through a 32 amp protective device and others as long as iz can carry a minimum of 20amps to satisfy regulation 433.1.1 for overload.
So yes mostly 2.5mm is used if kept away from insulation and ref method c is used then the ccc of each part of the ring is 27amps so will comply.
Use 4mm if the volt drop maybe an issue or cables encounter insulation or other derating factors and the ccc is below 20amps
 
The tables give ratings for a single cable, a ring final has two cables. That's the whole point, to allow a greater current carrying capacity than could ordinarily be handled by a cable of the same size.

You said it yourself that the CSA of a ring wired in 2.5mm is 5mm. Thus, the total current carrying capacity of the ring is the 2.5mm current carrying capacity x 2. But as has been pointed out, you need to factor in the overload handling requirements.
 
As I said read regulation 433.1.204 it states 2.5 mm as a minimum and can be supplied through a 32 amp protective device and others as long as iz can carry a minimum of 20amps to satisfy regulation 433.1.1 for overload.
So yes mostly 2.5mm is used if kept away from insulation and ref method c is used then the ccc of each part of the ring is 27amps so will comply.
Use 4mm if the volt drop maybe an issue or cables encounter insulation or other derating factors and the ccc is below 20amps

Thanks just found that reg and it makes it clearer. So ref method 100 it will have to be.
 
As I said read regulation 433.1.204 it states 2.5 mm as a minimum and can be supplied through a 32 amp protective device and others as long as iz can carry a minimum of 20amps to satisfy regulation 433.1.1 for overload.
So yes mostly 2.5mm is used if kept away from insulation and ref method c is used then the ccc of each part of the ring is 27amps so will comply.
Use 4mm if the volt drop maybe an issue or cables encounter insulation or other derating factors and the ccc is below 20amps

Thanks just found that reg and it makes it clearer. So ref method 100 it will have to be.
 
As I said read regulation 433.1.204 it states 2.5 mm as a minimum and can be supplied through a 32 amp protective device and others as long as iz can carry a minimum of 20amps to satisfy regulation 433.1.1 for overload.
So yes mostly 2.5mm is used if kept away from insulation and ref method c is used then the ccc of each part of the ring is 27amps so will comply.
Use 4mm if the volt drop maybe an issue or cables encounter insulation or other derating factors and the ccc is below 20amps

Thanks just found that reg and it makes it clearer. So ref method 100 it will have to be.
 
The tables give ratings for a single cable, a ring final has two cables. That's the whole point, to allow a greater current carrying capacity than could ordinarily be handled by a cable of the same size.

You said it yourself that the CSA of a ring wired in 2.5mm is 5mm. Thus, the total current carrying capacity of the ring is the 2.5mm current carrying capacity x 2. But as has been pointed out, you need to factor in the overload handling requirements.

Noooooo, apparently not. Doubling 17a its not; doesn't add up like that !!!!
 
The tables give ratings for a single cable, a ring final has two cables. That's the whole point, to allow a greater current carrying capacity than could ordinarily be handled by a cable of the same size.

You said it yourself that the CSA of a ring wired in 2.5mm is 5mm. Thus, the total current carrying capacity of the ring is the 2.5mm current carrying capacity x 2. But as has been pointed out, you need to factor in the overload handling requirements.
Ideally, the ring circuit acts like two radial circuits proceeding in opposite directions around the ring, the dividing point between them dependent on the distribution of load in the ring.
Noooooo, apparently not. Doubling 17a its not; doesn't add up like that !!!!
2 cable in parallel used to evenly distribute the load around the ring so a cable with a smaller ccc can be used with a 32amp mcb and nothing more
 
The point I was trying to make is that if one cable can handle 17A and they are both subject to the same reference methods, then the absolute maximum current carrying capacity of the ring is 34A due to the parallel paths.

That's it, nothing more.

If you can't change the reference method, or change the cable in those sections up a size to 4mm, then down rate the breaker.
 
The point I was trying to make is that if one cable can handle 17A and they are both subject to the same reference methods, then the absolute maximum current carrying capacity of the ring is 34A due to the parallel paths.

That's it, nothing more.

If you can't change the reference method, or change the cable in those sections up a size to 4mm, then down rate the breaker.

I can see where your coming from SC, but a 6mm only has a ccc of 27a (ref 101), so how can a 5mm (2x2.5) can have a rating of 34A ?

Looking into it I think the ring has more of a science to it, and thats why the reg quoted earlier is accepted. But how can a 5mm have a greater rating than a 6mm ? Must be something with the parallel paths and load distribution
 
Two 2.5 conductors do not result in a single 5.0 conductor but two separate 2.5 conductors in parallel. The current carrying capacity of the two 2.5 conductors is higher than the single 5.0 due to thermal considerations. In other words the two 2.5 conductors will dissipate heat better than a single 5.0, an approximate factor of 1.414 can be used.
This is often why large submains, think hundreds of amps are not very often run in a single cable because you can get reduced csa using two or three cables in parallel.
 
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