Well I don't think people want to be clipping direct so I think we can see why rings are still used !
New ring circuit or spurs?
- Thread starter dremario
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I know that's what you meant - but was adding an extra hypothetical 'idea' of my own!
Kind Regards, John
Buried in the wall counts as clipped direct.
I'm not quite sure what you mean - 'clipping direct' (in quotes!), aka Reference Method C, includes buried in walls etc - in fact, most 'common' installation methods that do not involve insulation or conduit.
AIUI, they had to 'fiddle' ('reconsider and revise') Table 4D5 to make 2.5mm² RFCs compliant with the regs (which require CCC ≥20A) with installation Methods 100 and 102!
Kind REgards, John ≥
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does loose under floor count is clipped direct ?
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There is nothing wrong will one ring for whole house. Every installation shall be divided into circuits, as necessary, to avoid hazards and minimize inconvenience in the event of a fault and that statement in the regulations means we often have more than one ring.
But there are also other factors one is the amount of cable which can be used to form a ring without exceeding the loop impedance or volt drop considerations. You can use more cable in a ring than 2 radials so although in some cases you can split a ring into two radials this is not always the case.
With the kitchen on it's own circuit then unlikely there will be a power problem using a single ring it's a lot of work to split a ring into two but clearly can be done.
Step one before doing any electrical work is to find what you already have. So I will list what we look for with some notes.
1) Is it already RCD protected if not then adding to the ring will depend on how easy it is to add RCD protection. It may be the consumer unit will take RBCO's but likely it would mean a new consumer unit.
2) What is the earth loop impedance this will show if you can add to the ring or if already at the maximum length.
For the DIY guy often the easy way is to add RCD FCU and from that a number of sockets as the 13A fuse raises the allowed ELI from 1.44 for B type MCB to 2.42 ohm for a 13A fuse so unlikely the limit will be exceeded and also you have the RCD protection required.
As to ring / radial with a ring you can use 106 meters of 2.5mm cable and be within the volt drop allowed using a 32A MCB but with a radial dropping to a 20A MCB the maximum length is just 32 meters which is why the idea of splitting a ring into two radials does not work.
I built an excel program when 17th Edition came out as I could not see where the 106 meters came from and it includes ambient temperature correction and a lot of data like factor of grouping and is not an easy calculation. The formula is given in 17th edition appendix 14 Correction for operating temperature 6.1 but what it does not state is the special allowance for using the British ring final where I went wrong was the design current for the circuit we are allowed to consider it as 26 amp not 32 amp for this calculation. Idea is 20 amp in centre and remaining 12 amp even drawn from around the ring.
This idea does however cause a problem when using smaller than 32A MCB as the 20 amp at centre or end remains so feed a ring with 25A MCB the figure used to calculate is still 22.5 amp so very little difference in max length still only 122 meters and there is also the point the ELI will be 1.65 not 1.50 with the 106 meters personally I think it should be 100 meters which gives ELI of 1.44 but it's not me writing the books.
I know 100 meters seems a lot of cable but the 1/3 rule for drilling beams means one does not just go up and down the wall but 1/3 way across the room for drops. To counter this problem some firms split the house side to side rather than up down which reduces cable used so gets a better ELI.
The idea of 16A radials one to each room does mean fault finding is easy but OK with MCB costing less than £10 but with RCBO's the £35 for each circuit really boosts the price look at a 10 way consumer unit and that's £350 for the RCBO's little point is splitting if then combined to same RCD with ring likely only need a 6 way for same house so RCBO saving around £140 on RCBO's that will buy a lot of cable.
To my mind the only problem with the ring is bad installation where some one makes it into two radials in error and as a result it can be overloaded but really can't condemn a system because some people get it wrong.
But there are also other factors one is the amount of cable which can be used to form a ring without exceeding the loop impedance or volt drop considerations. You can use more cable in a ring than 2 radials so although in some cases you can split a ring into two radials this is not always the case.
With the kitchen on it's own circuit then unlikely there will be a power problem using a single ring it's a lot of work to split a ring into two but clearly can be done.
Step one before doing any electrical work is to find what you already have. So I will list what we look for with some notes.
1) Is it already RCD protected if not then adding to the ring will depend on how easy it is to add RCD protection. It may be the consumer unit will take RBCO's but likely it would mean a new consumer unit.
2) What is the earth loop impedance this will show if you can add to the ring or if already at the maximum length.
For the DIY guy often the easy way is to add RCD FCU and from that a number of sockets as the 13A fuse raises the allowed ELI from 1.44 for B type MCB to 2.42 ohm for a 13A fuse so unlikely the limit will be exceeded and also you have the RCD protection required.
As to ring / radial with a ring you can use 106 meters of 2.5mm cable and be within the volt drop allowed using a 32A MCB but with a radial dropping to a 20A MCB the maximum length is just 32 meters which is why the idea of splitting a ring into two radials does not work.
I built an excel program when 17th Edition came out as I could not see where the 106 meters came from and it includes ambient temperature correction and a lot of data like factor of grouping and is not an easy calculation. The formula is given in 17th edition appendix 14 Correction for operating temperature 6.1 but what it does not state is the special allowance for using the British ring final where I went wrong was the design current for the circuit we are allowed to consider it as 26 amp not 32 amp for this calculation. Idea is 20 amp in centre and remaining 12 amp even drawn from around the ring.
This idea does however cause a problem when using smaller than 32A MCB as the 20 amp at centre or end remains so feed a ring with 25A MCB the figure used to calculate is still 22.5 amp so very little difference in max length still only 122 meters and there is also the point the ELI will be 1.65 not 1.50 with the 106 meters personally I think it should be 100 meters which gives ELI of 1.44 but it's not me writing the books.
I know 100 meters seems a lot of cable but the 1/3 rule for drilling beams means one does not just go up and down the wall but 1/3 way across the room for drops. To counter this problem some firms split the house side to side rather than up down which reduces cable used so gets a better ELI.
The idea of 16A radials one to each room does mean fault finding is easy but OK with MCB costing less than £10 but with RCBO's the £35 for each circuit really boosts the price look at a 10 way consumer unit and that's £350 for the RCBO's little point is splitting if then combined to same RCD with ring likely only need a 6 way for same house so RCBO saving around £140 on RCBO's that will buy a lot of cable.
To my mind the only problem with the ring is bad installation where some one makes it into two radials in error and as a result it can be overloaded but really can't condemn a system because some people get it wrong.
That's arithmetically true, but stating it as you have makes the difference sound a lot more dramatic than it actually is.
From the point-of-view of VD, the maximum cable distance from CU to a socket right in the middle of a ring is (at the design current of 32A, distributed around the ring as you describe) about 53 metres (i.e. half of the 106m total ring length), so it is this 53 metres (not the 106 metres total ring length) that one should really comparing with 32 metres (for a 20A 2.5mm² radial), not the 106 metres total ring length.
There is still a substantial difference (understandably, given that the effective CSA of the ring to the most distant socket is 5mm²), but nothing like as great as your implied difference of "106m vs. 32m" comparison might seem to imply. I would think that 32 metres (about 105 feet) is probably plenty for a radial, even supplying upstairs sockets, in the 'normal average' house, isn't it?
We tend to think of VD is terms of complying with regs, but it's worth noting that a maximal permissible (11.5V) VD at 32A would represent some 368W of 'wasted power' (at 230V), quite significant in both 'green' and financial terms - which is a good reason fro striving to keep cable lengths and VDs as low as possible!
Kind Regards, John
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