Ring main,going going gone

[cliffed]

My tutors told me to use this as a rule of thumb and you shouldn't go wrong, for a ring final circuit "1 roll of 100m cable = 1 ring final circuit", if that's wrong I stand corrected, but my tutors swore by this rule of thumb.
Found this remark on another forum,using 100m, that ring main ain’t gonna go far.
I usually have 2 rolls on the go.

 

[telectrix]

main thing is the number of sockets, and whether there are drops of 6 ft. odd or rises of 2 ft.

 

[cliffed]

Agree,& where the DB is positioned.

 

[Pete999]

I would suggest you look at Reg BYB 433.1.294 ans Appendix 15

 

[westward10]

I suspect there has been some confusion here with the maximum floor area for a 32/30A ring final being 100m sq. It is possible they could also be referring to the volt drop limit if you only use 100m of cable in the circuit bit I am pretty sure that is too high as I thought it was 79m for a well balanced circuit.

 

[telectrix]

also to do with the cable resistance .... Zs value. for a 32A MCB you need a R1+R2 < 1 ohm ( exact value depends also on Ze.). ain't got tables to hand, but from memory, 100m 2.5mm T/E wil give aroung 0.6 ohms for R1+R2.

 

[Wilko]

Hi - another slant - is the tutor meaning "a ring circuit from a single 100m drum of 2.5/1.5 is unlikely to exceed the max loop impedance of a B32 RFC on a TNCS installation" (?) ... Is so, then I agree.

From the OSG Appendix I, 100m of cable R1+R2 is a bit less than 2 Ohms at 20 C.
So r1+r2 is a bit less than 0.5 Ohms at 20C.
TNCS Ze is less than 0.3 Ohms.
Total Zs less than 0.8 Ohms at 20C, or 1 Ohm at 70C, which readily complies with Table 41.3 for B32 OCPD.

For TNS the max Ze is 0.8 Ohms, giving a total of 1.3 Ohms max, which is close to the max of 1.37 Ohms.

 

[Leesparkykent]

It’s based on volt drop using 20A at the furthest point and the remainder up to the rating of the OCPD spread evenly. So for a 32A OCPD the design current is 26A. The mV/A/m for 2.5 is 18.

18x26x100=46800

46800/1000=46.8

46.8/4=11.7V

Just a Gnatscock over 11.5V which is the recommended, maximum, permissible value given in the regs of 5% of 230V for power. So the max is actually more like 98 meters but you easily loose the couple of meters when tidying the job up and cutting cables to tuck them into back boxes etc on 1st fix.

 

[Pete999]

Hi - another slant - is the tutor meaning "a ring circuit from a single 100m drum of 2.5/1.5 is unlikely to exceed the max loop impedance of a B32 RFC on a TNCS installation" (?) ... Is so, then I agree.

From the OSG Appendix I, 100m of cable R1+R2 is a bit less than 2 Ohms at 20 C.
So r1+r2 is a bit less than 0.5 Ohms at 20C.
TNCS Ze is less than 0.3 Ohms.
Total Zs less than 0.8 Ohms at 20C, or 1 Ohm at 70C, which readily complies with Table 41.3 for B32 OCPD.

For TNS the max Ze is 0.8 Ohms, giving a total of 1.3 Ohms max, which is close to the max of 1.37 Ohms.

Wilko
Bit puzzled and not disagreeing in any way, but You mention R1+R2 for 100mts cable is less than 2 ohms.
Then in the next sentence r1+r2 is a bit less at 0.5 ohms both at 20degC.
It's probably me but can you explain what you mean ? sorry Mate just me wondering that's all.

 

[Leesparkykent]

Wilko
Bit puzzled and not disagreeing in any way, but You mention R1+R2 for 100mts cable is less than 2 ohms.
Then in the next sentence r1+r2 is a bit less at 0.5 ohms both at 20degC.
It's probably me but can you explain what you mean ? sorry Mate just me wondering that's all.

r1+r2 then divide by 4 because it’s a ring final circuit gives you R1+R2.

In the example above if R1+R2=2 Ohms

Then the expected value of r2 would be 2/1.67=1.19

Leaving 0.81 for r1

Obviously 1.19+0.81=2

2/4 because it’s a RFC gives you 0.5 for R1+R2

 

[Pete999]

Can't see the wood for the trees.

 

[johnduffell]

Just because the ring is a standard circuit and it's an easy to remember amount, there's nothing else special about the length of a roll. If you design it all correctly you won't end up far off so it's a rule of thumb.
It's just a short cut to avoid detailed design and calcs. You can just design a more suitable circuit if 100m isn't enough eg use 2.5mm or 4mm radials, or multiple rings, or even the feared lollipop circuit.

 

[Marcus Vaughan]

or even the feared lollipop circuit.

I don’t know about feared, but it was pointed out to me recently that lollipop circuits are not compliant. Reg 543.2.9 in the 18th. Not sure about previous editions to be honest.?

 

[plugsandsparks]

I think it is quite a useful , back pocket, reminder. I was wiring a large house that was a challenge for cable runs, UFH, high ceilings and the like. Client wanted sockets everywhere and i mean everywhere. Anyway when i had cabled 2 bedrooms and run off a 100m drum, i thought hang on and stopped the install. I ended up pulling in two extra SWA cables to SP Cu's just for VD and Zs. This house has ended up with 3 X TP boards and 2 X SP boards, not just because of socket wiring. its good to not just keep running more and more cable, something has to stop you and make you think. BTW at the design stage we did not have No of sockets and i did rather hope for easier shorter cable runs but in reality it was much harder.

 

[DPG]

If 543.2.9 is the one about conductors in ting finals being connected end to end then that still allows for a lollipop circuit. It's just that the ring is fed from another length of cable. Apologies if I've misunderstood your post.