Max cable length/run of 2.5mm T&E ring main

[Invisible-Man]

Hi,
Could anyone tell me if it's 50m or 100m max of 2.5mm T&E cable on a ring main. Many thanks

 

[Dellsmp]

Domestic ring main is limited to the area of the floor ie 100 square metres not 100 metre length, you can have unlimited sockets on it as long as the floor area is less than 100 square metres. And one spur of each socket or fcu.

 

[Invisible-Man]

Thanks for that,
I thought it was also the length of cable

 

[Adam_151]

Length will be limited by volt drop and Z's, I'll focus on volt drop here, because without knowing the Ze and the type and rating of protective device, I can't comment on Z's

There is an allowed volt drop of 9.2v (4%) 2.5mm² T&E drops 14mv per amp per metre:

9.2v / 0.014v = 657
657 / 32A = 20.5
20.5 x 4 (due to the ring nature of the circuit) = 82m

... at least I think thats right

 

[Invisible-Man]

Thanks, them calculations are beyond me

 

[plugwash]

as for Zs

assuming a B32 breaker and a worst case TN-S

Ze=0.8

fast trip current= 32*5=160A

max Zs = 240/160=1.5 ohm
max R1+R2 = 0.7 ohm

i don't have the figures for R1+R2 handy so lets make a pesimistic guess of 25 ohms per kilometer

0.7/25*4=0.112KM=112 meters

so as we can see the volt drop calculated two posts above is the dominating design factor on cable length.

theres probablly some correction factors i missed but that'll be in the right ball park