Cable calculations

Sparkingchip:

Yes there is a formula, it depends on the position of the spur in the ring, I will have to look it up to be accurate.

I sat down at my desk about twenty minutes ago and have been trying to find it in the IET books on my bookshelf, but can't. I know it is in one of the IET books, but cannot remember which one.


Basically you should not end up with a long spur on a short ring, say a 10 metre spur on the end of a ring with 6 metres of cable in it and the length of the spur depends on its position in the ring. I know it's in one of the IET books, but it's not coming readily to mind which one its's in ?

Mike Gorman:

Hi, additional the advice in other posts, outdoors your outside the building equipotential zone so will possibly need to consider appropriate protection via an RCD and safe trip current rating  housed in IPXX box. If this is not done well garden equipment ( which are notoriously dirty ) could take down the buildings ring circuit inadvertently and therefore any equipment connected to it. While it might be annoying it would be prudent to run a radial from the FB in  4mm2 depending on max load and VD and short circuit calc.        

There is no "equipotential zone" for protection against electric shock. It disappeared in the 17th Edition.


The requirements for RCD protection for general protection against electric shock in TN systems are in Regulations 411.3.3 and 411.3.4 ... RCDs may be required for other reasons, such as where cables are concealed in walls at a depth of less than 50 mm, or where necessary for special locations in Part 7.


That doesn't mean they can't be used if none of these requirements apply ... and I agree with Mike it's a good idea whether BS 7671 specifies it or not for the particular application.

MrJack96:

Hi guys i wanted to calculate the maximum run when spuring of a ring final circuit is there any equations or is it just as simple as calculating VD to the furthest point in the ring and then VD to the end of the spur. The reason for the question is I want to spur of an existing socket using a 10meter run of 2.5 for an outside socket. Just want to Double check it’s sufficient 


thanks guys 

Assuming you've no problem with too many spurs at roughly the same place round the ring, the formulas to use depends on whether the ring has an RCD at or before the protective device supplying the ring.


If it does, volt-drop is the only consideration, otherwise disconnection time would come into play as well.


Volt-drop can be a little complicated depending on whether there are other spurs off the ring, but if you want to slightly "over-engineer", you can assume the worst-case volt-drop as being the highest of either:


13*(R₁+R_n)+19*(r₁+r_n)/4 or 20*(R₁+R_n)+12*(r₁+r_n)/4


where:

r₁+r_n are the end to end resistances of the ring final circuit line and neutral conductors respectively

R₁+R_n are the resistances of the spur line and neutral conductors respectively

If you need to take loop impedance into account, then it all depends on where the spur comes off the ring, and whether the ring is at max loop impedance, as follows:


1. Work out how much lower Z_{s(measured) is} at the furthest point in the ring, than the rule of thumb times the maximum permitted for the protective device in Chapter 41 (0.8*Z₄₁).

2. Calculate the additional length L_spur(min) of spur you could have at the furthest point in the ring (at L_ring/2, where L_ring is the end-to-end ring length) using, for 2.5/1.5 T&S, (0.8*Z₄₁-Z_s(measured))/19.51 (19.51 is the R1+R2 per metre value from the OSG).

3. Use the following to estimate (adapted from Table 18.7 of the Commentary on IET Wiring Regulations BS 7671:2008+A3:2015):

• 1/8 way round the ring end-to-end (either way), L_spur=L_spur(min)+L_ring/8

• 1/4 way round the ring end-to-end (either way), L_spur=L_spur(min)+L_ring/16

• 3/8 way round the ring end-to-end (either way), L_spur=L_spur(min)+L_ring/64

• 1/2 way round the ring end-to-end (either way), L_spur=L_spur(min)