Cable calculations

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)

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)