Spur supporting spur

[ban-all-sheds]

OK -

In a ring circuit can the length of spurs be additional to the 50m maximum for the ring, or do all the spur lengths have to be counted as part of the 50m maximum?

Firstly, the limits on the length of 2.5mm² ring circuits range from 46 -91m, depending on the type of protective device and its rating and whether you have a TN-S or TN-C-S supply. I think securespark must have forgotten Table 7.1 in the OSG...

For a 30A Type B the limit is 88m, and for a 32A Type B its 84m. Although if you do the sums, worst case voltage drop kicks in at 68.1m and 63.9m respectively, so I think the OSG is only considering disconnect times.

In a ring circuit, in order to determine the safe length for any one spur cable does the voltage drop have to be calculated individually for each separate spur (taking the resistance of the ring itself into account, but not other spurs)?

Yes, with the caveat that you might hit a limit die to disconnection time first.

Is the resistance of all the spurs in a ring circuit discounted when calculating the voltage drop in the ring itself; but the resistance of the ring (but not of any of the other spurs) taken into account in determining the voltage drop at the end of each single spur in turn?

Yes.

In an existing ring circuit can extra spurs still be added by inserting (accessible) junction boxes, instead of inserting a socket in the ring? My 1908 house was re-wired with junction boxes serving a spur supporting a second spur, which I want to amend.

Yes, but bear in mind that for some reason you aren't allowed more spurs than there are sockets on the ring.

 

[biblio]

ban; many thanks. Thanks, AdamW, securespark.