Is there a maximum length for a spur cable?

Sounds perfectly sensible, and not worth reaching for the calculator for.

A free voltage drop calculator where you put your load in, and it tells you the thinnest cable you could use is
on the TLC website here .

Even if you put in double the length, you have plenty in hand - another (rather rough) rule of thumb is that if you need two reels of 2.5mm (i.e. more than 100m of cable) for a ring and its spurs, then voltage drop on load needs careful thought. Much less than this, no thought at all.


So you know what to expect, the TLC site has some info (based on an older version of the regs, so the RCD stuff is not right)here and linked pages

 

There is a small risk that the existing ring actually is not, and is really a centre fed radial - while the covers are off and it is isolated, it is worth making a ring-round test to verify that it really is a ring (an ohm meter test from L-to L at any place where the ring can be interrupted, and then N-N and E-E). Depending how much hacking has been done before you got there, not all 'rings' actually are...


Spurring from the consumer unit is fine by the regs, but be aware that many consumer units are a bit full, and in that case, placing a junction box nearby may be easier to do.

Sounds perfectly sensible, and not worth reaching for the calculator for.

A free voltage drop calculator where you put your load in, and it tells you the thinnest cable you could use is
on the TLC website here .

Even if you put in double the length, you have plenty in hand - another (rather rough) rule of thumb is that if you need two reels of 2.5mm (i.e. more than 100m of cable) for a ring and its spurs, then voltage drop on load needs careful thought. Much less than this, no thought at all.


So you know what to expect, the TLC site has some info (based on an older version of the regs, so the RCD stuff is not right)here and linked pages

 

There is a small risk that the existing ring actually is not, and is really a centre fed radial - while the covers are off and it is isolated, it is worth making a ring-round test to verify that it really is a ring (an ohm meter test from L-to L at any place where the ring can be interrupted, and then N-N and E-E). Depending how much hacking has been done before you got there, not all 'rings' actually are...


Spurring from the consumer unit is fine by the regs, but be aware that many consumer units are a bit full, and in that case, placing a junction box nearby may be easier to do.