Spur off a spur on a radial

[KWCFC]

Hi all I've just bought a new property and have noticed sockets are wired from 2x 30A fuses with each 30A fuse feeding 3 radials, (no end to end readings at all) and there are spurs off these radials via junction boxes that are feeding more than one socket... AKA spur off a spur.

I know that it's incorrect to wire them this way from a 30A fuse and in an ideal situation it would be rewired. Unfortunately I'm not in a position to get the property rewired currently so my question is could I effectively change the 30A fuse to a 20A to make the circuit safer for now?

I know that a 20A fuse feeding a radial is fine, but I'm unsure as to whether it is ok to have a 20A fuse feeding multiple radials which have spurs feeding other spurs?

 

[Taylortwocities]

I guess the wiring is in 2.5mm² PVC cable? If so, a 20A (or 25A) circuit protection device is what is required for a radial provided in that size cable.
Higher current fuses are used for larger cables.
A 20A fuse for a radial with any number of branches on a 2.5mm² PVC circuit is fine, but you have to remember that the maximum current will be limited at 20amps.

But you keep saying "fuses" do you really mean fuses? You'll not find 20A fuse wire in most of the 'sheds', but easily available on line.

 

[333rocky333]

It would depend on the cable size 30a radials are ok but likely need to be 4mm minimum

 

[chivers67]

is it 2.5mm T&E?

 

[KWCFC]

Yes it's 2.5mm T+E

 

[AndyPRK]

Fuse or Mcb ?
What brand are they

 

[flameport]

I know that a 20A fuse feeding a radial is fine,

What type of fuse?

With 2.5mm² cable, a 20A cartridge fuse is ok.
A 20A rewireable fuse is not.

 

[KWCFC]

It's a cartridge fuse. What difference would it make between cartridge and rewireable?

 

[333rocky333]

The Bs numbers are different, therefore it indirectly affects the current rating of a cable
Bs3036 Rewirables have a 0.725 correction facter applied to the cables current rating capacity.
Therefore 2.5mm is possibly to small for a 2.5 radial on a 20 amp rewirable

 

[flameport]

Rewireable fuses have a higher fusing factor (of 2) which means the cable needs to be capable of carrying a much higher current than if a device with a lower fusing factor was used (most devices 1.45).

2.5mm² cable has a maximum rating of 27 amps, which is obviously more than the 20A fuse.
Most cable calculations use the 1.45 factor, but as rewireable fuses have a factor of 2, a correction of 0.725 must be applied (1.45 / 2).
A 27A cable x 0.725= 19.5A, which is less than the 20A, so not permitted.

It's also why 20A rewireable fuses are rare objects compared to the very common other values.

 

[KWCFC]

So despite there being spurs fed from spurs the circuit will be safe and no chance of overloading if I was to change to a 20A?

 

[333rocky333]

The spur from spur scenario is for Ring circuits and does NOT apply to Radials.
You can still overload the circuit but the design should ensure the fuse ruptures prior to any damage to the cable, your existing 30a circuit will not do that

Changing to a 20 amp is also dependant on the Earth loop impedance at the furthest point, but likely you dont have a tester to do this.
Although it is more likely that the reading is suitable for a 20a than your existing 30 a
A 20 amp Hrc fuse is unlikely to fit your 30 amp carrier, is the make Wylex