Spur off a spur on a radial

K

KWCFC

Joined
6 Mar 2009
Messages
30
Reaction score
0
Country
United Kingdom
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?
 
Sponsored Links
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.
 
It would depend on the cable size 30a radials are ok but likely need to be 4mm minimum
 
Sponsored Links
It's a cartridge fuse. What difference would it make between cartridge and rewireable?
 
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
 
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.
 
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?
 
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
 
D

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
You must log in or register to reply here.
Sponsored Links

Similar threads

D
Cooker switch with socket
Replies
3
Views
983
Steve
S
B
Spur point
Replies
2
Views
944
bothyhead
B
T
Radial Spurs
Replies
3
Views
1K
lazygit
L
F
Cooker supply
2
Replies
20
Views
2K
securespark
securespark
C
Spurs on a radial
2
Replies
16
Views
3K
skenk
S
Back
Top