Spur off spur

[danny199]

Hi,

Just need to check something. Sparky installed two new plug sockets. To do this, cut into the main ring, put a junction box in, one cable out to socket. Then from there connected one more plug. I understand to spur off a spur you should use a fuse between them?

 

[Kidsolo]

It doesn't,

It clearly does say one single or one double. Look in the top/left square, my friend.

I said in my opinion there is no difference in practice because both allow the connection of any two 13a appliances with a 13a plug.

The trouble is, anyone can change that single to a double at a later stage... you now have two doubles.

If you reduce the supply cable to ONE 2.5mm with a spur off the ring then the load carrying capacity of that part of the circuit is effectively halved

You do know the current carrying capacity of 2.5 mm is a lot more than 13 amps, right? You can easily use a 20 amp breaker to protect a full set of double sockets on a single 2.5 mm circuit.

 

[Kidsolo]

both may trip under fault or overload conditions.

You say that like it is a bad thing. Even thou I believe only one would trip.

 

[fairlight]

It clearly does say one single or one double. Look in the top/left square, my friend.
You've misunderstood but I perhaps wasn't clear, when I said ' It doesn't', I was agreeing....it doesn't.


The trouble is, anyone can change that single to a double at a later stage... you now have two doubles.
That would be their problem, it is a requirement that before alterations are made the suitability of the installation is assessed. Anyone can connect a cooker to a lighting circuit and change the fuse to a 50a, you cant blame that on the person who installed the lighting circuit.





You do know the current carrying capacity of 2.5 mm is a lot more than 13 amps, right? You can easily use a 20 amp breaker to protect a full set of double sockets on a single 2.5 mm circuit.
It is more than 13a, it is around 26a. But the idea of using a 13a FCU to fuse down a spur from a ring circuit is to prevent a potential heavy loading at one point on the ring. Rings function most efficiently when loading is evenly distributed throughout the circuit, of course in practice 2x 13a heaters could be plugged into one twin on the ring and upset that balance. But if a number of points are to be wired on a single spur it would be bad practice and non-compliant to enable a heavy loading at that point by fusing it at 20a. You are correct that a 20a breaker can be used to fuse any number of points on a single 2.5mm circuit, but that is a radial, a separate circuit, a spur off a 32a ring circuit is a completely different thing

I hope this answers the points you have made, my answers in bold

 

[Pete999]

You say that like it is a bad thing. Even thou I believe only one would trip.

How do you work that one out Kidsolo?

 

[littlespark]

The fuses are there to protect the flx of the appliance as well, which will always be less than the 2.5mm ring cable (2 x 2.5mm!)

Different fuses for different appliances.
Most common are 13A for larger appliances, and 3A for lamps, tv’s etc.
If a fault develops in an appliance, and We didn’t have a fuse in line, the flex could melt before any 32A circuit breaker senses the over current.

The uk did used to have 15A and 5A unfused plugs but they went out of regular use decades ago.
( I will be corrected on that, I’m sure, as these unfused plugs are still used in some specialised places)

 

[SJD]

The uk did used to have 15A and 5A unfused plugs but they went out of regular use decades ago. ( I will be corrected on that, I’m sure, as these unfused plugs are still used in some specialised places)

Well 15A unfused round-pin plugs are quite common in stage lighting, but it is true they are not often found in a domestic property nowadays.

 

[Pete999]

The fuses are there to protect the flx of the appliance as well, which will always be less than the 2.5mm ring cable (2 x 2.5mm!)

Different fuses for different appliances.
Most common are 13A for larger appliances, and 3A for lamps, tv’s etc.
If a fault develops in an appliance, and We didn’t have a fuse in line, the flex could melt before any 32A circuit breaker senses the over current.

The uk did used to have 15A and 5A unfused plugs but they went out of regular use decades ago.
( I will be corrected on that, I’m sure, as these unfused plugs are still used in some specialised places)

Dorman and Smith I believe that's the makers name utilised the fuse as the live oin on their plug tops, some of the old Council houses had them, my Mum and Dad's house had them in Bath the houses were built at or during the end of WW2

 

[Kidsolo]

If a fault develops in an appliance, and We didn’t have a fuse in line, the flex could melt before any 32A circuit breaker senses the over current.

This is where the problem is. We would have 20 amp breakers on each power circuit, which allows 20 or more outlets (not connected in a ring). doesn't burn out the flex before tripping the breaker. You can piggy back off other outlets, connect 3 other outlets from one. You guys made it too hard and complicated.

 

[DPG]

This is where the problem is. We would have 20 amp breakers on each power circuit, which allows 20 or more outlets (not connected in a ring). doesn't burn out the flex before tripping the breaker. You can piggy back off other outlets, connect 3 other outlets from one. You guys made it too hard and complicated.

More than 20 outlets seems a bit excessive on one 20A breaker. But you seem to know better than us.

And don't worry, we design circuits so that the protective device trips well before cables start to overheat. Our regs are quite exacting on that matter.

 

[Kidsolo]

But you seem to know better than us.

It is part of our wiring rules. the idea is that not all of the outlets will be used at once. It is the standard in all domestic wiring and seems to be working fine.

 

[DPG]

It is part of our wiring rules. the idea is that not all of the outlets will be used at once. It is the standard in all domestic wiring and seems to be working fine.

Not saying it wouldn't work, but I personally wouldn't do it.

 

[Pete999]

It is part of our wiring rules. the idea is that not all of the outlets will be used at once. It is the standard in all domestic wiring and seems to be working fine.

So do ours !!