Understanding fused spur

markus2025 · 26 Oct 2025

Hi guys can anyone answer a question im intrigued by so on a house hold ring main on 2.5mm twin and earth protected by 32amp breaker you can spur off any part of the ring with only 1x double socket i presume this is because if you added more double sockets then just 1 you could theoretically overload the spur cable which say was 2.5mm would be rated at say 27amp max is it ?

so you could overload it by 6 amps before the breaker blew

however i believe its permitted that you can add a 13amp spur unit from the spurred socket and then run as many sockets as you want from that being protected by the 13amp unit

the bit i dont understand though if say you was maxing out the spurred double sockets with 2x 13amp on each point so pulling 26amp you would be almost at the max of spur cable

but then you could pull a further 6amp with the fused spur before the 32amp breaker went and you would be overloading the orginal 2.5mm spur cable would you not ?

EFLImpudence · 26 Oct 2025

markus2025 said:
however i believe its permitted that you can add a 13amp spur unit from the spurred socket and then run as many sockets as you want from that being protected by the 13amp unit

Yes,

markus2025 said:
the bit i dont understand though if say you was maxing out the spurred double sockets with 2x 13amp on each point so pulling 26amp you would be almost at the max of spur cable

But everything is protected by the ONE 13A fuse in the Fused Connector Unit (the cable is the spur).

markus2025 said:
but then you could pull a further 6amp with the fused spur before the 32amp breaker went and you would be overloading the orginal 2.5mm spur cable would you not ?

No.

pete01 · 26 Oct 2025

Use 4mm cable to to spur it avoids any future problems.

markus2025 said:
but then you could pull a further 6amp with the fused spur before the 32amp breaker went and you would be overloading the orginal 2.5mm spur cable would you not ?

No, because of the 2 x 13 amp plug fuses.

SUNRAY · 26 Oct 2025

NO

You can add a fused connexion unit (FCU) to the ring, either as part of the ring: after the FCU you can have as many sockets as you wish

OR on an unfused spur as the only item on the spur: after the FCU you can have as many sockets as you wish

EFLImpudence · 26 Oct 2025

markus2025 said:
however i believe its permitted that you can add a 13amp spur unit from the spurred socket and then run as many sockets as you want from that being protected by the 13amp unit

I took that to mean:

Is the one with three cables not the spurred socket?

markus2025 · 26 Oct 2025

Ow so if a double socket has already been ran on a radial spur in 2.5mm from a ring main circuit you cant then add a fuse spur unit from that socket ?

EFLImpudence · 26 Oct 2025

No.

Apologies if I misunderstood.

Jurassicspark · 26 Oct 2025

markus2025 said:
which say was 2.5mm would be rated at say 27amp max is it ?

Depends on how it is installed.

markus2025 said:
however i believe its permitted that you can add a 13amp spur unit from the spurred socket and then run as many sockets as you want from that being protected by the 13amp unit

Nope.

markus2025 said:
the bit i dont understand though if say you was maxing out the spurred double sockets with 2x 13amp on each point so pulling 26amp you would be almost at the max of spur cable

What is the likelihood of both appliance consuming 13A at the same time?

markus2025 said:
but then you could pull a further 6amp with the fused spur before the 32amp breaker went and you would be overloading the orginal 2.5mm spur cable would you not ?

An additional 3A, 5A, 6A, 13A? Where does this 6 number come from?

But you wouldn’t be allowed nor be able to, if you’d done the job properly, by only adding one spur off the original socket or from a FCU as the first point, from the ring, then the sockets after that added on.

One socket and full load = 2X13 = 26A

Ideal conditions of T&E 2.5mm clipped direct = 27A

Introduce a FCU = Whether if it's 13A, 6A, 5A or a 3A, the sockets connected to the load side of the FCU will be limited to that. Once the load from the sockets after the FCU exceeds the fuse rating in the FCU, the fuse will blow, cutting the power to all sockets, from the load side of the FCU.

Jurassicspark · 26 Oct 2025

pete01 said:
Use 4mm cable to to spur it avoids any future problems.

What if the sockets that are going to be added, or being thought of being added by the OP, whether hypothetical or in real life scenario, are in places where high current devices are more prone to be plugged in.

If, in this case, a 4mm spur on spur is being used in a kitchen or utility room for high current drawing appliances, would it be a good idea? The closer to the first RFC socket on one of the RFC legs, this spur is being taken from, the more affect it will have on that leg's smaller cable.

Fine for occasional light to medium loads but I'd be a lot more sceptical if these sockets were being installed for known heavy loads or if they are known to be closer to the origin of the ring.

But I know someone pedantic will quote this and say what about the first two sockets on the ring? What if both are grossly loaded?

The question here is has anyone yet come across a melted leg(s) of a ring due to imbalance?

pete01 · 26 Oct 2025

Jurassicspark said:
The closer to the first RFC socket on one of the RFC legs, this spur is being taken from, the more affect it will have on that legs smaller cable.

True, but if I'd have said extend the ring no one would have batted an eye lid...probably

Jurassicspark said:
But I know someone pedantic will quote this and say what about the first two sockets on the ring? What if both are grossly loaded?

That is true though

Jurassicspark said:
The question here is has anyone yet come across a melted leg(s) of a ring due to imbalance?

I haven't

JohnW2 · 26 Oct 2025

pete01 said:
No, because of the 2 x 13 amp plug fuses.

Not to mention the 1 x 13A fuse in the FCU

JohnW2 · 26 Oct 2025

Jurassicspark said:
What if the sockets that are going to be added, or being thought of being added by the OP, whether hypothetical or in real life scenario, are in places where high current devices are more prone to be plugged in. ... If, in this case, a 4mm spur on spur is being used in a kitchen or utility room for high current drawing appliances, would it be a good idea? The closer to the first RFC socket on one of the RFC legs, this spur is being taken from, the more affect it will have on that leg's smaller cable.

True, but not materially different from the situation in which those 'high current drawing devices' were plugged into sockets on the ring which were physically very close to each other (OR, having multiple 2.5mm² spurs, each supply one socket and all originating from points very close together on the ring).

Jurassicspark said:
The question here is has anyone yet come across a melted leg(s) of a ring due to imbalance?

I very much doubt it, particularly if (as is very common) the ring is wired in Method C 2.5mm² cable - since there's no way that a cable rated at 27A protected by a 32A OPD ever going to melt (or, I imagine, come to any harm at all).

Even if a full 32A load were connected to a single point (or very close points) on a ring (probably never going to happen!), that point has to be fairly close to one end of the ring for it even to be possible that more than 27A could to travel along the short leg of the ring.

markus2025 · 26 Oct 2025

maybe what im saying is being misunderstood so i have 1 double socket coming off a ring on 2.5mm which has 2x 13amp outlets then from that double socket there a fcu capable of taking another 13amp
so does that not mean i could pull 39amps all togeather if there was no 32amp breaker but cause of the breaker i can pull 32amp still 6amps over the 27amp rated cable ?
or am i looking at it wrong and a fixed double gang socket can only take 13amp ? across both its points not 13amp on each point

JohnW2 · 26 Oct 2025

markus2025 said:
maybe what im saying is being misunderstood so i have 1 double socket coming off a ring on 2.5mm which has 2x 13amp outlets ...

You mean as a spur from the ring? If so ....

markus2025 said:
then from that double socket there a fcu capable of taking another 13amp

That's not allowed - because, if it were, as you go onto say ....

markus2025 said:
so does that not mean i could pull 39amps all togeather .

pete01 · 26 Oct 2025

JohnW2 said:
Not to mention the 1 x 13A fuse in the FCU

Oh yeah

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Understanding fused spur

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