Why are 32A breakers used on 26A wiring?

wonderwire · 21 Dec 2016

Disclaimer: I am learning about wiring (not practicing!). Please entertain my questions.

I've seen a number of 32A breakers on consumer units that feed into 2.5mm cable.

I've also read that 2.5mm cables should only be rated up to 26A.

Please could someone explain why you would use a 32A breaker on a 26A cable?

When junction boxes or inline connectors are used, why are these usually rated below the breaker (e.g. 30A or 26A, respectively)? Are they safe to use on a circuit with a 32A breaker?

Yes I am only just starting to learn.

Thanks!

mfarrow · 21 Dec 2016

Those circuits are likely to be wired as a "ring", ie the cabling starts from the consumer unit, connects to each socket in turn, then returns to the consumer unit. So you should see 2 wires at the CU.

The reason its allowed is historic: a scarcity and the price of copper after the war led to an invention of this ring main with a 32A breaker supplying cable rated at 20A. The idea is that you'd unlikely get a leg on the ring pulling more than 20A, and so all the accessories and junction boxes needed to be rated to that as well.

Indeed, the reality is that you could probably get more than 20A on a leg of ring if, the ring was long enough and if, the load (socket) was close to the consumer unit. Hence rings are a bit controversial these days and a lot of people prefer Radial circuits, which don't form a ring.

You're also limited to socket outlets and fused spurs. So no cookers etc added on in the middle.

Taylortwocities · 21 Dec 2016

You will also see a single 2.5mm² cable powered from a 32A MCB typically when wired as a spur from a ring final. The spur could be from the back of a socket, or added to the MCB at the consumer unit. On a spur you are only permitted to put a single or double socket or a fused connection unit, so the maximum current that will flow on the spur would be 2x13 = 26amps.
This is the main reason what spurs from spurs are not allowed.

JohnW2 · 21 Dec 2016

wonderwire said:
When junction boxes or inline connectors are used, why are these usually rated below the breaker (e.g. 30A or 26A, respectively)? Are they safe to use on a circuit with a 32A breaker?

Your question about cable/breaker seems to have been adequately answered.

As for junction boxes etc., it's really essentially a historical issue. In the past, we used 5A, 15A and 30A fuses, but they have become 6A, 16A and 32A breakers, but the manufacturers of most junction boxes etc never changed the 'rating' they wrote on their products! That 'rating' is pretty arbitrary - I'm sure than a "30A" one could happily be used with 40A, 50A or more (if cables would fit iin the terminals) - so there is no problem.

However, to satisfy purists, it would be nice if these things were actually 'rated'/labelled as 32A! It is, after all, many decades since 32A breakers started taking over from 30A fuses!

Kind Regards, John

wonderwire · 21 Dec 2016

Thanks @mfarrow and @Taylortwocities . So if I understand this correctly, today's CUs won't have ring circuits set up with 32A, most likely 26A breakers? On the old circuits "protection" really comes from the fact that the sockets are fused at 13A each, is that correct? What's to stop 3 sockets being used at 13A each on a ring and overloading the wire?

Thanks also @JohnW2 - can you use a 26A inline connector or a 30A junction box on a 32A ring circuit then? I would assume so if the wire itself is 2.5mm 26A?

bernardgreen · 21 Dec 2016

wonderwire said:
I've also read that 2.5mm cables should only be rated up to 26A.

26 amps is the maximum safe continuous current that 2.5 mm cable can carry without the conductors becoming too hot for safe operation.

A 2.5 mm² copper conductor in standard PVC twin and earth cable can carry considerably more the 26 amps for a prolonged period without becoming hot enough to damage the insulation. The current and time period are affected by the way the cable is installed ( free air, surrounded by insulation, against a cold surface ( that the cable heats up ).

The 32 amp MCB doesn't trip instantly at 33 amps. A prolonged current several times the 32 amp rating a causes a heating element in the MCB to heat up and eventually it is hot enough to trip the MCB.

The time it takes the MCB to trip when over loaded is short enough to ensure a cable selected and installed in compliance with the standards will not become dangerously hot before the MCB trips.

JohnW2 · 21 Dec 2016

bernardgreen said:
26 amps is the maximum safe continuous current that 2.5 mm cable can carry without the conductors becoming too hot for safe operation.

I don't know where all this talk of 26A has come from (is it perhaps because 2 x 13A = 26A??). The maximum current capacity of 2.5mm² cable using the usual 'Method C' (on surface, or buried in a wall etc.) is actually 27A.

However, as has been said, there are all sorts of safety margins built into the figures. The characteristics of breakers dictate that it is deemed safe for 2.5mm² cable to carry about 36A indefinitely, and 46A for at least an hour - and I imagine that there are pretty generous 'safety margins' built in even to that.

Kind Regards, John

JohnW2 · 21 Dec 2016

wonderwire said:
Thanks also @JohnW2 - can you use a 26A inline connector or a 30A junction box on a 32A ring circuit then?

I'm not sure if/where you would find a "26A connector" but, yes, I'm sure that would be fine.

If you bought, say, a stepladder which said "maximum load 100kg", would you really expect it to collapse, or come to some other harm, if someone of, say 102kg used it?

Kind Regards, John

flameport · 21 Dec 2016

wonderwire said:
today's CUs won't have ring circuits set up with 32A, most likely 26A breakers?

Ideally they should be, although as 26A breakers are not available it would normally be 25A or more commonly 20A.
Unfortunately plenty of people still install them.

wonderwire said:
What's to stop 3 sockets being used at 13A each on a ring and overloading the wire?

Nothing, but as there are 2 cables on a ring rated to 26A each, the maximum current for the pair of cables is well above the 32A breaker.
The problems occur when people install spurs with multiple sockets and make alterations resulting in the ring being broken, both of which result in a single 26A cables 'protected' by a 32A device.

JohnW2 · 21 Dec 2016

flameport said:
Ideally they should be, although as 26A breakers are not available it would normally be 25A or more commonly 20A.

I think I understand what you're saying, but if one used such breakers there would be no point (other than CPC redundancy) in installing a ring circuit at all (which is what the OP was asking about).

I presume you were suggesting that, in your opinion, ring final circuits should no longer be installed, but I don't think that was very clear from what you wrote in response to the OP's question about breakers protecting ring circuits.

Kind

Iggifer · 21 Dec 2016

32A 4mm² radial final is the way forward for beefy loads

20/25A radial final for general purpose stuff

Less testing too!

JohnW2 · 21 Dec 2016

Iggifer said:
32A 4mm² radial final is the way forward for beefy loads. 20/25A radial final for general purpose stuff. Less testing too!

I'm sure that many would agree with that.

However, to put this into perspective ..... in over half a century of usage, I would imagine that there have been very very few cases in which a problem has arisen by virtue of use of a 2.5mm² (or similar) 30/32A ring final which would not have arisen with a 4mm² (or similar) 30/32A radial.

Kind Regards, John

Iggifer · 21 Dec 2016

I'm not so sure about that. I often have people say "oh that socket hasn't worked for ages/since I moved in" and it's invariably the live/neutrals have popped out of the circuit. If that happens on a radial, you obviously lose the rest of the circuit so people are move likely to get it fixed.

Granted it could be the EOL but in this day and age, ring finals have had their time. The amount of times a leg of a ring final follows the other cable back to the CU for the majority of it's run, it's just a waste of cable.

30A ring finals had their place when you had one or two socket circuits in a house and you needed that capacity on the circuit, now circuits are more segregated now

EFLImpudence · 21 Dec 2016

As I keep saying, the ring was only necessary because of the derating factor of BS3036 fuses.

Unless the (socket) circuit is actually physically a ring, today there is no advantage.

That you may today want to make a ring because of volt-drop considerations in, say, a lighting circuit does not allow you to use the special dispensations of the standard socket circuit.

endecotp · 21 Dec 2016

wonderwire said:
So if I understand this correctly, today's CUs won't have ring circuits set up with 32A, most likely 26A breakers?

No you don't understand correctly. The ring circuits on a modern CU will have 32A MCBs. This is fine, despite the max current in the 2.5mm2 cable being less than 32A, because it is a ring and the current will flow from the CU to thw sockets in both directions around the ring.

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Why are 32A breakers used on 26A wiring?

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