13 Amp socket tolerances

[Snip]

bear in mind that cable rating is based on voltage drop over a specific length. I'm not getting up to find my copy of the current Regs to check on the precise details. It follows that a much shorter length of cable will be quite happy carrying a much higher current.

Reply to
charles
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What regs would that be then?

Reply to
ARWadsworth

Its not quite that simple alas. The length does have an impact on voltage drop as you say, and there are limits to the allowable drop, which in some circumstances impose maximum cable lengths on circuits.

However the current carrying capacity is dictated by temperature budget. The maximum conductor temperature for a normal PVC clad cable is 70 deg C, and so the maximum current it can handle is that which causes the conductors to reach that temperature. The temperature of the conductor will be affected not only by the rate of energy input, but also by the rate of loss. Hence why the current carrying capacity is not a fixed figure, but varies with how the cable is installed and other environmental factors like ambient temperature.

Reply to
John Rumm

"Still argues that the iee know best, even though they are constantly changing the rules as they cover up previous mistakes.. like only fitting one socket to a 2.5 mm spur when they used to allow two."

I presume you know that you are allowed one double socket on an unfused spur, so how else are we to interpret your statement?

Reply to
John Rumm

14th, 15th, 16th & 17th Editions of the IEE Wiring Regulations

now called Requirements for Electrical Installtions BS 7671:2008

Reply to
charles

Indeed, I have two rings, one which includes the kitchen has 32 sockets and the other 26 sockets. Of course not all have items connected and even those that do have items connected do not have them all on at the same time. Doubt I have ever had 7Kw of load spread across both rings, let alone on a single ring. The breaker by the way is 30A, not 20.

Reply to
Old Codger

Probably the IEE regs, now marketed as BS 7671 and produced by the IET.

Reply to
Old Codger

But the heating effect is based on I²R losses - else where does the heat originate. So, voltage drop is a reasonably good measure of the current carying abilty. I agree that there is the effect of cooling on the cable as well - so, how it is installed has an effect, which is why 2.5mm² cable can carry up to 39A (Table 4E4A)

Reply to
charles

Any part of it relevant to you assumption?

Reply to
ARWadsworth

Correction. only one cable on spurs and you can have a lot of those. And there is a specific warning about unbalancing the rings, to such an extent that they ban fixed heating and other high current stuff.

But it isn't, that is the whole point. The overload protection relies on the plug fuse, a user supplied device that the user knows nothing about and is not always as specified. Even if it is as specified you can have two of them on a single 2.5 mm spur and they take 20 amps to blow. that means that a user can draw 40 amps through the 2.5 mm cable for hours at a time and there is nothing in the circuit to stop him. So much for it being overload protected.

So you admit that the 32A breaker will not protect a 2.5 mm spur and that there is no other mechanism to do so. I am glad that you finally get the point.

snip the rest as you finally get it.

Reply to
dennis

What's that got to do with what you said, everyone else agrees that two sockets were permitted and a double is still permitted.

Reply to
dennis

Its a 20A breaker on a 2.5 mm radial, a 32A doesn't provide the required protection. Just as a 32A breaker doesn't protect a 2.5 mm spur on a ring.

Reply to
dennis

In article ,

[Snip]

but what about the 32A MCB?

Reply to
charles

If you take into consideration that you are unable to use an induction hob without starting a fire why should anyone listen to you?

Reply to
ARWadsworth

You are correct for a radial but we are discussing ring circuits which have 30A breakers and either 7/.029 or these days 2.5 sqmm cables, both rated at 20A, in a ring which gives a total safe load of 40A so a 30A breaker is more than adequate.

As regards a spur, 2.5 sqmm cable will actually withstand the maximum of

26A for very long periods without damage or causing harm to either property or people, or do you think you know better than all the Chartered Engineers, fully qualified and highly experienced, who originally compiled and, over very many years, updated/modified the regulations. The ring main concept originated over 60 years ago, I think they should have it right by now.
Reply to
Old Codger

Why would you think that? The 32A breaker (or 30A fuse) will not trip until 40+ amps. So it is possible to get a sustained 40 amps down a 2.5 mm spur (just put two four way strips into a double and load them up, no tampering required, the 13 A plug fuses don't blow until 20+ amps).

How long will the 2.5 mm single cable last then? Is it guaranteed to melt and short causing the breaker to operate or will we have a fire?

Reply to
dennis

Can't remember which parts but the regulations specify maximum voltage drops under various conditions (ambient temperature, free air, enclosed, bunched, etc) and all these conditions should be taken in to account when specifying cable sizes. Tables giving cable sizes for various currents/uses are 'rule of thumb' under normal/average conditions. The actual conditions under which the cable is used always takes precedence over the cable size tables.

Reply to
Old Codger

You said: "like only fitting one socket to a 2.5 mm spur when they used to allow two". That says to me that you believe the regulations used to allow two sockets (twin or separately) and now allow only one. It appears that a twin socket, not two separate, is permissible so you appear to have got that one wrong.

Reply to
Old Codger

Not quite true - it's the core of Nevil Shute's 1948 novel, No Highway. Professionally Shute was an aeronautical engineer.

formatting link

Reply to
Tony Bryer

Indeed, and one of the designers of the R100 airship (the successful private enterprise one, not the unsuccessful R101 which was the 'government' one).

Reply to
Bob Eager

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