Normally when you see a maximum rating, you're lucky if you can maintain that for long! If the room's a bit warmer than they tested it at....
Someone decides they want to keep warm in winter when their central heating breaks. Two 3kW fanheaters....
They cycle on and off. Stick two of them in close proximity and they'll spend even more time off. Even though time off may not be nearly as long as time on (midwinter, 6ft snowdrift) I bet it's plenty enough to prevent overload.
The reason I'm interested in the 20A rating is not for overload but when deciding what the design current of a circuit is. Do you use 20A for each double socket before you apply diversity?
Alex
Perhaps. But diversity applied to a 30A ring main still has the correct fuse or breaker should diversity not take place as you thought. If the double socket only takes 20A and there's only a 30A fuse protecting it in the CU, it should have it's own 20A fuse.
It's the same situation. A 30A fuse doesn't blow at 30A, so the ring circuit can be made to carry more current than its 'rated' current, simply by plugging too much in.
Neither 30A circuit nor 20A socket have protection that kicks in the moment you exceed that rating. Both rely on making overloads hard, but not impossible, to achieve.
Alex
1363 part 2 1995. Its from the temperature rise test section, where you are supposed to load a socket with 14A on one side, and a further 6A on the other when testing a double. The load is then maintained for a minimum of 4h, and up to a maximum of 8h or when a stable temperature is reached (whichever comes first). To pass, the temperature rise must be limited to 52 deg C over ambient.
Check the test spec in BS1363, the minimum test duration is 4h @ 20A
100% duty cycle.Indeed. However many would still be alarmed by the temperature rise on the socket and that would probably serve to have them connect things differently. ;-)
You can have an unlimited number of sockets on a circuit, so it does not really factor. The guideline is to limit the floor area served by the circuit.
On Monday 01 April 2013 23:53 Alexander Lamaison wrote in uk.d-i-y:
It's printed on the back of one of mine.
Also, as regs allow a double on a spur and for nominal purposes the cabling in a ring main is defined to be good for 20A min under all conditions, it makes sense to limit an oversized accessory at the same.
On Monday 01 April 2013 23:00 Dave Liquorice wrote in uk.d-i-y:
Oh - in that case, you are right - it would be silly not to just insert a new socket :)
Yes, but you've now got a 20A socket with a 30A fuse, which is even worse. A
20A fuse would allow short term overloads already. Upping that to 30A is too much. Ir surprises me how many petty safety rules there are yet it's so easy to set fire to your wall simply by using two 13A sockets at 13A. How difficult would it have been to make them take 6A more?
That's pretty hot. Lets say you have a warm room of 23C. That's 75C. I wouldn't want 75C sat in my wall. And that's only at the 20A rating.
Depends what cable you use. If I was wiring a spur I'd use something rated at at least 30A.
I don't know who wired this house (it's a Bett home built in 1979), but the ring main looks to have been done with 4mm^2 cable, not the 2.5mm^2 you're referring to. The shower is wired with 6mm^2, and the lights with 2.5mm^2. It's bloody difficult fitting those big wires in I can tell you.
On Tuesday 02 April 2013 12:19 Major Scott wrote in uk.d-i-y:
Your house was wired by a looney - or you've measured wrong...
I can understand the shower - perhaps he just happened to have that cable available.
And a spur should have thicker cable than a ring, as it's only ONE cable.
The lighting seems over the top, but if it gets embedded in fibreglass.....
On Tuesday 02 April 2013 12:48 Major Scott wrote in uk.d-i-y:
That's the only bit that is likely correct.
No it should not. There are likely to be issues with terminal packing with mixed cable leading to unreliable joints.
That would be over the top! 1.5mm is heavy enough - 1.0 is often used.
Even if someone had a job lot of cable, to be so cheap as not to get the right cable would make me very suspicous of everything...
It least they bodged it in the right direction I suppose and didn;t use bell wire...
I don't want 30A going through a 20A cable and making it hot.
If you make sure it's in the grub screw firmly, there is no problem. Why would you think there's a difference between several thin wires and one thick one?
I typed the wrong number, it's actually 1.5mm^2, which is rated at 14A, a bit excessive for lighting.
I would agree if he had used too thin a cable. If you're doing a few jobs and need 2 different sizes of cable, why buy two reels when the big one will do both?
Well it's guaranteed the cable will never overheat!
On Tuesday 02 April 2013 13:27 Major Scott wrote in uk.d-i-y:
Because several thin wires and one thick one do not pack in the same way as several identical wires.
There is no accessory that can legitimately take 30A.
A 30A point load on a ring is bad design.
That's sane then and quite correct. I'm using 1.5mm - mostly to given a little more leeway on the L-E loop impedance.
Because it's a bloody bodge. Next question...
However the same cannot be said of the terminations.
You're obviously no good with a screwdriver.
Irrelevant. The fuse is 30A, so nothing should melt first.
Why would you need 14A for a 5A circuit? Think how much easier it would be to fit multiway lightswitches with 5A cable.
Overengineering won't do any harm.
If you can't fit the wire into the terminal block, then obviously it's no good, but it fits. And the fuller that block is, the less likely it will fall out.
On Tuesday 02 April 2013 14:58 Major Scott wrote in uk.d-i-y:
Because mine are 10A circuits, type C breakers to boot to reduce liklihood of spurious tripping. This suits the layout of my house and it not common, though it is a standard circuit.
In fact you can have a 16A lighting circuit as a standard circuit, though it is almost unheard of in domestic scenarios.
And you clearly did not understand the concept of loop impedance which is
*as important* (in terms of ensuring that disconnection times are met under full fault conditions) as not overloading the cable.I will explain further with numbers if you wish.
For the umpteenth time - it will do harm if it exceeds the terminal capacity or in any way makes the terminations unreliable. Not debating this anymore - it's your house, I don't care.
Are the grub screws beefy enough to hold the wire down. Stiff wire flexing when you push the accessory into the wall will cause a *lot* of strain on the terminal. Will one of the thin wires find a void between the terminal and the tick wire that it falls into, losing grip?
Big house? I've got two 5A circuits. So if you turn one off (or there's a fault) you can still see what you're doing with the other.
I'm not fussy on disconnection times - I've got fuses!
Anyway, if the current limit is less, you don't need as low an impedance surely? Say it was a 0.5 amp circuit....
Also my house was built before we had circuit breakers.
If it exceeded the terminal capacity, the wire wouldn't fit.
Well everything works, so obviously not.
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