30 amp off a ringmain trick

[piman]

Hello Georgie,

I was questioning the post by Wheresmejumper saying a double 13 amp socket is meant to carry less than 26 amps combined. The rating of the circuit breaker be it RCD protected or not is dependant on the circuit, if it's only rated at 15 amps then clearly you can't fully load a 13 a twin socket but on a 32 a circuit you can.

Talking about 11 Kv, I saw the aftermath of a situation where a breaker feeding a pump wouldn't trip and they needed to stop this pump. The rather crazy solution was to rack the breaker out, what a mess that made, melted the breaker and the spouts. But as it was underground I guess they couldn't just open the incomer to the bars.

Alec

 

[madkayaker]

The 360 will have provided a very good path to earth when the cable was cut so the protection will have detected it very fast and disconnected it. Where as the bloke that cut the cable would not have provided such a good path so the protection (maybe not set as well) would have been slower to react. Had a case at work last year where some 11kv tripped out when the electric board closed a breaker onto a fault 5 miles away! The current shoved into the ground was enough for our protection to detect a earth fault (which lead to half a day of paperwork and testing to check the cable was clear as we're not allowed to close onto a fault like the electric board) its that ground current/ voltage that would have set fire to your tree!

 

[wheresmejumper]

Hello Georgie,

I was questioning the post by Wheresmejumper saying a double 13 amp socket is meant to carry less than 26 amps combined. The rating of the circuit breaker be it RCD protected or not is dependant on the circuit, if it's only rated at 15 amps then clearly you can't fully load a 13 a twin socket but on a 32 a circuit you can.

Alec

in an average installation,you will very very rarely have two items drawing 13 amps continuously from a single circuit,less so from one double socket.
you have to remember that they are a domestic outlet(even though used in industry).
it mightnt be common practice in the UK,but I never* fit anything over a 20 amp mcb on a 2.5mm ring main socket circuit.it rules out running more than 20 amps from one double socket
you can never have too many socket circuits in an installation.never.

* sometimes a 25 amp fused supply is okay for special circumstances like welders etc

 

[tigler]

Thats in conflict with what I'm used to seeing in most domestic ring main circuits. The 2 in my house are 32 amp MCB each, same applies to my mothers place and all others I've noticed for as long as I can remember.

 

[wheresmejumper]

in the UK?

 

[GeorgiePorgie]

The 2 in my house are 32 amp MCB each

Are these not the Cooker point and Shower MCB's?

 

[tigler]

in the UK?

Yes.

Are these not the Cooker point and Shower MCB's?

No. Ring mains are served by 4mm2 cable and the MCB is rated at 32amp. Radial circuits are served by 2.5mm2 cable and the MCB is rated at 20 amps.
A cooker spur is by 6mm2 cable and a 32 amp MCB.

 

[wheresmejumper]

i said in my original post it mightnt be common practice in the uk^^

the only reason i wire ring main circuits in a domestic supply is to cater for a large number of sockets permitted in a 100m squared area,but still on a 20 amp rated supply

 

[GeorgiePorgie]

you can't fully load a 13 a twin socket but on a 32 a circuit you can.

Not disputing with you on the 32a circuit but there's no mention of circuit amperage in the OP's post.

If the poster wants to run his welder from someone else's house he should use the cooker point which is rated at 32amps

 

[wheresmejumper]

4mm2 cable for a ring main is overkill anyway,only if the circuit was mega mega long

 

[tigler]

i said in my original post it mightnt be common practice in the uk^^

the only reason i wire ring main circuits in a domestic supply is to cater for a large number of sockets permitted in a 100m squared area,but still on a 20 amp rated supply

Didn't notice you were in Ireland but why would practice over there using the same hardware be different to the UK ?
BTW, I've just read that most 13 amp twin sockets according to BS1363 are rated at 20 amps combined load. Would be interesting to do a controlled load test at 26 and 32 amps to determine the heating affect .

 

[wheresmejumper]

our rules state(i think,cant find them now) that you can rate a ring main to 40 amps,as you have two 2.5mm cable feeding each socket,(2 x 20 amps current carrying capacity)

just not that common in domestic installations

 

[CRB]

These give some idea !! :-

https://www.youtube.com/watch?v=-iClXrd50Z8

No sound on this next one but it's bl00dy nasty !!!

https://www.youtube.com/watch?v=W6Im7PLduwc

Shudddderrrr

I am always careful with electricity and with good reason.

 

[tigler]

4mm2 cable for a ring main is overkill anyway,only if the circuit was mega mega long

Hmmm, not too sure about that, current rating for 2.5 seems to vary between 23 and 27 amps depending on manufacturer.

 

[wheresmejumper]

but you have 2 supplies feeding each circuit
which equates to 1/2 the resistance of the cable,which doubles the current carrying capacity

 

[tigler]

but you have 2 supplies feeding each circuit
which equates to 1/2 the resistance of the cable,which doubles the current carrying capacity

True. I'm only stating recommended working practice in the UK though. I think it's playing safe with power demands in modern homes now.

 

[wheresmejumper]

really the rules are very similar(and change every year!),but practice and habits dont change as frequently from county to county,region to region,as long as they dont break any rules

 

[wheresmejumper]

and its just an indicator of what people mean when they say 'leave it to an electrician if you dont know....'

 

[tigler]

I always find it strange how many technically very capable people find electrics and electronics so much of a mystery !! Even before I trained in electronics I was doing electrical stuff in my parents house. It's easy really. Power transmission and 3 phase is a bit more difficult to grasp with power factors, diversity etc but I reckon most tech minded people could pick it up pretty quickly.

 

[Drains]

I always find it strange how many technically very capable people find electrics and electronics so much of a mystery !! Even before I trained in electronics I was doing electrical stuff in my parents house. It's easy really. Power transmission and 3 phase is a bit more difficult to grasp with power factors, diversity etc but I reckon most tech minded people could pick it up pretty quickly.

I think the issue is not the basic knowledge of the theory of electrics (I find electronics a bit more obscure), as it's not hard to work out current/resistance/cable capacities etc. What's more specialist is knowledge of best practice. Anyone can wire up a house with a bit of practical knowledge, but doing it correctly with an in-depth understanding of the risks (and let's face it, regulations), is specialist.

The other practical point is that a correct installation will be efficient in terms of materials, but still safe. If Johnny DIYer is doing it to be safe then he'll probably spend more on cable etc. From the posts above I gather that cabling and fusing is rated by the likelihood of loads, not the actual load capacity of the number of outlets.

It's an issue I have at work (though the other way round) with our occasional Sparky who won't put in more 3-phase sockets than the cable/breakers/incomer will carry, regardless of the fact that inly two machines are likely to be running at one time. It's fantastically inconvenient and I think he's wrong, but it's not up to me.