Random- how many sockets?

[guineafowl21]

So my dummy question - how come it's ok to have the single radial cable just daisy chaining loads of sockets? I must be missing something, as that feels to my tiny brain, a bit like a spur?

A radial circuit can be thought of as just that - spur-on-spur-on-spur. As long as the circuit breaker is rated correctly, the 2.5 can never be overloaded.

Radials have lower current ratings than equivalent rings. What rating you use (say, 20A or 16A) is partly governed by the loop impedance - essentially the resistance of the wiring that will limit fault current. You want a low enough ‘resistance’ to ensure about 5 X the rating of the breaker to flow for a typical set-up.

For example, a 20A B breaker would require about 100A to flow to disconnect reliably. A loop impedance tester will check this is possible.

 

[JohnPW]

I've got 7 available at my own workbench at work:

work lamp
baby food warmer for hide glue
hair dryer (for warming wood before gluing)
usb charger
rechargable battery charger (AA, AAA)
soldering iron (only plugged in when using)
workshop communal vacuum cleaner (only plugged in when using)

that's using two 4-socket extension leads, not wall sockets. One has an on/off switch so I can switch the whole lot off at the end of the day.

Obviously the workshop fixed machines have their own sockets.

 

[Phil Pascoe]

As an aside - years ago I had red plug tops and green ones, green ones were used on things meant never to be switched off and red ones on things never to be left plugged in when not in use. I thought this quite sensible, but I've not seen the coloured ones in years.

 

[Jacob]

I've got a ring of 8 double sockets around my workshop, on a board at about 40" high. I've fixed a 4" shelf to the top edge of the board which protects the sockets from things being leant or falling against them, and is a useful shelf - battery charger, radio, other bits n bobs all above work surface height.

 

[Jonm]

I'll preface this by saying akirk above probably knows a little more than me about electrical installations, so bear with me!

I kinda understand the logic of ring main circuits. But I've just read, (before reading MikeJhn above about socket limits in France, so maybe irrelevant question now), that radials can have unlimited sockets on 2.5mm cable (under 50sqm area, I think it said). It did explain this would be for normal home type usage, not large draw such as a line of tumble driers.

So my dummy question - how come it's ok to have the single radial cable just daisy chaining loads of sockets? I must be missing something, as that feels to my tiny brain, a bit like a spur?

If it's just too complicated to explain, just tell me to wind my neck in and concentrate on something I understand

Basically if you are using 2.5mm of cable as a radial you would put in say a 16amp Breaker. If you install the same cable but as a ring main then the electricity flows through two wires to the socket so you would put in say a 32 amp breaker. It is then a case of the number, size and type of room which determines how many circuits you put in. The assumption is that just because a bedroom has six sockets you will not draw more current than one with two sockets.

In the case of the French standard you referred to, if the room is a bedroom with a 16 amp radial then it will normally be fine, it would just about take a large fan heater and hair drier. If the room is a kitchen with fan heater, washing machine, tumble drier, electric kettle, toaster and microwave then it will be a full time job resetting the breaker?

In uk if you had three bedrooms on 32 amp ring main, and switched on a 3 kw fan heater in each room (36 amps) then the breaker would probably trip.

 

[Spectric]

But please note that BS7671 is not a "statutory instrument" therefore not the electrical regulations as they are often referred to, but a British Standard.

Yes thats always been a double edged sword, there is nothing that says you must follow this guidance but when you electrocute someone and the HSE takes you to court they will ask you if you have followed Bs7671 and if not you better have a bomb proof answer otherwise you will get a free holiday.

When designing an installation you often run 2.5 radials fed from a 20 amp PD to bedrooms and a 4.0 radial fed from a 32 amp PD to kitchens, utility rooms and living rooms and run ovens and fridges/freezers on their own circuits. But Industrial is much easier, no floorboards/carpets and wall chasing just trays, trunking and conduit, plus these days electric conduit benders and threaders without the tallow.

 

[Timthetangent]

4 x doubles and a gang of four on the ceiling. Workshop 18 feet by 9 and inherited from the previous owner (moved in last November). Using just the table saw, one Sonos speaker and an oil-filled radiator just before Christmas, it appeared to trip. On inspecting the consumer unit, neither MCB's not RCD's had tripped. After significant investigation, it transpires the whole workshop is powered from an external plug socket at the back of the house (all the wiring hidden under the deck). The workshop effectively is powered from an extension lead. It is similar to many other things within the new home. The previous owner was a very enthusiastic DIY merchant, which would have been lovely if that enthusiasm was matched by skills, knowledge and/or competence. It should have been sorted four weeks ago but the rather brilliant electrician who came to survey and quote for the job to be done properly has had a series of mishaps causing delays. Thankfully I have no deadlines to meet because until it's safe I won't be using it. Insurance companies come down quite hard on retired firefighters who set light to their own homes.

 

[Phil Pascoe]

We moved thirty years ago and I was painting everything white just to clean up. I came across a loop of 2.5mm where someone had obviously intended to put a socket. I said to my wife I'd finish the job then paint. The consumer unit was in the basement so I plugged in a radio and turned the volume up. I pulled the fuse I thought it was and the radio went off. I pulled them all and started to put them back .... it came on again. All the tails were mixed up - the immersion and the basement lights on one fuse, the cooker and the upstairs lights on another ............
I said I'd look at them a few days later when she went away on a course. I took all the tails out, checked what was what and replaced them, labelling the fuses properly. I was in the pub a few days later when a friend said well, one thing you won't have any problems with there is the electrics. Why's that? I asked. Because the guy you bought the house off was a qualified electrician.

 

[Jonm]

Radials have lower current ratings than equivalent rings. What rating you use (say, 20A or 16A) is partly governed by the loop impedance - essentially the resistance of the wiring that will limit fault current. You want a low enough ‘resistance’ to ensure about 5 X the rating of the breaker to flow for a typical set-up.

For example, a 20A B breaker would require about 100A to flow to disconnect reliably. A loop impedance tester will check this is possible.

Presumably you are talking here about disconnecting instantaneously under a short circuit. My understanding (I am not an electrician) is that for sustained loads the breaker should trip at 1.2 to 1.3 times the rated load. So for a 20 amp class B breaker it will trip under a sustained load of 24 amps to 26 amps but it will take a couple of hours to trip.
https://www.electricalclassroom.com/b-c-d-k-and-z-mcb-trip-curves/#1-class-b-trip-curve

 

[MikeJhn]

Yes thats always been a double edged sword, there is nothing that says you must follow this guidance but when you electrocute someone and the HSE takes you to court they will ask you if you have followed Bs7671 and if not you better have a bomb proof answer otherwise you will get a free holiday.

Not quite, the HSE will not ask any questions on BS7671, but their Barrister may ask what you did to ensure the installation was safe without leading you to an answer, but as you say your answer must be convincing or the holiday will ensue, or as is more common you will get a slap on the wrist and a fine, especially if you are a public house landlord in Essex.

 

[MikeJhn]

the immersion and the basement lights on one fuse, the cooker and the upstairs lights on another

Diversification is quite normal to ensure that if a fault occurs on one circuit the other circuit does not go off as well, useful on lights where the whole floor is not plunged into darkness, the advent of LED's has minimised the need now days as they are not prone to short circuits/overload when the go, unlike our friend the old tungsten bulb.

 

[Phil Pascoe]

Yes, but presumably not running lighting circuits off the largest fuses.

 

[Jonm]

Diversification is quite normal to ensure that if a fault occurs on one circuit the other circuit does not go off as well, useful on lights where the whole floor is not plunged into darkness, the advent of LED's has minimised the need now days as they are not prone to short circuits/overload when the go, unlike our friend the old tungsten bulb.

Lighting radial connected to 40 amp breaker for the cooker may be fine provided the lighting is wired with 6 sq. mm. cable and the fittings are similarly rated. Otherwise it looks like a recipe for an electrical fire to me in the event of their being a fault. My new house has 6 amp breakers on all lighting circuits.

 

[mccpe]

In the case of the French standard you referred to, if the room is a bedroom with a 16 amp radial then it will normally be fine, it would just about take a large fan heater and hair drier. If the room is a kitchen with fan heater, washing machine, tumble drier, electric kettle, toaster and microwave then it will be a full time job resetting the breaker?

The French regs have the tumble dryer and washing machine on independent 2.5mm circuits with a 20A breaker for each one. General purpose kitchen radials are 2.5mm, 20A breaker and max 6 sockets per circuit. I guess you might trip that with a fan heater, kettle, toaster & microwave, but the French regs are pretty sensible.

 

[Jonm]

The French regs have the tumble dryer and washing machine on independent 2.5mm circuits with a 20A breaker for each one. General purpose kitchen radials are 2.5mm, 20A breaker and max 6 sockets per circuit. I guess you might trip that with a fan heater, kettle, toaster & microwave, but the French regs are pretty sensible.

My post was in relation to a question about how radials worked which I was trying to explain. The reference to French standards and redials was included in the question.

Your response has clarified what the French standards actually say which is interesting and all looks very sensible. Presumably this results in a bit more cable and a lot more breakers. The advantage presumably is that if there is a break in the circuit then some sockets stop working and is fail safe. Our system of ring mains means that if there is a break in the circuit (like a broken or disconnected wire in a socket) we effectively have two radials on one breaker rated at twice the correct value, Ie not fail safe.

 

[NormanB]

I'll preface this by saying akirk above probably knows a little more than me about electrical installations, so bear with me!

I kinda understand the logic of ring main circuits. But I've just read, (before reading MikeJhn above about socket limits in France, so maybe irrelevant question now), that radials can have unlimited sockets on 2.5mm cable (under 50sqm area, I think it said). It did explain this would be for normal home type usage, not large draw such as a line of tumble driers.

So my dummy question - how come it's ok to have the single radial cable just daisy chaining loads of sockets? I must be missing something, as that feels to my tiny brain, a bit like a spur?

If it's just too complicated to explain, just tell me to wind my neck in and concentrate on something I understand

The basic reason is that the ring starts at MCB runs around looping into each socket and then returns to the same MCB - so if you plug say a 10 A load in the exact centre of the the ring (in terms of cable length) then 5 A goes down one leg and 5 Amps down the other and this was the reason they were introduced because the current loading is shared you can use a smaller cable. It is also a weakness because if a fault occurs and say one of the line conductors come adrift then the current drawn can exceed the now single cable current carrying capacity and cause overheating and potentially a fire. The current sharing nature of a ring allows, the protective device (MCB/RCBO) to be 32 Amp whereas with same cable size 2.5mm a radial would have a maximum of MCB of 20A to protect the cable.

 

[doctor Bob]

I had a count up today, 52. Thats the workshop and sprayshop and office and showroom / client area.