Design current ring main

[GaryMo]

Not any appliance, surely, only fixed or non-mobile ones? Or is it only ovens & hobs? It would be a tad impractical to have to provide 3 dedicated circuits for the kettle, toaster & iron...

It's advised that cookers, ovens and hobs with a rating over 2kW are connected to their own dedicated radial circuit.

Also it's advised not to supply an immersion heater, comprehensive electric space heating or loads with a similar profile to a ring final circuit.

 

[1john]

Hi Gentlemen
right if a lighting circuit has 10 lights 100watt each = 1000watt divided by 240 volts = 4.2 amps rounded up, so a 6 amp mcb to be used i believe
this to be correct

so if i got 15 double sockets on 70 meters of 2.5mm t/e how do i work out the design current.

thank you

basically you need to work out the current carryign capacity of the cable after all factors have been applied and select your protective device from there, if you deem that after that calculation that the protective device would be too small to serve the circuit then you need to re-calculate with a bigger cable.

Circuit design shouldn't start with the cable size, it should start with the assumed load / design current, which as stated previous is very hard to do on a socket circuit. You have limited your design immediatly by stating that you are using 2.5mm which AT BEST is good for 27 amps and can quite quickly become de-rated to under 20Amps, im which case it is NOT suitable to be used to create a rinf final circuit (ring main).

Things to consider are how the cable will be ran, will it be in trunking or conduit, will it be ran with other cables, will it be touching them, will it be ran in hot locations, will it be surrounded by insulation, all of these factors limit the ability for the cable to dissipate heat which mean that if you dont select the correct cable for use under it's intended load that the cable can become very hot and infact cause a fire!!

design current > protective device > choose assumed correct cable size > adiabatic equation > check cable size will disconnect > select adequate cable size and re check with adiabatic equation if required.

 

[mikhailfaradayski]

15 double sockets on 70 meters of 2.5mm t/e is what i am try work out
on a kitchen and utility room

15 x 2 x 13amps = 390amps. Easy!

then 390A x 'diversity factor' = 32A, magic

If you are using a ring final on a 32A mcb, there is guidance to suggest that this circuit should serve no more than 100m^2 of floor space.

I guess this limitation serves to replace any diversity calculations, perhaps using the thought that you would only expect to see a certain amount of appliances in use in a limited space, in 'normal' circumstances.

Also, if you embrace the whole spirit of using 'standard circuits' and follow the guidance that fixed appliances over 2kW should be supplied by its own dedicated radial circuit, then its not very likely that you will even see a ring final pulling near 32A

But of course, this is just 'guidance' to a non-mandatory set of regulations.
(just thought id drop that little chestnut in, for a laugh - cue uk schuko installers and wannabe lawyers)
It is still the responsibilty of the designer to take a view on the intended use of the installation and install something appropriate.

IMO, ring final circuits are becoming less practical due to the greater requiremnen for circuit segregation.

Radials - its the future!

 

[Niblok]

so lates say washer,dryer,iron and boiler are on in utility room from looking at my appliances i get 24 amp so putting a kettle on in kitchen as well would make 33 amp not good

so put utility room on a radial 2.5 t/e rated at 27 amp 32amp breaker cable approx 10m

 

[LoveRocket]

so put utility room on a radial 2.5 t/e rated at 27 amp 32amp breaker cable approx 10m

Erm, have you read anything that's been said in this thread?

How can you have a circuit cabled with a current carrying capacity of 27A (and that's if it's clipped direct!) on a 32A protective device?

If you're going for a radial (which is a great idea) and want to use 2.5mm, then you'd be looking at 20A or 25A MAX (depending on installation method)

I really am concerned that you think it acceptable to run a 2.5mm radial on a 32A MCB.

 

[ban-all-sheds]

2.5 t/e rated at 27 amp

32amp breaker

 

[Niblok]

mmmmm so if all the appliance are on using 24 amps would it not trip a 20 amp breaker

 

[LoveRocket]

YOU CAN'T USE A PROTECTIVE DEVICE WITH A HIGHER RATING THAN THE CABLE!

 

[Niblok]

yes sorry the breaker needs to trip before the cable set on fire

 

[ban-all-sheds]

mmmmm so if all the appliance are on using 24 amps would it not trip a 20 amp breaker

See p249 of BS 7671:2008 for the answer to that question.

 

[mikhailfaradayski]

so lates say washer,dryer,iron and boiler are on in utility room from looking at my appliances i get 24 amp

maximum demand. This, practically speaking, will probably never happen (due to thermostats cycling the load)

so putting a kettle on in kitchen as well would make 33 amp not good

its not quite that straight forward

so put utility room on a radial 2.5 t/e rated at 27 amp 32amp breaker cable approx 10m

Nope, don't do that, thats bad

if you want to use 2.5mm for a radial, limit the mcb to 20A max.
(you probably could use a 25A, but that means having more knowledge of installation methods and de-rating factors, if you don't already know all that good stuff, avoid!!!)

 

[LoveRocket]

yes sorry the breaker needs to trip before the cable set on fire

That's what I'm talking 'bout!

Consider a 4mm radial on a 25A MCB.

In fact, try to use all radials - ring circuits are a bad idea, especially in high-load situations like kitchens.

 

[OwainDIYer]

The fact is, socket circuits are impossible to calculate design currents for.
This is due to the nature of them having ANY number of sockets on them, with ANY rating of appliances plugged into them.

There's no need to calculate diversity for a 32A ring final circuit - it's a standard circuit and the design current is effectively 32A (with 20A per leg or unfused spur feeding one twin 13A socket).

Obviously you have to establish whether a 32A ring final circuit is *appropriate* for the purpose.

On the other hand if you have a small workshop and are putting in 16A or 32A sockets for a portable tool, you might have several sockets on one 16A or 32A radial, with a design current of 16A or 32A on the grounds that there will only ever be one tool and it will be moved around for convenience.

 

[aptsys]

YOU CAN'T USE A PROTECTIVE DEVICE WITH A HIGHER RATING THAN THE CABLE!

Strictly speaking you can.

 

[ban-all-sheds]

This thread has always been about final circuits with an indeterminate number of BS 1363 socket outlets, so no, you may not.