Design current ring main

[Niblok]

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

 

[Spark123]

It is a real pain to calculate design current for a RFC as you cannot say what will be plugged in where - unless all equipment is fixed. What are you trying to work out?
The protective device is normally a 30A or 32A for a RFC.

 

[Niblok]

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

 

[ban-all-sheds]

Say you managed to work it out.

What then?

 

[Steve]

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

Its a ring final. Why would you put this circuit on anything other than a 32A MCB? IMO the "design current" is irrelevant and impossible to calculate here.

 

[ban-all-sheds]

Its a ring final. Why would you put this circuit on anything other than a 32A MCB?

And if you did, on what basis would you claim compliance with BS 7671?

 

[Chri5]

If you are considering an overload situation then only you will be able to decide.

If you consider the items to be plugged in to the circuit, the use level and time of use in tandem with other items on in the circuit that should give you a heads up.

If the utility room is running washing machine, tumble drier and a heavy duty steam iron and at the same time your are making use of the toaster, sandwich maker, kettle and microwave you would have an overload issue.

Either consider a separate circuit for each room or live with the possibility that such things happen occasionally.

Better still dump the ring and provide 2 circuits- 4mm radials to each area

 

[LoveRocket]

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!

 

[OwainDIYer]

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

Its a ring final. Why would you put this circuit on anything other than a 32A MCB? IMO the "design current" is irrelevant and impossible to calculate here.

Is the design current not then 32A? It can't be any less, because the load could be increased at any time, and it can't be any more because the circuit would not be adequate.

 

[jaysean1]

If I can remember from college you would use diversity

 

[Steve]

If I can remember from college you would use diversity

Go on then.... show us your working....

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.

The design current could be anything from 2 watts to 7360 watts, depending on the connected devices at that time.

So we just have circuit designs - radials of 16/20A on 2.5mm, radials of 32A on 4mm, rings of 32A on 2.5mm. Any other ring final circuit is not recognised in the regs.

 

[Spark123]

32A 4mm ring final
32A 1.5mm ring final in pyro

 

[ban-all-sheds]

radials of 16/20A on 2.5mm, radials of 32A on 4mm, rings of 32A on 2.5mm. Any other ring final circuit is not recognised in the regs.

1) Table 7.1 in the OSG lists 5, 6, 10, 15, 16, 20, 25, 30, 32, 40 & 45A radials with cable sizes from 1mm² to 16mm².

2) You're always free to design any circuit that works for Ib ≤ In ≤ Iz and voltage drop.

3) The only fixed aspects of ring finals are the 30/32A OPD and an Iz of at least 20A - if that means 6mm² then so be it.

 

[securespark]

As 7671 advises any appliance >2kW has its own circuit, that would lessen the load on a general socket outlet circuit.

 

[ban-all-sheds]

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...