Can anyway help with Radial circuit maximum floor area help?

[King84]

Hi everyone

I am doing level3 design andI have a circuit to design but I have confusion with radia circuit maximum floor area to understand?

I have a socket to feed on the design and floor area according to my knowledge is Length multiply width so in that case it exceeds 75 m square so I am unsure what Circuit breaker to use?

How we calculate floor area? Does it start from CCU or it only counts where sockets are located which means from first socket to last?
as in ON site guide it says we can use 2.5 cable for radial but when I work out with grouping factor and ambient temperature then I am compelled to use big cable but still unsure what circuit breaker to choose and the culprit is maximum floor area?
Any help is much appreciated thanks

 

[radiohead]

If for example the sockets are to serve two offices then the floor area served would be the combined floor area of the two offices, you would not include the passageway that the wiring ran through on it's way to the offices.
As far as radial circuits go for 13a sockets you can use standard circuit arrangements, ie a 20a radial in 2.5mm, or a 32a radial in 4.0mm. The OSG gives maximum length of run for those circuits. .Obviously in each case either a 20a or 32a device would be chosen.With a standard circuit arrangement no calculation is required as long as the circuit length does not exceed the maximum and the circuit is not installed in a way that would require the application of rating factors, ie ambient temp,insulation or an SE fuse.
If the radial simply serves one socket, then you can ignore floor area, as such an arrangement is clearly not intended to serve multiple usage in a large area

 

[Ian1981]

The OSG tables for maximum lengths of circuits is just a guide, it gives pessimistic lengths due to assumed loading and the voltage being exactly at the minimum required ie either 3% or 5% of 230v.
If you know the loading then these ‘max’ lengths can be manipulated as the volt drop will be less.
If the cable/circuit is known to carry less than its ccc then the volt drop will be less due to the cable resistance being less( lower conductor temperature) .
See table 6 page 381&382 in bs7671.

Obviously there’s things like maximum earth fault loop impedance etc to consider as well.

 

[King84]

Thank you for reply guys, I really appreciate your help.

This idea is abit confusing for me so let me post the attachment of my design and one of the circuit I highlighted is socket that is feeding sockets around the pool area which is L3/C5.

I have also attached picture of the calculations for this circuit and as this circuit also have requirement of Ambient temperature Ca which is 0.91 for 40 degrees for 90 Thermosetting and also the requirement is Cg 3 circuits which is 0.70.
So when I put all these into calculations I end up with 16 mm square cable which you can see from my calculation.

Could you please point out what am I missing from my calculation

 

[Ian1981]

Thank you for reply guys, I really appreciate your help.

This idea is abit confusing for me so let me post the attachment of my design and one of the circuit I highlighted is socket that is feeding sockets around the pool area which is L3/C5.

I have also attached picture of the calculations for this circuit and as this circuit also have requirement of Ambient temperature Ca which is 0.91 for 40 degrees for 90 Thermosetting and also the requirement is Cg 3 circuits which is 0.70.
So when I put all these into calculations I end up with 16 mm square cable which you can see from my calculation.

Could you please point out what am I missing from my calculation

Look to change your overall design current, I doubt your going to need a load of 32 amps.
Also your chosen cable choice should reflect the maximum operating temperature of 70 degrees and not 90 unless any switchgear and accessories can be connected to such a cable operating at this temperature.
Also bear in mind that 90 degree cables if running at that temperature need to be segregated from 70 degree operating cables

 

[King84]

Look to change your overall design current, I doubt your going to need a load of 32 amps.
Also your chosen cable choice should reflect the maximum operating temperature of 70 degrees and not 90 unless any switchgear and accessories can be connected to such a cable operating at this temperature.
Also bear in mind that 90 degree cables if running at that temperature need to be segregated from 70 degree operating cables

Thanks for the reply however, I was given design specification to choose and it was not my decision to select 90 degree thermosetting cable.

The specification was in the pool side at 40 ambient temperature, your circuit should be Cg 3 which was 0.70 factor in conduit and also the cable needs to be 90degree thermosetting which gives us 0.91 Ca at 40 temperature..

so by considering these factors I had to choose 16mm cable or do I have another choice to reduce the cable size by using these specifications?

 

[SJD]

Looking at the table you posted, I don't understand how you worked out that you need 16mm2 cable, which is quite a ridiculously large size for what I assume are 13A sockets? Possibly the working out is under the blanked-off boxes?

 

[Ian1981]

Thanks for the reply however, I was given design specification to choose and it was not my decision to select 90 degree thermosetting cable.

The specification was in the pool side at 40 ambient temperature, your circuit should be Cg 3 which was 0.70 factor in conduit and also the cable needs to be 90degree thermosetting which gives us 0.91 Ca at 40 temperature..

so by considering these factors I had to choose 16mm cable or do I have another choice to reduce the cable size by using these specifications?

Where did you get 75 meters from as your spec or circuit details in the 2nd picture says around 25.70meters?

 

[pc1966]

When you say "a socket to feed" is that a single 13A outlet? A double 13A outlet? a 32A commando? A few 13A at different places?

 

[King84]

Thank you guys for taking some time.
I have attached Pool scenario picture along with the specification that I had to follow. Also the calculations I did to choose the cable.

This is a poolside design as you can see from the attachment and Ze was 0.11 ohm also I have attached the specification due to which I had to use thermosetting cable 90 degrees at 40 ambient temperature that would give me 0.91 factor.

Also you can see from specification I have highlighted that circuit should be grouped with nomore than 2 other circuits so that makes 3 circuits and cg would be 0.70.

Considering these factors It which is In/Ca*Cg would give me 50.24 due to which I had to choose the next available IT which was 68 for 16mm cable.

I have worked out my calculations please clarify what can I do or this is the correct procedure in this instance?

And yes these are normal socket outlets but by considering the specification they have given how can I reduce the cable size and In?