Circuit design - voltage drop

[pencilpusher]

Hi all

Scenario:

Small 2 bed terraced house. Ze 0.70 ohms on a TNS. One RFC ,PD 32amp type B. RCD protected.

Outdoor socket and lighting required in small rear garden.

Spur from kitchen socket to take power outside for one IP66 double socket and from there to 3 lights via FCU IP66 (3 amps max for lighting) Wired in 2.5mm three core SWA. Last part of lighting wired in flex.

I am after advice on VD calcs. I am scratching my head re Ib. The PD at the origin is rated 32 amps, it is fused down to 13 amp as the spur and then down again to 3 amps for the lighting. Its one radial circuit from the RFC to the lighting points.

I am after advice on Ib and if it's 3% or 5% as its a RFC and not a lighting circuit or as a radial is it worked out with both?? There are lights on it but fused down.

This is how this is to be done. This will not be taken from a spare way in the CU.

Brain ache with the calcs here though!

I like to understand everything you see.

Thanks

PP

 

[telectrix]

unless you are loading up the external supply, i can't see any problem. bang it in, the measure the volts with full load applied.

 

[pencilpusher]

Cheers Tel, that was bloody quick. I did think of that and I am generally happy with it but am wanting to know how this circuit is viewed from a VD angle.

PP

 

[telectrix]

so use your design current and work it out for the cable you have selected. a lot of these domestic jobs are over thought. but it's good to actually calculate things like VD and Zs before installing.

 

[pencilpusher]

Tel, this is confusing me!

You suggest I should bang it in and then measure on full load and then to calculate VD and ZS before installing!? I am trying to calculate it.

My question was what are the views on the design current you tell me to use? 32amp for PD, 13 amp for spur or 3 amp for lighting or a mixture of all of them?

Cheers

PP

 

[kingeri]

It's no different to any situation where there is a change in CSA in the circuit, e.g. a fused spur off a ring for a boiler or light. Calculate in sections. TBH I wouldn't worry about it too much in the environment you're describing....

 

[telectrix]

those are my thoughts. a short run of 2.5mm with a low loading will not give any appreciable volt drop. it's not worth bothering calculating unless you're installing a long run with a load close to the ccc of the cable.

 

[Rockingit]

If you're really bothered about doing calcs beforehand, just allow for a 3kw load at the socket and 120W for the lamp as your load, then use the smallest CC cable over the full run distance - so that's 16A for say 30 to 40m. Do the maths yourself, but there's a pint says a 2.5mm T&E cable will cope with that.

 

[Guest55]

Tel, this is confusing me!

My question was what are the views on the design current you tell me to use? 32amp for PD, 13 amp for spur or 3 amp for lighting or a mixture of all of them?

Cheers

PP

Well the 32a mcb isnt any sort of design current , its the rating of the cpd , which has to be higher than , or equal to , Ib.
your design current is the load that can be reasonably be expected to be drawn from both the socket and lights combined together with diversity applied.