Advice for apprentice

[ziddygango]

Ok sorry if I sound clueless here but I am doing college work designing a circuit. I want to use a radial socket circuit in twin and earth that covers an area of 60 m^2, the osg says in table h2.1 that a 32 A breaker with minimum 4 mm^2 CSA cable.


So I then do calculated known loads and assumed 200W loads for rest of sockets then I do design current Ib, this gives me a value of 17 A, which would mean a 20 A breaker is more appropriate, I don't do diversity for cable size since the loads can be unpredictable on socket circuit. So I use derating factors for grouping etc etc and that gives me a Iz of about 22 A. Then from that with in table 4d5 in the regs with method 103 I get It of 32 A with a correspong CSA of 6 mm^2. So this is where I am confused. one do I use, will it be the reccomended OSG values of 32 A and 4 mm^2 for areas upto 75 m or is it what I calculted as 20 A breaker with 6 mm^2 which sounds really to large.



What takes priority, can I just use the osg reccomendations or do I need to use the sizes I calculated? I mean what are the assumptions used in the OSG reccomendations?



Is OSG for a particault method (clipped direct) and no de rating factors etc etc. It's very confusing? The question would apply if I chose a ring circuit, what takes priority the OSG reccomendations or your calculations?

 

[Rockingit]

OK, so..

Firstly the tables in the OSG and BS7671 are there to make our lives easier by having done most of the commonly found maths already - they don't replace actually applying the various formulas and equations which can often give slightly different results.

Secondly, we build what we design. So if the design requirements for our circuit are (x), we don't carry on an blindly building (y) because it happens to fit a table, we build (x), in accordance to the criteria of BS7671.

Thirdly - and I hope this is slightly more helpful to your design situation - there's nothing wrong in over-engineering a solution (to a point!). So if your tabulated values suggest using 4mm and 32A protection, that will be equally valid for a circuit that actual maths suggests you only need 2.5mm and 20A protection.

We make a policy on here of not doing actual homework for people, so have a think about these points and your question and get back with your thoughts.