# Design Current Calculation

I know the 16th edition on site guide gives advice on the area a ring main may cover, the size of cable and the size of protecion device, however I am going to re-wire a house in spain and would like to perform some calculations for cable and protection device selection.

I understand that Ib<In<Iz, where Ib is the design current, In is the protection device and Iz is the cable rating, please correct me if wrong. I am assuming the design current is not the maximum current as this would be massive, do I apply some diversity factors or do I make assumptions based on what appliances will be on at any one instance.

If I wire using a ring main, I am assuming that the load is split, how can I then calculate the split currents.

Also the regulations state that a cable length of 50m is acceptable, is this to restrict the number of sockets or is this related to volt drops across the cable length.

spud
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Hi Spud, you do like to get complicated!!!
The whole purpose of the domestic ring circuit with it's inbuilt limits ie cable size, floor area, MCB max value is so you don't have to work all this out!!!
By stating you need one ring per 100m2, & you must try to spread the load evenly over the circuits you automatically apply diversity factors, & as regs say no other calcs reqd.
I assume this rewire is going to use UK sockets for Brits. to use as no one else uses rings & you certainly can't use continental 'Shuko' type unfused outlet on a ring!!
Spanish rules & wiring methods totally different to ours, using flex conduits looped from 'arquettas' JB's
all fed from 4pole RCD to banks of MCB's each feeding radial circuits.
Also as of early this year Spain now has the equivelent to Part P regs, unless you are licenced by local authority you could be in trouble if DIYing electrics, all will be well until 'Civiana' asked for meter connection.....
regards SPARX
sparx
Rank: Project Manager
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Location: The fifth continent.

The reasons I was asking the questions was because I assumed that the guidelines given are based on 240volts and was worried that they would not be compatable with the Spanish supply.

I hadn't intended to use UK sockets as I thought this would be a give away that it hadn't been wired to Spanish regs, but I was not aware that you couldn't put the unfused outlet's onto a ring, so thanks for that. Is it easy to explain why this is the case.

I am using single core conduit cable for the rewire as advised by someone who lives in Spain, and they are currently attempting to translate what I believe is their regulations. What are 'arquettas' JB's, is this a type of junction box.

The Spanish wiring seems to be very condensed where power and lighting seem to be on the same circuit. I had intended to split circuits up, as in the UK, which should be less complicated and easier to fault find. I can not see this being a problem, but I will take on board your information regarding the ring main.

I had intended to get the wiring certified once completed.
spud
Rank: Labourer
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Joined: Tue Jun 26, 2007 10:01 am

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