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CSA on a Radial, what length would you reach with a 4mm?

Don246

So I have been down a rabbit hole for some time now and need a bit of clarification.

Possibly someone might be able to explain Fig 15B - Radial final circuit arrangement

it's suggested that 4.0/1.5mm cable could serve a 75 m2 on a 30A or 32A CB

what would the length be of that cable?

when I perform a voltage drop calculation the length of the cable is 32m which is no where near 75m.

The OSG mentions that the max length for a radial on a 32A CB is 43m with an assumed load of 26A.

Anyone able explain how 75 m2 in fig 15B has been worked out, surely there should be a not stating what length cable its equivalent to.

on a separate note what if the radial circuit was supplying 1 x 13A socket, is there an equation to work out the overload allowance of a circuit or is it simply Ib < In < Iz?

if so then what would the maximum load be expected on a 13A socket? there's nothing stopping someone plugging in an extension lead or adaptor to increase the load.

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  • when I perform a voltage drop calculation the length of the cable is 32m

    I presume you're assuming that the entire 32A load is concentrated all at the furthest point of the circuit - there are other valid possibilities - e.g. the load could be evenly distributed along the circuit - which yields much lower maximum voltage drops (as there more remote lengths of cable are then carrying less current). Or something in between. If you look back over past versions of the OSG you'll see that the recommended circuit lengths can very quite considerably - not because maths or physics as changed (although the precise requirements for v.d. are more than a little arbitrary and have varied over the years) but mainly because of the assumptions, especially about the distribution of loads, have been "refined" over time.

       - Andy.

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  • when I perform a voltage drop calculation the length of the cable is 32m

    I presume you're assuming that the entire 32A load is concentrated all at the furthest point of the circuit - there are other valid possibilities - e.g. the load could be evenly distributed along the circuit - which yields much lower maximum voltage drops (as there more remote lengths of cable are then carrying less current). Or something in between. If you look back over past versions of the OSG you'll see that the recommended circuit lengths can very quite considerably - not because maths or physics as changed (although the precise requirements for v.d. are more than a little arbitrary and have varied over the years) but mainly because of the assumptions, especially about the distribution of loads, have been "refined" over time.

       - Andy.

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