SWA 4sq.mm electrical supply to new Garden Cabin

by Editor on May 25, 2013

An electrician’s question about a SWA 4sq.mm electrical supply to new Garden Cabin with RCD protection.

Q- I’m a newbie and going to register with NICIEIC. Have taken a job to power up a cabin and submit as one of the two job’s required. An existing SWA (20ft) is partially buried and partially surface run under the cabin floor. Which reduces the amp’s down from 40Amp to 35/30Amp at the cabins CU. my confusion is I’ve used a 30ma RCBO at the main CU (TNCS) on the non RCD bank of the CU to supply the sub main to cabin and a 63Amp double pole 100ma  RCD unit in cabin. Total Ib is 30a. A 20a ring main and a 6 amp lighting circuit with two smoke detectors of the lighting cct. There a point of isolation for the smoke detectors. I’ve earthed the SW at the main CU but not at the cabin. I have supplied the cabin with it’s own TT system which the ZE reads 93 ohms. In your opinion do the figures add up. I’m not sure I have the 30ma RCBO and 100ma CU  round the wright way.

A-The TT earth connection sounds good at less than 200 ohms. For discrimination between the two RCDs you would normally put the highest rated at the supply end so that the 30mA in the cabin would trip first if an earth fault occurred at that end. It could be rather a nuisance having to go to the house to reset the 30mA.   The cabin supply RCD protection should be ok as a 100mA 32A or time delayed 30mA (S).  If the cabin sockets are wired in 2.5mm cable then they could be configured as a 16 amp radial.


{ 1 comment… read it below or add one }

Rusty Curry June 6, 2013 at 02:46

Your design must ensure the maximum voltage drop allowed between source and point of use is not exceeded when at full load. Under the 16th edition wiring regs this was usually defined as 4% of the nominal supply voltage (about 9.2V at 230V AC). Under the 17th edition it has been relaxed to 5% for most circuits, but tightened to 3% for lighting circuits.

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