Patrick asks this question, which is about a TT-earthed housing development, meter boxes, consumer unit tails, and the size of these. It has been answered by the NICEIC (Theme – Distribution Equipment including Cable Management):
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Question: I live on a Park Home site where up to now, a very dubious situation exists for the supply of electricity to each home - and all earthing is TT.
This is now being rectified by Western Power putting in new supplies to each home, and where a meter box exists in the wall of the home, a new supply is being taken direct to it, which is fine.
Where a home does not have a meter box, a box is being fitted at the boundary of the property and then it is up to the site electrician to connect the consumer unit to the double-pole switch in the meter box that Western Power has fitted.
My question is what size should the cable be?
The tails from the consumer unit are 25mm; the meter to cut out and switch are 25mm. My view is that it should be 25mm as the cut out fuse is 100A. There must be a length limitation on the cable because, assuming a short at the consumer unit, sufficient current must flow to blow the cut out fuse within the specified time.
Also, should the earthing be left TT or changed to PME, which is provided by Western Power? The homes would be compliant because of the earth cable sizes being for PME, even though they are currently TT.
Again, my view is the earth cable should form part of the SWA used to connect the meter box to the consumer unit in the case of the remote meter boxes. The SWA should be earthed, but not used for the main earth. This cable is left lying on the concrete pad that forms the base of the home and is protected by a brick wall surrounding the home it is not fixed.
What do you think?
Answer: The cable supplying the unit should be of an adequate size to provide capacity for the maximum demand for the home.
The protective device must be capable of providing overload and fault protection, and also take into consideration volt drop. A 25mm cable would be a standard size for a 100A supply – however, all of the factors need to be taken into consideration before a final cable size is determined. This decision should be made by the designer of the installation.
Before using the PME earthing facility supplied, we would suggest that the designer carries out a risk assessment to determine whether an equipotential zone can be established inside the unit, and that any metalwork of the unit and the mass of earth are not simultaneously accessible.
The type of cable used would, again, be the decision of the designer. SWA is a common cable used in this situation.
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