Do you have a burning issue you'd like to discuss on Voltimum and to your industry peers? Get it off your chest and give us an email. This month's letter is in response to September's Speakers' Corner and is from Senior Electrical Design Engineer, Bob Dawson.
My experience of new build houses built in the last twenty five years is that ring main cables are most likely to be installed in the ground floor ceiling/floor void, rather than the roof void (to save cable).
However where this is not the case, my recommendation, to save expensive rewiring of otherwise sound circuits, would be to downgrade the protective device from 32A to 25A. This would offer least the same, if not better, protection to the down-rated cable. This would be a low cost option. The situation could them be reviewed to see if there were any tripping issue because of the load, before more expensive options were pursued.
Bob Dawson
Derry Building Services Ltd.
Original letter from Arthur Day (electrical contractor) partner, Kevin Day:
Having had a recent NICEIC annual inspection visit, discussions with the inspecting engineer made it very clear that many installations will no longer meet BS 7671 standards with regard to conductor size/ current carrying capacity (ccc) and protective device coordination. Many existing installations will have been installed to BS 7671 16th edition. For instance, a 32 amp ring main would normally have been wired in 2.5mm twin + 1.5 mm earth cable. Especially in a bungalow having a solid floor, this cable would have typically been covered with a thin layer of insulation and would have met the regulations with regards to coordination.
Many loft insulation firms have recently started upgrading loft insulation to 200mm with little or no regard to the effect said upgrading will have on electrical circuits. Because of the 0.51 factor which has to be applied to the ccc of the cable in this instance, it is clear that when using installation method 101, that the cable will only effectively carry around 12.5 amps. This results in a fairly well loaded ring main posing a potential fire risk (for this reason, a 4.0 mm twin + 2.5 mm earth cable would now usually be installed).
If a periodic inspection is carried out on the property, a deviation code of 2 would have to be applied to the ring circuit described and drastic action recommended to be taken to amend the situation (ie rewiring with larger cables), moving the cables out of the insulation or lifting the insulation above the cables. I am surprised that there seems to be no one taking action to prevent insulation installers from burying cables in ths manor or that the issue is not raised more frequently by building control inspectors.
I would be interested in feedback from voltimum readers. Are there similar experiences and any remedies which could be cheaply applied retrospectively by anyone finding themselves in this situation.
Sincerely,
Kevin Day (Arthur Day Electrical).