by ericmark »
Sun Apr 13, 2008 3:40 pm
Appendix 10 of BS7671:2008 (17th Edition) covers parallel conductors full of do’s and don’t and calculations. Basically you must ensure neither cable can become over loaded. So in general must be of equal length and size. 11kw is 47.8 amps and 6mm² cable clipped to wooden joist etc with no more than 100mm insulation is method 100 from table 4A2 so rated at 34 amp table 4D5 but how does it get to loft space if through insulated stud wall pushed down through centre then we move to method 103 and then 6mm² drops to 23.5 amp put two together then rating factor of something like 0.8 so down to 18.8 amp per cable giving 37.6 amp for pair and I haven’t started on air temperatures. But it become apparent you must know the route the cables take as although they could be big enough for use in loft space you have not said how they get there. If all the way buried in plaster or loft then should be OK. At 7.3mV/A/m and 14 meters two cables only 2.5 volts volt drop so that’s OK but each cable will only be approx 0.1Ω each and any slight variation could cause one cable to overload using two MCB’s will ensure if one does overload it will trip out, but the MCB’s have resistance and could be the cause of imbalance, so I would tend to use one, but use a clamp on ammeter on first use to check balance which needs to be within 10%. The regulations start at 433.4 Overload protection of conductors in parallel. Out of interest I put the figures you give into my excel program to select cable sizes and it selects 16mm² not 10mm² at 46 amp selects 10mm² so two 6mm² cables would be better than 10mm² and since I only list up to 16mm² in look-up tables if insulation is over 100mm or if in centre of stud wall methods 101 and 103 it fails. I look at regulation 433.1.1 and I read this as saying 1.45 times the lowest of the current-carrying capacity of each cable so 49.3amp with method 100 so 50 amp MCB would seem to fit the bill. Although I realise you may not be able to read the 17th Edition wiring regulations we give the regulations so anyone else reading my reply can verify in case I have miss read one. I would use the two existing cables after being satisfied the route was OK but would include in my installation certificate the clip on ammeter reading as well as the earth loop impedance etc. This way in the future electricians can compare readings when checking for any bad connections. Prospective short circuit current between line and neutral is also important as electricians may not have a clip on ammeter but will have psc meter when doing the 10 year or change of ownership inspection and tests. Since the circuits are already there it is possible you will not need Part P but I recommend you follow links and read yourself as sorting out latter can as we have seen on here be quite a problem.
