Scott asks this question about a new eco house and whether the installed 10mm three-core SWA cable is adequate for the load. It has been answered by the ECA (Theme – Cables, Wiring and Wiring Accessories):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: Today I attended a job on a new build property. It has 10mm three-core SWA cable running from the DNO cutout to the house (about 20m).
There is a 4kW solar PV system, two 3kW immersion heaters, an electric induction hob (8kW) and an electric oven (3kW). All lights are LED. And there’s no heating system as it’s an ‘eco house’ having a heat recovery system (6A).
Will the 10mm sub mains be OK?
Answer: A 10mm multicore 90 degree centigrade thermosetting SWA cable clipped direct has a current carrying capacity (before other factors are applied) of 85A for one two-core cable and 73A for one three- or four-core cable.
In free air on perforated cable tray, these figures rise to 90A and 78A respectively (figures taken from Table 4E4A of BS 7671:2008, as amended).
The loadings are a question of diversity being applied to the sub-main circuit. A general table for domestic installations can be found in Appendix A of the IET On Site Guide.
The two immersion heaters will be the total full load of each = 2 x 3kW = 6kW = 27A; the oven and hob loading total 11kw = 48A, but 27A (after diversity is applied); the eco heating = 6A; the solar PV will be a constant load (refer to installer/manufacturer); then the lighting load must be added to all of that.
So, the total loading we have figures for, with diversity applied, is 60A.
You will need the additional load figures to be added to this, when they have been obtained. You will then have to compare this figure with the current rating of the cable, after it has been calculated, taking into consideration any applicable 'C' factors, voltage drop, fault level capability and Zs value limitation.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link. Experts from leading organisations provide online answers to your technical questions on a broad range of subjects. Our searchable database of existing Q & As now contains over 3,500 entries; you can browse through them here.