Ray asks this question about whether the sub-mains arrangement is acceptable for the loading of the new ‘eco-house’ he’s working on. It has been answered by the ECA (Theme – Distribution Equipment including Cable Management):
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Question: I attended a job today – a new build property. It has 10mm2 three-core SWA cable running from the DNO cutout to the house (about 20m).
There is also a 4kW solar PV system installed, plus two 3kW immersion heaters, an electric induction hob (8kW) and an electric oven (3kW). All lights are LED. There is no heating system as this home is an ‘eco house’ fitted with a heat recovery system (6A)?.
Will the 10mm2 sub-mains be ok?
Answer: A 10mm multicore 90oC 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; 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’s ‘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 installation will be a constant load (refer to the installer / manufacturer); then the lighting load must be added to all of that.
So, for the total loading, we have figures for - with diversity applied – 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 system loop impedance (Zs) value limitation.
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