Instant water heater

7 Sep 2007
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United Kingdom
Want to fit a 12KW instant water heater to service 2 en-suites. Is 10mm cable sufficient? , the length is about 10m. What size MCB should I fit, the house master fuse is a 60 AMP, and I do not want to overload it if the cooker (electric is on) when this 12KW heater is switched on.
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10mm is right on the edge, if it is clipped direct or burried in ordinary building materials then it's ok. If it is in conduit or trunking then it is not. Also your protective device would be bigger than the cable rating with 10mm which opens up a whole can of worms of it's own. I would strongly reccomend using 16mm and keeping it well away from any insulation. At this size split con will probablly be easier to handle than T&E.

For protection I would reccomend a 60A switchfuse (e.g. ) henly'd off the tails.

Usually the best approach with service fuses is to wait until they blow. Usually when this happens they will uprate the fuse to 80A or 100A. Also a 60A fuse blows very slowly under a 100A load. Unfortunately if network upgrades are needed to give you a higher service fuse rating it will be expensive.

If you haven't already bought the equipment and the option is availible to you i would STRONGLY advise you to investigate using gas.

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