Will a MCB protect my additional circuit.

7 Sep 2007
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United Kingdom
Moved into an old terraced property recently and want to add an additional 9.5 KW Instant water heater for remote en-suite. I've chosen an instant heater as it will supply both shower and hand basin with hot water. The existing supply is through a 60 Amp fuse and the meter tails feed 2 fuse boxes, 1 off fuse box for ring main, electric oven and lights and 1 off fuse box for existing 10.5 KW shower and Electric Hob, fused at 45 Amp. I now want to add to the second fuse box a 9.5 KW Instant heater. This will bring this fuse box potential loading up to 109 Amp if all the appliances were switched on fully for several minutes. Is is permitted to route these 3 circuits through a 45 Amp MCB to protect the current line fuses and main fuse. Help please, have tried to find electrician to do the work but they all seem to be only interested in very large jobs for lots of money. :rolleyes:
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not an attractive idea.

When you say you have two small fuseboxes, I'm visualising a very old installation that is overdue for renewal. Can you post some pics?

if you have gas in the house, a Combi boiler or a Multipoint would be a better bet. You could also have a gas hob which is better than an electric one.

If not, consider a hot-water cylinder and immersion heater.

Do you understand the need to Notify this work, and pay for testing and inspection?
I have taken an image but do not know how to post it to this forum. If you could let me know you email i could send it direct??

It is a fairly old system, but was upgraded about 7 years ago when the original shower was fitted and the kitchen overhauled.

There is gas in the house but way at the other end, this is a very long old terraced property, same with hot water. The current hot water tank and header tank is on the middle floor and i want hot water on the top floor, this would mean moving the header tank. Please explain further "Multipoint". immersion heater good idea but its not instant. Maybe your suggestion of Combi boiler is the way forward. Thanks for the interest.
there are a few about posting pics on page 1 here


A Multipoint is a gas water heater (not a boiler) that will supply several taps but does not provide central heating, so they are cheaper to buy and install. they hang on the wall. You might have seen gas water heaters above the kitchen sink with one tap, a multipoint is a bit bigger and can feed several taps including a bath. they are not very widely used these days as almost everyone has a gas central heating boiler.

A hot water cylinder (whether heated by a boiler or am immersion) is "instant" as soon as you turn the tap on ;) provided the heater was turned on e.g. by a timer earlier. With a modern foam insulated cylinder, or a good jacket, they will stay hot for 24 hours. A gas boiler will take a cylinder from cold to hot in 20 minutes.

A gas boiler or combi can typically provide about 30kW of heat (equivalent to 120Amp of electricity), which is far more than you can get from any domestic electric shower or water heater, so they are faster, as well as gas being cheaper than electricity.
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I would go with JohnD's idea of a small, immersion heated cylinder but on one condition. You need to get its cold tank, which can be a small one, higher than the shower head, at least 1m higher and preferably more. The cylinder itself can be on the floor in the bottom of a nice little airing cupboard.

All you need is a mains water supply, a 15 amp radial (or, at a pinch, a spur off an existing, lightly loaded ring) and an overflow. You use a mixer shower with its cold supply taken from the same tank as the cylinder. The basin hot comes from the same cylinder but who else will be using it while you're in the shower? Probably no-one. This means none of those annoying 'cold' spells when somebody turns on a hot tap somewhere else in the house!

The only drawback with all of this is the cost of electricity compared to gas. Also, using high grade energy to make low grade heat is not environmentally friendly - if you care about that.

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