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Fuse overriders and new fuses?

Postby cgs05 » Tue Aug 17, 2010 10:18 pm


I'm looking at installing a new 9.5kw shower and need to install a 45A fuse to cover the load.
I have a 100A overrider switch and currently have only 8A spare.
Do I need to increase the overrider to cover the extra load or link the fuse to a separate overrider?

I'm a little confused as to how this works and haven't found any articles referring to the total fuse capacity.

I'm also wondering how much the average quote is for a Part P plumber to install a new fuse and cable for the shower?
Someone did mention they had been quoted approx. £250 which sounds quite reasonable.

Many thanks

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Postby sparx » Tue Aug 17, 2010 10:58 pm

I have read this post 3 times and don't believe it's a wind up?!?!
Overider? presume you mean main isolator on consumer unit, they don't come any larger than 100A as that's the max size fuse in supply co's 'service head cutout'.
Since you are obviously not a registered electrician you cannot legally do any of this yourself.
There is no such thing as a 'Part p plumber' wtf?
How do you get to 8A spare?
If you are adding up the total rating of the protective devices eg 32+16+6+32+6 etc, then it doesn't work like that!
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Postby cgs05 » Thu Aug 19, 2010 6:41 pm

Sorry not a wind-up. I got my terms a little mixed up and am definitely not planning on attempting any of this work on my own.
I was unsure how the fuse system works and I couldn't find any useful explanations on the forum. I was adding together the fuses in the box with gave me a total 92A usage.
Hopefully this means I should be able to install a new 45A fuse to cover the new shower without having to change anything else? The only issue now is trying to find something compatable with a 15yr old fuse box made by Square D (heard somewhere they are no longer around).


Ps. I wasn't sure if part P referred to plumbers or electricians as was flicking between forums at the time!
I'll stick to wiring plugs for the time being. :0)
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Postby kal » Thu Aug 19, 2010 10:12 pm

Is that quote to change the consumer unit for a new one? That’s a good quote and I am an electrician. Please do get an electrician to do the work.
He or she will calculate the maximum demand for you too and if anything needs to be upgraded, i.e the tails.
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Postby ericmark » Fri Aug 20, 2010 2:43 am

The fuse box has in the main been replaced with a box holding miniature circuit breakers (MCB) look like switches and so has changed name to consumer unit which is a type tested distribution board.

New rules mean every item in a bathroom must be RCD protected and it is normally considered not worth fitting a special for bathroom only and so normally the whole consumer unit would be changed.

There are two basic methods one is to use two RCD's and the other is to use RBCO's which are RCD and MCB combined.

The latter is considered the best option as the more RCD's that are used the less likely it is that they will trip without due cause.

The RCD measures power out and power back and if within 15 to 30ma of each other stays closed. It is assumed if they are not the same then some power must be leaking to earth so the device opens.

The fuse can take quite a long time to blow and to ensure it will blow in time if there is a fault the electrician has to measure the resistance of the wires. The MCB has two trip devices a slow acting thermal device and a fast acting magnetic device the latter is given a letter B, C, or D and with B it takes 5 times the marked power to trip. So a B45 MCB will trip with 225A in 0.1 seconds for that to happen we have to ensure 225A can flow and we used Ohms law to work out resistance which we can measure.

With AC it is not called resistance but called impedance so we measure the Earth Loop Impedance ELI.

Although as electricians we do this every day without thinking about it many DIY people don't test what they fit and in the past this has caused mistakes resulting in shocks or fires.

As a result Part P building regulations try to ensure any DIY work likely to cause electrocution is tested either by an electrician who is in turn tested or by the local council however when the rules were written no one seems to have considered electric work is often a lot cheaper than building work and the minimum charge was based on £1000 worth of work so the council charge is over £100 plus vat.

Hence the need to ensure the electrician is a member of a scheme and can sign off the work without the need for a council inspector.

Anyone having to pay to be a member must of course charge more for his work than those who are not members and the quote you give of £250 is on the low side. You need to ensure he is a member of a scheme or you will need to pay council as well as him.

At the end you should have two documents an Installation Certificate which gives all the readings and a completion certificate which shows the council has been informed.

Does that tell you what you wanted to know?
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Postby cgs05 » Wed Aug 25, 2010 9:54 pm

Thanks for the advice Eric.

As a non-electrician some of the terms are quite confusing and I like to get all my facts straight before requesting quotes so I know someone isn't taking the mickey.

Will look for a registered Part P electrician in the area via this website and go from there. I'm hoping to get the whole bathroom done around the same time so any digging out of wires in the wall can be tiled over properly rather than patched up.

Much appreciated, cgs05.
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