MCB Rating Help

[Turbo]

I'm installing a 2kw under sink 10 litre water heater. It is 3m from the fuse box, fed with 1.5mm2 3 core pvc flex inside some mini trunking.

My calculations say that it should draw under 9 amps full loads, would a 10a type B MCB be enough to feed it? The other choice I have is a 20a type B, which I think is too high. I think I already know the answer - a 16a type B but I just want to check with the proper sparkies. Unfortunately I don't have a 16a in my spares box so I would actually have to buy one!

 

[dan.taylor.1]

The mcb is there to protect the cable, so find out what the rating of the cable is in the installation method you've used and that's what size mcb you want or less.

At the appliance end you need a fused spur and a correctly sized fuse to protect the device, so by your calcs a 10a fuse.

I'm not an electrician but that's my understanding of it all!

 

[Dee J]

Yourre going to need a double pole point of isolation for the heater, not just a single pole mcb. Do it properly. Terminate your flex near the heater with a 13A double pole switched, fused spur. Then install a 2.5mm T&E in the mini trunking, protected by the B20 mcb.

 

[Hotstuff]

It’s surprising how much you have to consider with circuit design and installation. It gets more confusing with guidance offered from laypeople.

After seeing so many dangerous DIY/amateur electrical installation jobs over the years, I would always recommend that people employ an electrician to carry out electrical installation work.

Rant over….is the sink located in a bathroom?
If yes then you will need to consider 30 mA RCD protection and suitability of any accessories used for their location.

Supplementary 30 mA RCD protection may be required for cables buried (chased) into walls.

I would suggest that this circuit is wired with 2.5mm twin & earth cable, supplied by a 16 Amp MCB or RCBO.

At the appliance end, you should provide local isolation and could use a 20 Amp double pole switch or switched fuse spur with a suitably rated cartridge fuse.

Please do bear in mind that the installation’s earthing system and the new circuit should be inspected & tested before being energised and as this is the installation of a new circuit, it is considered as notifiable under part P of the building regs.

 

[Turbo]

To answer your questions-
The heater is in a kitchen, hidden in a false cupboard to stop folk messing with the thermostat setting.

The fuse box has a 30 mA RCD already.

The supply comes out of the fuse box, into a 20a double pole isolator, then through a digital timer and then to the water heater. Due to the heaters location that's the only way I could do it. To fit the isolator above the worktop would mean it would be too near the sink.

The heater instructions said to use 1mm2 flex, that I thought was a bit light so I went for 1.5mm2, plenty enough for a 2kw load. There's restricted room at the back of the timer so 2.5 T&E would be very difficult to use.

So back to my original question 10a or 16a MCB?

 

[Dee J]

It's not about the operational current, it's about the fault current and the cable rating. Heat resistant flex is generally rated up to 16 A continuous - but I don't have the regs in front of me any more. A B16 trip doesn't trip instantly the moment 16 amps is exceeded. It takes time and more than 16 amps. Under overload fault conditions will your cable burn before the trip operates? That's the question. If you're not going to go with the conventional circuit designs from the on-site guide then you'll need to do the design maths for your set of circumstances. On site guide will give radial circuits based on T&E sizes. A B16 or B20 protected radial would be 2.5 T&E.
Once more. Please follow conventional arrangements. T&E from the mcb to a switched fused isolator. Then use your flex from the switched fused isolator to the timer and the appliance.
Anything else will be seen as unconventional and require your design backup.

 

[Hotstuff]

Please don’t be offended by my lack of positive input. If I lived local to you I would call in to talk it through in person.

Through the course of my job I’ve seen too many dangerous electrical installation jobs over the years carried out, mostly, by laypeople.
From your descriptions of the job at hand, it’s clear that you have limited knowledge or experience of electrical installation work and it just reminds me of things I’ve seen and news items I’ve read where innocent people have died as a consequence of negligent electrical installation work.

I have only taken time to explain my views as this forum is generally so friendly & helpful and I really do not want to be seen as the cause of any unpleasantness.

Good luck

 

[brightspark]

personaly id wire it from db with 2.5mm cable to a 13 amp double pole switchfused spur fed from a 16 amp b type mcb then 1.5mm flex from spur to timer then heater . then your well covered

 

[Dave-sparks]

So back to my original question 10a or 16a MCB?

10A MCB for a 2kW heater is fine, I've done it many times.

 

[Dave-sparks]

At the appliance end you need a fused spur and a correctly sized fuse to protect the device, so by your calcs a 10a fuse.

This is not necessary, the fuse in a switched fused spur is only there to protect the outgoing cable, any protection for the appliance should be built in to the appliance.

As the circuit will be protected by a 10A MCB the flex to the appliance is already protected and an additional fuse is unnecessary and just adds another potential point of failure.