Size of MCB

Hi, this is my first posting.

We are about to start installing a new kitchen which up till now has been gas fired. i.e. no electrical cooking equipment.

In keeping with modern trends we shall be installing a variety of electrical cooking equipment which has a total power requirement of 14.85kW.The equipment will comprise of, one combi oven @ 3240w, one single oven@16amp, larder fridge@90w, larder freezer@120w, induction hob@7200w, downdraught extractor hood@285w,

The actual length of cable run between the consumer unit and the proposed position for the distribution board is 18 metres but allowing for any deviations due to obstructions I allowed 25 metres (cable drum length).

We are proposing to run a new sub main from the existing consumer unit which is a 10 way 100 amp RCD protected board to a new distribution unit within the kitchen. The cable run is just under 25 metres and according to our calculations a 6 mm2 T+E cable will be sufficient for this load.

The cable will be routed as follows: Rising vertically on a wall to enter the first floor void, and then route under to floor within the floor joists unclipped to drop into kitchen with the consumer unit at high level. It is not anticipated that the ambient temperature within the floor void will rise above 30 deg C, as there is no pipework near the run of cable.

I have not allowed any diversity as I was not sure what to allow. Normally I would allow a diversity for other sorts of design work of 0.6, but not sure whether that would be appropriate for cooking equipment.

Looking back at my calcs, and using an online cable sizing checker (Doncaster Cables), I have revised my selection to a 10mm2 T&E cable.

So the question remains, what size MCB do I need?

Hope this will enable someone to give me a definitive answer? Another forum did not seem to be able to help.

Thanks

GBH

I fear you may again be disappointed.

DIY is a misleading term in that, while you want to DIY, you don't know how to DIY so you want someone here to do your design and tell you everything that you need to do.

Your plan is not ideal insofar as your sub-main if done with T+E will likely require RCD protection in which case one little fault will shut down your kitchen.

The MCB rating is determined by the load and then a suitable cable is chosen taking into account the length, volt drop, EFLI, installation method etc.

Is there any chance you can, which you should, employ an electrician who will design the circuits, let you run the cable under his supervision, and then certify and notify the work?

GaryBH said:

We are proposing to run a new sub main from the existing consumer unit which is a 10 way 100 amp RCD protected board to a new distribution unit within the kitchen.

Click to expand...

You do not need a submain or a separate consumer unit for the kitchen.
The loads described are no different from the majority of domestic kitchens.

A 32A circuit for the cooking equipment and suitable socket outlets for the other items should be adequate.

flameport said:

A 32A circuit for the cooking equipment and suitable socket outlets for the other items should be adequate.

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Lest the OP be confused, I think that is probably somewhat ambiguous. Are you suggesting 32A circuit for the cooking equipment, that circuit also supplying suitable socket outlets for the other items (noting that kettles, toasters etc. have not been mentioned) OR a 32A circuit for the cooking equipment and a circuit for suitable socket outlets for the other items?

Kind Regards, John

I would expect you can draw 45 amp on 10mm² and likely that is the largest MCB you can get anyway.

So likely volt drop is around the 4.3 volt mark and the loop impedance with be 0.15 ohms less than supply.

So I would select 45 amp and use local MCB's to reduce the local supplies. But I am sure some one will point out your signing for design and as such you must design not ask some one else to do it for you.

I would think its reasonable to assume there is an adequate socket circuit already. But maybe the OP can confirm

AndyPRK said:

I would think its reasonable to assume there is an adequate socket circuit already. But maybe the OP can confirm

Click to expand...

It might seem reasonable to assume that, but one wonders when one sees that the 14.85kW total load calculated by the OP includes a fridge, a freezer and an extractor hood. As you say, clarification from the OP is needed.

Kind Regards, John

AndyPRK said:

I would think its reasonable to assume there is an adequate socket circuit already. But maybe the OP can confirm

Click to expand...

Exactly - this is an existing kitchen, so rather hard to believe it has no electrical items in it at all.

Existing socket outlets for the smaller appliances, perhaps adding/relocating a few sockets.
New 32A circuit for the cooking items.

flameport said:

AndyPRK said:

I would think its reasonable to assume there is an adequate socket circuit already. But maybe the OP can confirm

Click to expand...

Exactly - this is an existing kitchen, so rather hard to believe it has no electrical items in it at all.

Click to expand...

As I implied, I essentially agree - but started wondering when I noticed that the OP had included, for example, a fridge and freezer in the 'new load' calculations. Whatever, I still think that what you wrote was not necessarily going to be entirely clear to the OP, so I'm grateful to you for having now clarified.

flameport said:

Existing socket outlets for the smaller appliances, perhaps adding/relocating a few sockets. New 32A circuit for the cooking items.

Click to expand...

If there is an existing (and persisting) sockets circuit, then, needless to say, I agree. However, if that were not the case then, with about 25A of cooking appliances even after diversity, the 32A cooking circuit would not have much spare capacity for anything else.

Kind Regards, John

ericmark said:

I would expect you can draw 45 amp on 10mm² and likely that is the largest MCB you can get anyway.

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BS EN 60898 circuit breakers are readily available in 63A variants.

GaryBH said:

We are proposing to run a new sub main from the existing consumer unit which is a 10 way 100 amp RCD protected board to a new distribution unit within the kitchen.

Click to expand...

Do you propose to apply for Building Regulations approval?

Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

Who will sign this:

I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2011 except for the departures, if any, detailed as follows:

?

EFLImpudence said:

I fear you may again be disappointed.

DIY is a misleading term in that, while you want to DIY, you don't know how to DIY so you want someone here to do your design and tell you everything that you need to do.

Your plan is not ideal insofar as your sub-main if done with T+E will likely require RCD protection in which case one little fault will shut down your kitchen.

The MCB rating is determined by the load and then a suitable cable is chosen taking into account the length, volt drop, EFLI, installation method etc.

Is there any chance you can, which you should, employ an electrician who will design the circuits, let you run the cable under his supervision, and then certify and notify the work?

Click to expand...