Correct Size For Mcb

[GaryBH]

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 at 240/1/50.

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 existing consumer unit has spare ways on it, and my question is what size MCB will be required for this load?

Some information on the internet seem to suggest a 32 amp MCB would suffice, but I am not sure.

Any advice/comments will be much appreciated.

Thanks

 

[SPECIAL LOCATION]

A 32A MCB should give adequate protection for a 6.0mm T&E....

But..

it may well not provide sufficient current for your load...

Is your 14.85kW before or after diversity ??

What about your volt drop...?

What are your calculations..?

Is this a DIY project.. or are you charging a customer for your "electrical" work..?

:C

 

[NozSpark]

I saw a DIY CU change today,,,,, so many things wrong with it :shakehead

 

[Murdoch]

And this is notifiable under Part P too.

 

[kerching]

I saw a DIY CU change today,,,,, so many things wrong with it :shakehead

Pics?

Just asking

 

[steptoe]

I saw a DIY CU change today,,,,, so many things wrong with it :shakehead

it might not have been DIY,

it may simply have been one of mine,   :C

 

[NozSpark]

it might not have been DIY,

it may simply have been one of mine,   :C

They did say that their "lad" did it

 

[ProDave]

Not where PD lives  :innocent

now If I had said that.......

 

[Doc Hudson]

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 at 240/1/50.

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 existing consumer unit has spare ways on it, and my question is what size MCB will be required for this load?

Some information on the internet seem to suggest a 32 amp MCB would suffice, but I am not sure.

Any advice/comments will be much appreciated.

Thanks

Welcome to the forum, it looks like you are doing some of your calculations back to front. You should be calculating your design current for the circuit, then selecting an appropriate protective device by rating and type, so that it can supply the design current without running continually overloaded or tripping due to power-on surges from the load. Then select a cable big enough to ensure that the protective device is the item that trips and not the cable melting. Various correction factors due to the installation methods and environment must also be taken into account such as those mentioned by Canoeboy in post #2 etc. It would probably be best if you can post more detail on the type of loads, (appliances connected), your full calculations so far, then someone can give more constructive help.

Without actually doing any calculations and guessing that you have just added up some total power ratings with no diversity applied. I would think a 10.0mm from a 50A MCB could be an answer.  What are your RCD protection requirements with this sub main. is it going to be buried in plaster?

Doc H.

 

[GaryBH]

Thanks everyone for the responses. Some are helpful, some are not!

I calculate that the total connected load for all appliances is 14848.6 watts to be precise about it.

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).

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?

Many thanks,

GBH

 

[Murdoch]

Thanks everyone for the responses. Some are helpful, some are not!

I calculate that the total connected load for all appliances is 14848.6 watts to be precise about it.

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).

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?

Many thanks,

GBH

Are you an electrician? This is notifiable under Part P.......

 

[Andy™]

Thanks everyone for the responses. Some are helpful, some are not!

I calculate that the total connected load for all appliances is 14848.6 watts to be precise about it.

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).

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?

Many thanks,

GBH

you havent allowed any diversity, yet you have decided to use a 32a breaker when the total load is way above it?

 

[binky]

You don't turn everything on at once, and even when things like ovens are running they don't pull full load for very long, which is where diversity comes in to play.

AS you are planning a mini CU in kitchen a 10 mm feed on a 40A MCB would probably suffice - I hasten to add I haven't read your postings very carefully before the regulars correct me :^O

 

[Blue Duck]

Cooker diversity:

Say your cooker is 10kw / 43a

Set aside the first 10a then add to 30% of the remainder eg; 43-10=33÷100×30=9.9 +10 = 19.9a.

add 5a if there is a socket on the ccu

Note this is for a free standing cooker!

 

[Evans Electric]

Diversity is very , very difficult to work correctly  , for instance if you were commercial kitchen the result would be different as it could be functioning for 8 hours a day .

I don't know what all your cooking things are but doubtful you'd be going over 40A  TBH  at any one time . .    

I think most guys on here would be aiming for a seperate ring main  and a cooker supply .  You could consider that .

To cover your ass ,  a 10mm  with a 40A MCB   would most likely suffice...any problems , upgrade it to a 50A  . 

 

[Blue Duck]

You could stick in a 4mm radial.

plenty of kitchens on 2.5 /20a

Isn't there a good book on diversity / commercial circuit design?

 

[binky]

trying to get 4mm in back of socket terminals is biggest issue these days

 

[Murdoch]

trying to get 4mm in back of socket terminals is biggest issue these days

Especially if there are 3 cables!

 

[binky]

that's a non-starter unless you trim a few strands :slap

 

[Andy™]

thats why larger cable is stranded, isnt it? so you can easily remove a few to make them fit