How do I work ou the ratio between the total amperage of MCB's in a consumer unit ot the incoming supply?

Question

i.e. I have 96amps of MCBS in the unit and only a 60 amp supply, Do I need to feed the unit with 100amp supply or can I get away with a lower incoming supply as not all of the MCBS will be used a the same time?

Answer 1

I understand from your question that you have added up the rating of all the mccbs fitted in your consumer unit and compared that with the rating of the main fuse. This is not necessary or appropriate. The total load you need is calculated using a subject called “diversity”. As you question implies, a simple way to look at it is that you do not allow for the total mccbs ratings, as all of the mccbs will not be fully loaded simultaneously. However taking it a step further, a qualified person can look at the number and type of circuits you have and calculate the diversity factor to be allowed to the total loading of each circuit. The total loading in some cases will be the mccb rating but in others it will be based on the equipment/devices that mccbs is connected to. By then adding the calculated diversity loading for all of your circuits you arrive at the maximum demand you need. As long as the rating of your electrical board fuse is larger than this maximum demand there is not a problem.
Providing you do not have lots of high rated circuits such as an electrical shower, old style electrical oven (not modern convection oven as these only take a relatively low amount of current), hob and immersion heater all operating at the same time, a 60A fuse should be OK.
But always remember working on electrics should be left to qualified persons!

Answer 2

This is called discrimination.... The rating of the MCB’s must always decrease the farther you go away from the supply. e.g. if you have a main supply fuse rated at 100A feeding a sub-circuit with a MCB rated at 110A the main fuse will blow first taking down everything on the sub-circuit board.... In your case, I think what you've actually got is a number of MCB's that you've added up (haven't you?) to get 96A. It doesn't work that way, each circuit is independent, i.e. a 20A MCB will trip when 20A is exceeded, the remaining MCB's will be unaffected...... Confused?

Answer 3

Ideally you would need a 100Amp supply - that way you could run all your appliances at one time.

However as others have alluded to, electricians and engineers often apply a 'diversity factor' to reduce this figure. Applyng a diversity factor of 20% would imply that the no more than 80% of the equipment would run at any one time.

In your case you would have a 40% diversity factor - this would at first glance seem high to me. But if you look at all your loads you might be able to reconcile the difference - for example if you use night storage heaters, you wont be running the oven at the same time so only one of those loads need to be kept on.

Bear in mind though if you exceed the mains MCB rating, it should trip out, for safety, but also turning everything off.

Answer 4

As I understand it, you have a MCB rated at 96A while your mains supply (fuse presumably) is rated at 60A. If that is the case, your MCB will handle 60A comfortably. What you should watch for it how much all your appliances and electrical fittings (lights, etc.) are drawing. For most average homes, a MCB rated 96A is OK, but your home may not be average. Btw, you need a qualified electrician to do any electrical work these days.

Answer 5

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Answer 6

the mcbs are there to protect the cable.

look at the load that you expect to pull at one time.
start with the shower (7kw?) (1kw =roughly 4.5A)
cooker ?? (see cooker manual)
lights 60W x 10 likely to be on at any one time? 600 w=0.6kW
sockets tv video etc!! a few amps each.
etc..


a normal domestic property in the uk will be less than 60A usually shower that can make it close or more.

note any domestic dwelling in the uk needs a part p approved electrician to carry out work.

Answer 7

if you are a qualified person you would know,don't risk insurance and harm to people issues.