C U overload query

2 Mar 2007
Reaction score
United Kingdom
I have recently upgraded my shower which I protected with a 45amp mcb. As a result my split load CU now comprises :- 1 x 100amp main switch + 3 x 6amp and 1 x 16 amp mcb's + 1 x 63amp 30ma RCD protecting 2 x 32amp and 1 x 45amp MCB's.

However my query is that the note on the CU states 'The Total load must not exceed the rating of the main switch ' which is 100amps and yet the total MCB amp rating adds up to 143 amps. Can anyone explain this anolamy in laymans terms please. Is there a formula for calculating the total load allowable in terms of fuse rating for any given main switch amp rating?
Sponsored Links
You'll get a more detailed answer from one of the pro's on here but the basic answer is diversity.

You're unlikely to be using your shower at the same time as running a hairdryer, oven, iron, kettle, toaster, microwave etc.

I think the total load applied to the total load applied at one time rather than the potential total load should every MCB be maxed out.


Diversity it is. What is the actual rating of the shower, and what is the 16A circuit supplying?
Sponsored Links
Thanks Crafty and Fredflintsone. I understand the principal that not all circuts are in use at any given time but just how far do you take that. Surely there must be an equation or formula to go by otherwise its anyones guess as how much load you can put on the CU. I know you mentioned Diversity but I have no idea what that stands for. If it is the title of a formula, can you advise me where I can look it up.
i wouldn't worry about it, there is almost certainly a fuse rated at 100A protecting the whole install so if you go over 100A for too long it will blow.

working out diversity is something of a black art, there are some guideline fomulas in the regs but really the only way to do it properly is to look at how an installation is actually used.
Many thanks Plugwash. I fully appreciate the reasoning on this but am surprised the Regs have not come up with some sort of equation or limit rather than leave it up to the individual to make a judgement but now I know the answer I will die happy !! Thanks again.
The only thing I'm not overkeen on is a 63A RCD having the 45A electric shower on along with 2 ring socket circuits, imo a 45A RCBO on the non-RCD side would have been a better option for the shower.
Might be a Crabtree, you can get an 80A 30mA to fit instead. I don't know why (apart from pennypinching) they ship even their 15-way splitloads with a 63A RCD.

Of course, you don't get that problem with RCBOs :LOL:
JohnD said:
Might be a Crabtree, you can get an 80A 30mA to fit instead. I don't know why (apart from pennypinching) they ship even their 15-way splitloads with a 63A RCD

Hager do, too. :cry:

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links