Questions About Crabtree Consumer Unit


Postby bf_uk » Tue Sep 25, 2018 2:22 pm

We have a Crabtree Consumer Unit and I have a couple of questions if anyone is able to help please?

1. Is there a maximum number that all of the breakers should not exceed when added together?

2. We need a couple of dedicated sockets added, each running off its own 16A breaker. What are the options if all of the Neutral connections are already in use?
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Postby ericmark » Fri Sep 28, 2018 1:13 pm

A consumer unit is a type tested distribution unit, the type testing is to ensure it is suitable for use by non trained personal although it is assumed they will not use tools. i.e. all they do is reset a braker.

However it means they are no bigger than 125A in real terms 100A and so are fed from a DNO fuse no bigger than 100A.

So with a RCD and main switch rated 100A you can have 16 x 32A MCB's as clearly the total can't exceed 100A without blowing incomer fuse. However fit a 63A RCD then all MCB's added together should not exceed 63A as the 100A DNO fuse is going to allow 100A. But in practice it is unlikely this would happen, so it is quite common to see a 63A RCD feeding way over 63A worth of brakers.

Industrial sparks were taught one terminal one wire, however domestic it seems common to fit three wires in one terminal as you would with a socket on a ring final with one spur. In the main we aim with consumer units to match neutral to line, so forth MCB along will have neutral in forth terminal along on the neutral bar, same with earths, but there is no rule to say this must be the case. Two or three wires of same thickness in a terminal is normally OK, however mixing thickness can cause problems.

Rules say I can't form a new circuit unless I pay LABC even when I am an electrician, I am permitted to add to existing so where a CU is already fully populated even if the MCB/RCBO is not used, there is some ambiguity.

Every time you switch on a light you form a circuit, but not a new circuit, and when you fit a FCU that is technically a new circuit, but it seems is allowed without notifying LABC, so until some one is taken to court it is a little vague, however I do not want to be involved in making case law, so in a domestic premises, I would not add anything to a consumer unit as it is likely should anything go wrong, I would end up in court.

Clearly in your own house no one is likely to report you for braking the law, however if either house is rented, or you have rented the house, or have non family members living in the house, then you need to consider if worth the risk, same as braking 30 MPH speed limit, OK you may get away with 35 but is it worth the risk?

I am careful on the forum, on one hand don't want to frighten people so they don't ask questions, but neither do I want to tell them to do stuff which is technically illegal. I really don't care if you brake the law, that's up to you.

The big problem today is old consumer units, there is nothing to say you need to upgrade as the regulations change, so when the RCD was required by the regulations you did not need to add RCD's to every house, however there are very few new circuits which can be installed without RCD protection, so it is common to be asked to add just one socket, and realise to do that really needs a consumer unit change.

OK there are ways around it, using ali-tube cable with RCD sockets for example, but often the alternative is more expensive to changing the CU. When I wanted to rent out this house, it was cheaper to employ a firm to re-wire the house to DIY as I would have to pay the LABC so much. Had I not intended to rent I would likely have taken a chance, but even then it would have caused problems when I came to sell.
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Postby Mr White » Sat Sep 29, 2018 12:00 am

1) It does not matter what brand of consumers unit an installation has, so long as it and the MCB's are suitable for the required task.

If for example a premises had nothing but lighting circuits it would not be un reasonable to "fill" every vacant slot with 6A MCB's.
On the other hand if a premises had one light and nothing but sockets, it would be possible to "fill" every vacant slot with 32A MCB's and install a Fused connection unit for the single light.

The reason it works is diversity, not every circuit will be drawing the maximum amount of current that it can at the same time.

2) If you have room for 2 more MCB's then there should also be room for 2 x neutrals. As a consumers unit should be wired in such a way that counting (Lets say the 4th MCB) the 4th neutral along should be that of the 4th MCB along. If it is not the case then that means it was not connected as most electricians would have done, but it should not pose a problem to remedy.
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Postby bf_uk » Mon Oct 01, 2018 9:26 am

Thank you for the responses.

I took the cover off the CU and was planning to post a photo but, after looking at it properly, I realised there are actually spare Neutral connections. For some reason the first bank of Neutral connections have not been used and the 2nd bank is full. Not sure why it was wired this way.

The new sockets will be used for equipment that is already in use on the current sockets, so there won't be any extra load. They are also somewhat temporary wiring, in the same room that the CU is located, so they could be taken out if we do move.

I just wanted to understand a bit more about this myself, before getting an electrician in to do the work, so thanks again.
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