What do you call a multiple CU "trunk and branch" wiring scheme?

[joth]

(Mostly to help me in upcoming conversations with the various trades)

I'm likely to end up with a lot of rooms having 1 or more individual lighting circuits. (especially if moving away from DALI to centralized mains dimmers and contactors, as seems to be the recommendation)

Rather than bring back 2.5 floors plus garage to a single central CU, I'd rather have a distribution "sub board" per floor, plus another in garage, and in loft for PV inverter and batteies etc.

 

Is there a generic name for this "multiple CU" hierarchical topology? 

Presumably RCBOs all go on the sub-distro boards (as you want different ratings per local circuit, and earth leak trip isolation between them) and then the central board just has an incoming breaker and a number of MCBs for each of the branches? Regs don't require an RCD at that head location or anything?

 

Any general resources anyone can recommend on specifying and designing residential "structured wiring"?

 

[Jeremy Harris]

You need to look at the best way to do this efficiently, whilst protecting all the cables/circuits, and that means looking at the maximum demand from each CU, allowing for diversity.

 

I'd be inclined to have a small master CU, fed from the meter tails, with DP MCBs feeding the cables that supply each of the CUs on each floor, garage etc.  That way the cable supplying each of the distribution CUs can be sized for the demand.  I'd also opt to fit all-RCBO CUs everywhere else, with just a single DP isolator in each CU. 

 

The way this would work is that the 25mm² meter tails are OK coming in to the main CU with just the company fuse for protection (assuming they are less than 3m away from the meter).  The cable to each distribution CU would be protected by a suitably rated DP MCB (I'd opt for DP ones to allow isolation of each distribution CU using the main CU MCBs).  Each individual circuit from the distribution CUs would have the mandatory over-current and residual current protection by the RCBOs. 

 

One slight issue with this configuration is the need to allow good access to every CU, something that could eat up a bit of space.

[ProDave]

Such an arrangement is common in larger buildings. It's called a Sub Main to feed the second layer consumer units.

 

You could avoid the use of rcd's at the distribution board by using something like SWA for the sumbains and then have conventional CU's at the second layer.  In theory there would not be much need to ever visit the distribution board.