Can you feed a consumer unit from another consumer unit?

[Adam00]

Can you feed a consumer unit from another consumer unit, It would be supplied by a non RDC protected MCB. Is this allowed as I have always seen switched fuse disconnection units.

 

[littlespark]

Depending on the circuit design, then yes…. It’s allowed.

More details please for more explanation.
CU sizes, cable type and size, breaker size, environment….



Welcome to the forum

 

[Dustydazzler]

I often do it this way for small extensions / kitchen refurbs etc especially when the exisitng board has a nice spare way.

 

[pc1966]

Just to add to the above, it is a perfectly good way of feeding a few circuits elsewhere, such as garage, etc.

However, you get poor fault selectivity on MCB feeding MCB. Basically if your final circuit has a PFC that is able to hit the upstream MCB's "instant" magnetic trip point then both MCBs will fire, and so you also loose the other healthy circuits in the sub-CU.

In many cases that is not important enough to worry about, but if it is an issue then your alternative is to spilt the tails and feed the 2nd CU from a switch-fuse (typically in the region of 40-63A).

Also you need to consider RCD protection, ideally in the sub-CU as you suggest, but in turn that means you need cable or routing (i.e. not buried/hidden) that permits no "additional" RCD protection. Usual safe bet is SWA. You might still need upstream RCD protection if you can't disconnect on the sub-main PFC (usually TT earth case) but that can be a 100mA/300mA selectivity (short delay) type that gives your final circuit's RCD protection the chance to act first.