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Posted By Topic: Final Sub-Circuits

Jun 17 2019 15:04

Hi Guys,

I keep coming across MCBs with more than one circuit in the load side in residential installations and I wanted to ask the question of, what actually is deemed a final subcircuit?

I read the definition in 3000 as, for example, if you have 2 wires in an MCB, its 2 final sub circuits but I thought I would ask the question as under 2.6 around the arrangement of RCDs and only supplying 3 final sub circuits for one RCD, I have found multiple times that people will have 3 MCBs off an RCD but around 5 circuits on the MCBs.

Thanks in advance =)

Jun 17 2019 15:14

Final sub circuit originates at the load terminal(s) of a protective device. Final sub circuit can branch at any point, including at the origin of the circuit.

Jun 17 2019 15:30

So if there's 3 cores (3 circuits) in an MCB but its still only supplying say 5 GPOs in total, you would class that as 1 final subcircuit?

Jun 17 2019 15:33

It could be seven wires supplying twenty sockets, fifteen lights, two motors and a partridge in a pear tree. If they're connected to the load terminal(s) of one protective device then they are one final sub circuit.

Jun 17 2019 15:37

Haha cheers mate

Jun 17 2019 15:39

As mowgli says above, the "final subcircuit" originates at the protective device (MCB) terminal.
1.4.88 Subcircuit, final:- "load terminals of the circuit protective devices"

Note for example, that this is different from the wording in "more than one lighting circuit is installed" - so two lighting cables connected to one MCB would be two "lighting circuits" but one final subcircuit.

Jun 17 2019 15:54

DougP that's not quite right. Circuit includes the protective device.

So in they're saying that where there is more than one lighting circuit (MCB + final sub circuit) then these shall be distributed between RCDs.

Jun 17 2019 16:08

Ahh, ok, you're right again on the definition.

I'm not sure if that matches the intent of though...?

Surely if someone has wired two or more feed cables for lighting in a property, they would consider that best practice would be to distribute them across more than one RCD, rather than call it one "circuit" because they are all connected to one MCB.


Jun 17 2019 17:13

Regardless of the intent, by using defined terms, those clauses really only have one interpretation. Lately, I've been putting lighting on one or more RCBOs and only using RCCBs for groups of non-lighting final subcircuits. I guess I'm only one step shy of putting everything on RCBOs. Very little push back from customers. No one seems keen on their house going dark, even partly.

Jun 18 2019 07:32

I did my refresher last week, and part of the board-issued information stated "only three MCB's or fuses can be fed from an RCD".
Obviously referring to this post won't help you if you're ever dragged before the board, but gives you an indication of what they're thinking.

Jun 18 2019 09:19

that's the difference between "good practice" and "minimum acceptable". The requirements set by the Standards are "minimum acceptable".
I'd agree that good practice would be, if there are two cables leaving for lighting, they should generally be configured as 2 final subcircuits (ie 2 mcbs used). But the rules that drives number of subcircuits are 2.1.2 & 2.2.1