two circuits in one mcb.

vectraj · Jul 13, 2015

Hello

Just opened up a consumer unit, to find two circuits (lights and extractors) coming out of one 10 amp mcb. This isn't right is it? What should I do come the time to test and record results? Where would I write them down?

I've always thought its 1 fuse/breaker per circuit? head scratcher for me.

golfpaul · Jul 13, 2015

Not ideal.

But 2 cables into one mcb, wouldn't that be one circuit...

Darkwood · Jul 13, 2015

As above its classed as one circuit - if it feeds 2 seperate loads then test both legs and record the worse of the readings.

Its common to have a lighting circuit and then a E/M light supply out of the same mcb, your only limitation to the no' of cables in one mcb is the ensuring the mcb clamp is tight on all conductors.

Risteard · Jul 13, 2015

I wouldn't really agree that it's "not ideal". After all, feeding two legs of a radial circuit from the distribution board will reduce circuit length.

telectrix · Jul 13, 2015

also frequently have a smoke alarm circuit combined with a lighting circuit in 1 MCB. as said before, it then becomes 1 circuit. as long as the MCB is rated for the total load, no problemo.

davesparks · Jul 13, 2015

vectraj said:
Hello [emoji4]

Just opened up a consumer unit, to find two circuits (lights and extractors) coming out of one 10 amp mcb. This isn't right is it? What should I do come the time to test and record results? Where would I write them down?

I've always thought its 1 fuse/breaker per circuit? head scratcher for me.

If you look up the way 7671 defines a circuit you will find that everything supplied from a single OCPD is defined as one circuit.
As far as testing goes it's no different to any other circuit, test it and record the highest results for impedance.

vectraj · Jul 13, 2015

Thank you guys for some good points and making me see it as simple as it looks. :detective:

baldsparkies · Jul 14, 2015

Way to many old wives tales on this one.
Is it unsafe or dangerous in any way ??
If its considered non compliant because of interpreting one of the many regs that mis-interpretation can be so easily applied too, then let logic dictate and as you say, it becomes as simple as it looks.
Its one circuit by definition so treat it as such, simple as.