Lighting in MCB's

Postby Sparkydelux » Mon Jul 09, 2007 9:50 pm

I am hoping someone will shed some light (excuse the pun) on a few questions I have:
My questions:

Q1.How many lamps / radials can be wired onto an MCB?
eg: 8 x 50W halogens (circuit 1) + 4x Pendant lights rated at 100W each (circuit 2) +8 x20W halogens (circuit 3).I am looking at running all 3 circuits off 1 breaker.if the total wattage of the lamps do not exceed the 6A x 230V =1380W would it be viable to wire it up as i have explained?

Q2:Does the lighting radial have a maximum length and will it affect the scenario I have highlighted as above if the runs are quite long?

Q3: I have to run external perimeter lighting circuit.I am aware that it has to be RCD protected and the fittings have to be IP rated.I am wondering what i can do to minimize the excessive use of too many single circuits.Unfortunately the house is very big and the cable run is very long which brings me to having to split the circuit.Can I do the same thing with the MCB and external circuit?

Q4:Furthermore,there is a motorised septic tank which is being installed.I am to run a SWA from the motor to the house Consumer Unit.The cable run to the CCU is well over 40m.So,I was wondering (inorder to shorten the run) if it would be possible to take a feed (Twin&E) from the the CCU across the house via the joist access and feed it through the wall cavity to exit outside.I will then take the SWA to the place of (Twin&E) exit where I will connect the cables within a IP rated JB.The SWA will be burried and labelled up as should.Am I right in doing this?
PS:Motor circuit will be RCD protected.!

Many Thanks for your time and comments.

Until next time.
Rank: Tradesman
Progress to next rank:
Posts: 42
Joined: Tue Jul 03, 2007 9:32 pm


Simply Build It

Postby sparx » Tue Jul 10, 2007 7:34 pm

1) normal limit is recommended 1200W / cct. but if 1.5mm2 cables then no prob of load but may need 10 A mcb.
2)Yes limit is set by volt drop calculations often over looked, eg,
from table 4D5A in regs. 29mv/Amp/Mtr for 1.5mm2 cable +
29 X 6A X 30m= 5220mV /1000= 5.2 Volts dropped (max allowed 4%) =9.2V so in limit,
BUT 35 M of 1.0mm2 gives 44mV X 6A X 35Mtr/1000=9.240, which does not comply so it's quite complex, if in doubt u can run in 2.5mm2 from 6A breaker to 1st fitting then drop down to 1.5mm for rest of circuit!.
If all your new load is at one end of house why not run a sub-main to a local consumer unit? say a 10mm2 on a 40A mcb to an RCD board then MCB's for local circuits.
as for pump supply, I would try to get SWA into loft space & change to twin/E there. using the new CU I just suggested, (bet it's wrong end of house) gloom !!
regards SPARX
Rank: Project Manager
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides

  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!


  • Related Topics