10 amp mcb ok for a single lighting circuit?

[Simon_]

My in laws house has a single lighting circuit for the whole house and the consumer unit is being updated. The lighting circuit has downlighters in the bathroom and kitchen. As well as the the RCD protection that's obviously needed would a 10a mcb be best for the circuit while a rewire is not an option at the moment.

 

[Deleted member 26818]

What's the maximum load, and what's the Zs on the circuit?

 

[Simon_]

Ok thanks for that. There's no hard wired smoke alarms in the house. Circuit has not been tested yet.

 

[Deleted member 26818]

As a rough quide, most people rate individual lamp holders at 100W, so a typical 3 bed semi will have about 9 or 10 lamps which will be about 900 or 1000W.
So your looking at a maximum load up to about 4.5A. Obviously sometimes there are multi-lamp fittings, and sometimes there are spot lights in kitchens and bathrooms, which wil increase the calculated load.

 

[Engineer54]

I'd personally prefer seeing a 10A MCB on a 1.5mm cable, rather than the standard 1mm cable..

 

[Simon_]

I worked it out and it comes to 4.45 amps for all the lighting on the circuit.

 

[Simon_]

Will do mate. The main earthing needs upgrading when the CCU is changed. Bonding is fine though.

 

[Simon_]

Will do mate. The main earthing needs upgrading when the CCU is changed. Bonding is fine though.

 

[Simon_]

Doing the IR test is going to be a pain as there is quite a few downlighters on the circuit with transformers. Disconnecting them would be very time consuming. Any suggestions other than disconnecting them? Could I do L,N to E test and put notes in the limitations box on the cert after testing?

 

[shanky887614]

glen, would those power surge extension leads run amuck of them as well? or would they just negate the effort to remove them?

 

[Ponty Massive]

Remember the MCB size is protecting the cable and not the appliance!!!!

At the design stage, you should determine the load requirements of that circuit, then select the cable as required. There is no law saying you have to use 1.0/1.5 for lights and 2.5 for rings. You could design your lighting system and use a 10mm cable with a 40A MCB if you want (you may have trouble terminating the connections though :teeth_smile

As the circuit is existing, find out what size cable has been used, method of installation etc then select your MCB from this information. If after calculating the load demand for the circuit, you find the MCB will not be big enough, then it's time to run another circuit.

 

[malcolmsanford]

Will do mate. The main earthing needs upgrading when the CCU is changed. Bonding is fine though.

As you are upgrading the CCU then really the Zs of an existing circuit that can not meet the disconnection times for basic protection as in reg 411.3.2.1 then additional protection by an RCD with the characteristics of reg 415.1 can be used and therefore the Zs value of 1667ohm can be used from Table 41.5