max number of lights on a circuit, including double wall mounted lights

[faretra 1]

If I have an exam question that asks - an architect asks for 2 more wall mounted twin luninaires to be added to the lighting circuit, The lighting circuit already has 10 lights on it, will I need to put the 2 new twin wall mounted lights on a new circuit or can they be added to the existing one. (do we count each double wall lamp as 200w) The circuit is a new standard lighting circuit it has 6A mcb protection, in an extension above an open car port, we can used one of the two remaining spaces on the consumer unit circuit. the other is used for the sockets, there are no other spaces on the c.u.. the exam question also asks to show calculations to determine if we need a new circuit, I know it is 12 lamps at 100w max, so was wondering about the twin lamps and how we rate them....any help most appreciated,

 

[a1]

Which Exam are we talking here?

 

[Doc Hudson]

I understood theoretically you had to assume a user could put 100watt lamps in each holder. But considering you can no longer purchase 100watt bulbs, I am unclear as to what the realistic largest lamp is or if the exams or regs now take this into account? Maybe someone else can enlighten us.

Doc H.

 

[NozSpark]

and without any info as to the wattage of the existing lamps, you will not be able to answer this question with any certainty..

But yes.. it used to be 100w per fitting unless it's a fixed load (i.e. fluorescent)

 

[faretra 1]

Thanks for reply's this is the problem we are not given the wattage of the new lamps, it is one of those questions where you have to make a judgement call do you go with the reg all lamps are based at 100W or add the 2 double wall mounted lamps, as in reality the existing lamps will not be 100w...also very unlikely the double wall lights will be 100w, dont know if regs count double wall lamps as 1 or 2 lamps ???? (also is it possible to add the lamps on same circuit and up the breaker to 10a ?????

 

[PC Electrics]

There are no hard and fast answers to questions like this. Each installation needs to be assessed on it's own merits.

You would have to look at the existing circuit and see what kind of lighting it is already serving. If it's serving living areas it may have one or more multi-lamp lighting units (or the dreaded downlights). If it's mostly serving bedrooms it may be supplying primarily pendants, which these days are more likely to be fitted with CFLs than 100W filament lamps.

So, having assessed the existing loading and applied a diversity factor of 66% you can assess whether there is headroom to add more lights (max load for 6A = 1440W).

The primary factor in this decision, however, is going to be whether the existing circuit is safe and adequate to extend. A test of IR (L+N / E) and R1+R2 to ensure cpc continuity. Also check for shared neutrals with other circuits along with main earthing and bonding.

You will almost certainly need to add RCD protection for buried cables, so an RCBO or off-board RCD will be required.

So, if you consider the loading to be okay and the circuit is safe, go ahead and extend. If the circuit is safe, but loading is on the limit then increase the OCPD to 10A provided you've check that all cabling is at least 1.0mm (as far as you can) [some DIYer may have bodged in some 0.5mm flex] and is properly clipped to any loft joists to give adequate rating under insulation (method 101 = 10.5A).

 

[meerkat]

Had a similar question at college how many lighting points can a 5Amp fuse and a 6Amp fuse feed,

formula is P over V X I

transform this into V X I = P

230 x 5 = 1150W, so 11 x 100W lamps,

230 x 6 = 1380W, so 13 x 100W lamps, you must assume that the homeowner at some point may change all lamps to 100W lamps.

Sorry cant help with the twin wall lamp ratings, cant these 2 twin lights be fed from a spare way in the CU, or fed by FCU with a 5 Amp fuse fitted from the ring circuit, fitting them to the existing lighting circuit would compromise the 100W per light fitting reg.

 

[Risteard]

100W should still be assumed. (It's written somewhere - perhaps OSG.)

Of course this may no longer be realistic with CFLs etc. but the 100W rule hasn't changed as of yet.

I still use 150W lamps in the bathroom to help murder the environment. Have a nice 500W flood too.

 

[Mark2spark]

I don't think they do the 10 lights max rule anymore.

559.6.1.1 & 314 basically cover it now. Design currents and de-rating factors etc I suppose have done away for the need for such a finite rule.

Probably still good practice though.

 

[SPECIAL LOCATION]

On site Guide page 96

Table 1A whilst discussing Diversity

the item relating to lighting circuits still says

"current equivalent to the connected load with a minimum of 100w per lamp holder"!

This is also mentioned in Appendix 'H' of guidance note 1 selection & erection.

so although its a carp figure.. I think you would need to use it in your exams & refer it to OSG!

Guinness