Since it states lamp holder, not fitting, if you have a fitting which takes 5 lamps then it would see you would need a 500W supply! So living room alone in my house needs a 1000W supply for 10 x 5W bulbs. That is clearly rather silly. However I have seen a kitchen which needed a 5A supply without considering the rest of the house where the silly MR16 spot lights made it look like a planetarium.
Where GU10L2 lamps are used, or other lamps which are designed to comply with the L2 building regulations so tungsten lamps will not physically fit then clearly there is no need to allow 100W per fitting, and even with tungsten GU10 are made as 75W but they are rare and 50W is normally the largest lamp found. But the On-Site-Guide is for electricians who are wiring a house for often an unknown owner who could put any lamp in the fitting. So it needs to allow for some of the daft things people do.
The volt drop is something again which varies according to the lamp fitted. A fluorescent or LED lamp with a HF ballast or a pulse width modulated power supply will likely work with same light output from 150 volt to 250 volt. However what can seem to be the same lamp with a magnetic ballast or a capacitor to current limit will behave very different. Fluorescent is the worst where is fails completely once the voltage drops below what is required, the LED gets gradually dimmer.
With tests I did with a fluorescent in this case a 110 volt version I found below 100 volts they can fail to strike, but at 127 volt they could use twice the power rating on the fitting. Since then I have noted the ballast can be rated 220 volt, 230 volt or 240 volt and selecting the wrong ballast can really change what power it uses, answer is to use one with a HF ballast.
The ability of the user to swap what lamp he uses is becoming a problem. I swapped a 65W fluorescent tube for a 24W LED tube and if some one was to find a 65W tube (no longer made) and was to replace the LED tube with it, then some thing would go pop unless the wiring was also returned to original.
This raises the old question, should one wire with the idea an idiot may come latter and assume without testing, or should one wire with the idea any future work should be done by an electrician who knows what he is doing? To my mind if an as built plan is left in the consumer unit then it's up to anyone in the future to read it and alter as required.
It does seem that if one admits to the LABC that you have done emergency work which needs a completion certificate under Part P that in the main they will accept an EICR done by an electrician they select and will issue the certificate. As to date I have not heard of a LABC inspector insisting that the work is re-done from scratch, although they could in theory do just that.
Since I may have been renting out my mothers house and to do that I would need all the certificates, I had to consider if DIY was really the way to go. In theory I can inspect and test my own work and issue an installation certificate, however my test gear is faulty so likely will cost £500 for new meters. I would also have to pay the LABC, so have to consider it will likely cost £1000 to DIY. It would also likely take me a month. Using a scheme member would cost more, but not that much more, and he could complete the job in a week. So even with my C&G 2391 and 2382 I still decided that using a scheme member was the best option.
Maybe I was wrong, I have had problems, but if I when I have a degree in electrical and electronic engineering considered using a scheme member was best option for a re-wire, then one really does need to question any notifiable re-wire being done by some one who does not feel he has the skill or equipment to do his own testing and inspecting. I did do one notifiable job at my mother's house, and it did take some time to convince the LABC inspector that I had the skill to inspect and test. In the main they select an electrician they trust to do the inspection and testing and you pay for their time. My son insisted that anyone doing the inspection and testing should be higher qualified than me. I don't think they had an electrical engineer on their books who could show higher qualifications to mine.
