Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning how to achieve compliance with 17th Edition requirements on lighting voltage drops, and the reason for the new regulation, is answered by the ECA:
Since BS7671:2008 (17th Edition) has been implemented, I have been having trouble understanding why we are now limited to using only 3% voltage drop for lighting circuits.
As you can imagine, in an office environment, there is a higher usage of lighting and I am now limited in terms of what I can work with in the design. This is causing some lighting control systems to be difficult to use.
The DALI system that I am trying to use, for instance, is not actually worth using because I either have a HUGE sub-main cable to enable my final lighting circuit to be in a useable size, or - alternatively - I have a sensible size sub-main cable, but then have to use sizes that an not practical supplying my final circuit.
My question, therefore, has two parts:
Firstly, what is the theory behind limiting lighting circuits to 3%?
Secondly, can you think of any ways of complying with the new Regulations but without the designs suffering as a consequence (apart from asking the customer to buy their own HV/LV transformer)?
Answer: The theory behind the reduced 3% figure is that some lighting control gear will not function correctly if the voltage is on the low side. This is more of a problem where the voltage at the origin could already be as low as 216V.
I would suggest that you ascertain from the manufacturer the lowest voltage their lighting will operate at. So long as you have designed the installation so that the voltage at the terminals of the equipment is above this value, you have complied.
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