Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, about sub-mains feeding distribution boards having both power and lighting loads, is answered by both SELECT and the ECA:
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| What volt drops when feeding light & power? |
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Question: What does
BS7671: 2008 (the
17th Edition) require regarding volt drop percentage for a sub-main feeding a
distribution board, which - in turn - serves both power and lighting?
For example: A 100A TPN supply to (say) a six-way TPN distribution board, which serves six mixed power circuits (5% volt drop and one lighting circuit 3% volt drop) - is the volt drop in the sub-main 5%, and then the final circuits power 5% lighting 3% or 3% overall?
Furthermore, if we were serving (say) six TPN distribution boards from an MCCB panel board, and each of these was serving a mix of lighting and power final circuits (as in a normal installation), then what is the volt drop in the sub-mains - is it 3% or 5%? If it is 3%, then the option for 5% is never going to be used, is it?
Answer by SELECT: In Appendix 12 of BS 7671: 2008, the volt drops given are between the origin of the installation and the load point - ie: the end of the final circuits. Where the installation has sub-mains, the volt drop split between sub-main and final circuits has to be determined by the designer of the installation.
Question 2: The NICEIC says that it would expect the overall volt drop to be no more than 3% from the origin, regardless of the designer's input, and that if the regulations state 3% from the origin, then in a court etc, the regulations would stand.
This means that the designer does not have any say on the 3/5% volt drop. If this is the case, then surely there is no point in having both 5% and 3% volt drop options as - in most cases - distribution from the origin of a supply will have both lighting and power circuits connected. This means that most installations will require more copper and cost more. Please clarify.
Answer by the ECA: The overall voltage drop limit is 3% (6.9V) single-phase, if the sub-main is feeding lighting circuits. The three-phase voltage drop in the sub-main should be worked out first. If (say) that came to 4.5V, the single-phase voltage at the distribution board would be 400V - 4.5V (395.5V) divided by root 3 = 228.3V. This, therefore, leaves a maximum voltage drop of 5.2V available for the final circuit.
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