Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about how to calculate the voltage drop for an external lighting system, is answered by SELECT:
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| How to calculate volt drop for outside lighting? |
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Question: Can anyone explain to me the way to calculate voltage drop calculations for external lighting systems?
To define our lighting system: We have a line (3-phase 4-core cable) coming from a panel inside an electrical room. From there, the underground line goes to a junction box (distance to the junction box is 70m). After the junction box, the line is distributed to two separate lines, but actually they are coming from the same phases.
The first line supplies eight lighting fixtures; the distance from the junction box to the first lighting pole is 60m and the distance between each lighting poles is 40m. From the second line, this supplies 10 lighting fixtures - the distance from the junction box to the first lighting pole is 50m, and the distance between each lighting poles is 40m. The height of the each pole is 12m and the power of the each lighting fixture is 250W.
Which formulas I can use and how?
Answer: With regard to BS7671: 2008, the voltage drop from the origin of the supply to the current using equipment should not exceed 3% of the nominal voltage.
To calculate the volt drop, use appropriate tables from volt drop tables in Appendix 4. Then use formulae Vd = mV x Ib x L/1000
mV = volt drop from appendix 4 tables
Ib = design current
L = length of cable run
Max permissible volt drop = 3% x 400 V = 12V
Max permissible volt drop = 3% x 230 V = 6.9V
Calculate the volt drop for the circuit and ensure that it does not exceed above values.
NB: We are not sure as to what regulations apply in Oman.
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