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Guidance on Long-Run DC Cable Design for Lighting

Pabindu Gamage

Hi all,

I’m designing a greenhouse lighting system with a centralized DC PSU feeding multiple luminaires. The DC voltage varies between 200 V and 500 V.

The distance from the centralized panel to the first luminaire is around 50 m, and the total circuit length is approximately 100 m.

I have experience with AC cable calculations and design, but this is my first time working with long-run DC cabling. I would greatly appreciate expert guidance on:

  • Any special considerations when selecting cables or performing cable calculations for DC circuits — what is different from AC design?

  • Relevant standards or documents I should refer to.

  • Use of overload and RCD protection: do I need a specific type of RCD for DC circuits (Type B / RDC-PD), especially since I must provide minimum 300 mA RCD protection for greenhouse lighting?

I’m trying to expand my experience into DCcable design, so any advice or pointers would be greatly appreciated.

Thanks in advance for your support!

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  • +1

    Any special considerations when selecting cables or performing cable calculations for DC circuits

    Not really. AC calculations are done on the basis of "r.m.s." values - i.e. the d.c. equivalent - so Ohms Law and related resistance based calculations should be just the same. (For larger cables inductive/reactive effects will be rather different of course, but for smaller cables typically only the resistive component is considered anyway).

    Relevant standards or documents I should refer to.

    If it's in the UK or simialr, BS 7671 would be a good starting point I would have thought.

    Use of overload and RCD protection

    This bit is tricker. Fuses and MCBs should properly be d.c. rated, as breaking d.c. arcs is much more difficult than for the equivalent a.c. situation. RCDs I suspect will be very problematic - I don't know of any available ones that work on a d.c. supply. B-type ones for instance are usually good for spotting d.c. residual currents, but sill need an a.c. supply as far as I know.

       - Andy.

  • 0 in reply to AJJewsbury

    Thank you very much for your support.
    Yes, this project is based in the UK.

    Regarding RCD protection : do you think it would be advisable to install an RCD upstream of the AC/DC converter (i.e., the power supply unit)? I don’t want to create any issues by doing so, but I’d like to confirm whether such an RCD would actually be effective in detecting and protecting against leakage currents on the DC side of the circuit.

    Again, I greatly appreciate your support. 

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  • 0 in reply to AJJewsbury

    Thank you very much for your support.
    Yes, this project is based in the UK.

    Regarding RCD protection : do you think it would be advisable to install an RCD upstream of the AC/DC converter (i.e., the power supply unit)? I don’t want to create any issues by doing so, but I’d like to confirm whether such an RCD would actually be effective in detecting and protecting against leakage currents on the DC side of the circuit.

    Again, I greatly appreciate your support. 

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