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Cable Sizing for 2 nos x 400W Floodlight

Qaslun

I'm trying to design one single line diagram (just for learning session) and one of my circuit is having 2 nos. x 400W Floodlight with 1.5sqmm Cu/PVC cable, with 30m cable length, MCB rating 10A all the calculation such as load current Ib, MCB rating In, derating factor, cable carrying capacity and voltage drop is shown in the above photo.

Based on my calculation, my Ib < In < Iz is  3.9A < 10A < 17.5A respectively, with voltage drop of 3.933V @ 1.147% from 230V, 

However, after discussing this with several knowledgeable individuals and referring to previous single-line diagrams that I've encountered, they've indicated that 1.5sqmm might not be sufficient, instead they requested me to change it to 2.5sqmm. (I do understand 2.5sqmm is better than 1.5sqmm in terms of cable-current carrying capacity, and voltage drop).

My query is, where might my calculation have gone wrong in this context?  



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  • With the changes to the C&G EVSE courses there is a sudden resurgence in interest in doing full circuit design calculations as electricians are failing the domestic and commercial course exam, because they can’t do the full circuit design calculation.

    I did it all properly twenty years ago when I did the old C&G 2400 Design, Erection and Verification course, but have rarely done a full calculation since, so I am out of practice and having to think about it.

    Looking at your calculation it seems you have not multiplied the wattage of the lamps by 1.8 as recommended for discharge lighting in the IET guides, such as the IET On-Site Guide on page 136.

    You have simply said they are 400 watt floodlights, not exactly what they are, multiplying the wattage by 1.8 may need to be considered, we would almost have certainly been doing it twenty years ago.

  • in reply to Sparkingchip

    not multiplied the wattage of the lamps by 1.8

    I thought that was to account for an unknown, possibly quote poor,  PF (plus control gear) - the OP seems to have used PF=0.9 (presumably accurate for those particular fittings).

       - Andy.

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  • in reply to Sparkingchip

    not multiplied the wattage of the lamps by 1.8

    I thought that was to account for an unknown, possibly quote poor,  PF (plus control gear) - the OP seems to have used PF=0.9 (presumably accurate for those particular fittings).

       - Andy.

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