Size of cable for SON 150w floodlight

Hi all,

I am not really a novice at electrics, I am pretty clued up on terminology and domestic installation etc, however it is more of my hobby :D

I am installing some SON 150w floodlights, I will have 2 running.

First question, would it be better to run them on a ring? Or will a radial be ok?

Second question, they are 40m away from power source, am i right in thinking that a 1.5mm2 3 core flex cable would normally suffice for running the cables, but as its so far away it would be best to uprate the cable to a 2.5mm2 flex?

Also I would appreciate it if somebody could post the equations on how to calculate the size of cable for future references!!!!

Kind Regards


You might be better posting this on or searching the IET forum website, there are some discussions there about ship electrics.

ok, let me see if I got this right then.....

The ships electrics are continental and running at 220v and 380v from a 30kva genset.

I=P/U 540/220= 2.25 - so the current is 2.25A

and according to Table 4.7 - Current ratings and volt drops for sheathed multi-core p.v.c.-insulated cables

I could get away with a 1.0mm2 twin core cable? As that is rated for 11amps?

the voltage drop for that size cable is 44.0mv/A/M

so 44 x 2.25 x 40 / 1000 = 3.96V

which is within the permissable drop of 4% as it equates to a drop of 1.8%

Something doesn't look right there to me, I would have thought the cable size should be bigger than that?

Any help will be appreciated thanks.

I'm not sure which reference book you are using for Table 4.7 but you should be referring to

If you are referring to BS7671 then the cable calculations/voltage drops are generally based on the assumption of TN (C) (S) (TT) earthing system is available. As I understand it a lot of ships have full or partial IT based earthing systems .

This is definitely not a Diy (not) job. :)
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