# 1.5mm Flex length

Is there a maximum length in regs for using 1.5mm high heat flex in a lighting circuit? If so, what is it!?

Cheers,

Theta.
Theta
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For a lighting circuit the max volt drop at the premises is 3% this is to ensure discharge lamps will start.
Also it should be ensured that the protective device will trip with an overload.
We are given 29 mv/A/m as volt drop and 3% = 6.9v so for a 6A supply that's just under 40 meters.
To trip a B6 MCB with the magnet part will require 5 x 6 = 30A and at 230 volt that means a resistance of 7.6 ohms minus the 0.35 impedance of incoming supply = 7.3 ohms giving over 200 meters.
So going by the shortest of two then around 40 meters is max length.
However with 3A load you could double this and with 12A load half this so much depends on exactly what one is feeding.
Common sense also comes into play. For example with a switch mode (HF) florescent lamp able to run with a voltage of between 100v to 250v with a load of 60W a huge length of cable could be used. But with 6 x 250W sodium lamp using induction start gear then one may be limited to 20 meters and even then need staggered start.
ericmark
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Brilliant. Thanks Ericmark. Much appreciated.
Theta
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