How are max circuit lengths calculated in table 7.1

[n180]

Something I'm not quite getting.

In table 7.1(i) of onsite guide the max circuit length of (for example) a lighting circuit on 6 amps, with rcbo and wired in 1.5mm is 106 meters.

Where does this 106 meters come from because if max volt drop allowed is only 3% for lighting, 106m works out to 8% volt drop?

same sort of case for ring circuits.

What am I missing?

 

[Ian1981]

For lighting circuits it assumes an evenly distributed load where the average current is half the total load current (Ib goes from 6 amp as is the mcb to 3 amp) so 3 amps is used in the calculations and not 6 amps.
Of course the on site guide gives the most onerous results, you don’t need to take them as gospel as your loading thus calculations maybe totally different giving you more length or sometimes less than what’s given in the on site guide.
Limiting VD can be given as
6.9 X 1000/ Ib/2 X ( mV/A/m)

 

[n180]

Thanks, I didn't realise Ib can be divided by 2 in the calcs.

So for any circuit we design we can claim even distributed load by dividing any Ib by 2?

 

[Ian1981]

Thanks, I didn't realise Ib can be divided by 2 in the calcs.

So for any circuit we design we can claim even distributed load by dividing any Ib by 2?

Generally it’s limited to lighting circuits as the load isn’t all lumped at the extremity of the circuit and the loads to each luminaries are generally even ie similar sized lamps used or the actual individual lighting loads are negligible unlike say a radial socket circuit where anything upto 13 amps could be plugged into a individual outlet or a fixed load radial like a shower.