Voltage Drop

[Traineeboy]

I understand that in a domestic dwelling single phase you are allowed 5 % power and 3 % lighting voltage drop. 

Just some questions I wanting to clarify .....

1. I assume that voltage drop is from the main incomer not the distribution board the circuit is coming from. 

2. If you had to wire a lighting circuit in 2.5 mm2 cable due to voltage drop but then at the switch go off to the light in 1.5 mm2 ( to fit the conductors into the light)  would be OK ?  Remember the reason for the larger cable isn't because of CCC or other reasons but purely because of the voltage drop. This would obviously be wiring at the switch not loop method. 

 

[binky]

1/ as I understand it, voltage drop is from the main incomer, but you will tned to find Zdb is generally very low, so doesn't make that much difference.

2/ commercial lighting, like supermarkets, tends to be wired like this. There is also a long protracted argument as to why lighting drops in smaller cables such a s 1mm 3 core is perfectly OK on MCBs such as 16A - short version is that a light of say 100w is never going to overload the 1mm cable, so MCB becomes effectively irrelevent but will still protect against a dead short. 

 

[Doc Hudson]

I understand that in a domestic dwelling single phase you are allowed 5 % power and 3 % lighting voltage drop. 

Just some questions I wanting to clarify .....

1. I assume that voltage drop is from the main incomer not the distribution board the circuit is coming from. 

Whilst technically correct that 525.202 refers to the origin of the installation, (usually the supply terminals), if you are considering an average domestic dwelling, the real-world effects meter tails have on compliance with volt-drop is generally negligible.  Try calculating (mv/A/m) on the "tails part" of a circuit compared to the actual final circuit itself.  e.g. a 45m radial supplying a 30A load, with 3m long supply tails. 

If tails are 16mm (2.8x30x3)/1000 = 0.25v   or  If tails are 25mm (1.75x30x3)/1000 = 0.16v

If circuit is 6.0mm (7.3x30x45)/1000 = 9.86v   or  If circuit is 10.0mm (4.4x30x45)/1000 = 5.94v

So if volt-drop was a problem, any potential improvements you could make by upgrading the tails are so small (less than 0.1v), whereas the final circuit can get significant improvements by cable size selection. (even if tails were running at 80A or 100A you are still only looking around half a volt if you had 3m tails. Do the calculations with 1.5m tails and you will see how small and insignificant the values become.

Doc H.   

 

[Traineeboy]

Thanks chaps I love learning this stuff. Sounds sad I know. 
 

 

[binky]

Thanks chaps I love learning this stuff. Sounds sad I know. 
 

not sad at all, and will come in handy one day.

 

[Andy™]

Thanks chaps I love learning this stuff. Sounds sad I know. 
 

not rally. makes a change having people who want to learn how to do the job properly instead of just lashing in anything they think suitable. at least you can say why its suitable and compliant