Voltage Drop

[Golden_Boy]

Why all the complicated formulea for working out voltage drop?
Why not just connect the load and measure the voltage with a meter?

 

[Jonny66]

Sorry for raking it up again but it's interesting. Table 7.1(1) latest OSG gives a maximum length of cable as 59m for 1.0mm with ref method 101 on a type B 6a mcb. Table F6 OSG gives ccc of 1.0mm with ref meth 101 of 10.5a and vd of 44mV/A/m. But the cable length permitted doesn't work out if you calculate the vd with an Ib of 6a and cable length of 59m. 44 x 6 x 59 / 1000 = 15.58v which is way in excess of 6.9v (3%). Or have I picked this up all wrong?

What I'm trying to say is that the values in Table 7.1(1) for maximum length have probably..... taken into account that the lighting load will most likely be a lot less than 6amps and that this load will be spread out along the length of the circuit and not all lumped at the end. Hence using the standard volt drop calculation with full load and maximum distance will not equate to this standard circuit value. At the end of the day it is only a guide and doing your own calculations will give you a much more accurate value.

 

[Rob Smith 643]

There's nowt straightforward in this field...

 

[Knobhead]

This may explain it.
The voltages and % are cumulative
View attachment 14749

 

[Rob Smith 643]

The drop increases along the cable run dependent upon the load applied to it i.e., lighting points and longer cable unlike a shower circuit where all the load is applied at the far end? And the tables for vd in lighting circuits account for this probability?

 

[Knobhead]

You've got it. As you go along the cable the load drops in stages, each stage gives a lower voltage drop and %