EF: Voltage Drop Calculation vs OSG

[Watkins88]

Voltage Drop Calculation vs OSG

Hi All,

Hoping to get opinions on Vd calculations for a 1.5mm2 downstairs lighting circuit with an estimated length of 128m!
Length is due to loops at switches (neutrals required) and switched lives feeding various strings of lights.

Parameters:
Mv/A/m - 29
ib - 2.38
L - 128

Method 1:
Using the standard forumla Mv/A/m x ib x L / 1000 we get 8.82v (3.84%).
This seems unrealistic as it assumes the full load is at the end of the entire circuit length.

Method 2:
An adaptation of the standard forumla is to halve the length provided the load is evenly distributed. Example.
Using Mv/A/m x ib x (L/2) / 1000 we get 4.41v (1.92%).
Given this is a domestic circuit and not a car park, I'm not convinced the load can be assumed to have an even distribution.

Method 3:
Calculate Vd between each point and derive the sum. Example.
Loads have been totalled with loops and corresponding switched lives treated as one length between each point.
The current is adjusted to account for the voltage drop at each point. The result is a small increase from 2.35A to 2.38A.
From the table below, Vd comes out at 4.35V (1.89%).

OSG Table 7.1(i) suggests a max length of 106m for a 1.5mm2 lighting circuit with a 6A Type B CB on a TN-S supply with 30ma RCD.
The question, Is 128m acceptable provided both EFLI & Voltage drop are within max Zs/3%?

For completeness the max Zs is 5.83Ω and calculated Zs at 2.67Ω. See table below:

The circuit could be split but with only 2x 6A MCBs for upstairs/downstairs lights, no spare ways and a new CU out of question, this would inevitably result in two radials off a single MCB. Another option could be to run the loops with 2.5mm2 and the switched lives with 1.5mm2. This could limit choice of accessories as terminal sizes typically accept upto 1.5mm2.

Comments and suggestions appreciated!

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