Voltage drop calculator
Current has to get to the load and back, so a 30 metre run is 60 metres of conductor. That is why the single-phase formula carries a factor of two. Three phase is different — the return currents largely cancel in the star point, so the factor is the square root of three instead, which is why three-phase distribution wins over long distances.
Bands follow the common 3% lighting / 5% total limits. Your local regulations set the figure that actually applies.
Estimates for planning and study. Fixed wiring must be designed and installed to the wiring regulations that apply where you are, by someone competent to do it.
Voltage drop is 2 × ρ × L × I ÷ A for single phase, where ρ is resistivity (0.0172 Ω·mm²/m for copper), L is the one-way length, I the current and A the conductor area. Sixteen amps over 30 m of 2.5 mm² copper drops 6.6 V — 2.9% of a 230 V supply.
How to calculate voltage drop
Voltage drop rarely trips anything; it just quietly degrades everything downstream. Lamps run dim, motors run hot and draw more current to make the same torque, and heating elements deliver less than their rating. The classic 3% and 5% limits exist because equipment is designed to tolerate roughly that much and no more. On low-voltage systems the problem is far worse in proportion: a 2 V drop is under 1% at 230 V but nearly 17% at 12 V, which is why campervan and solar wiring is so much thicker than it looks like it needs to be.
Questions
Commonly 3% for lighting and 5% total from the origin to the furthest point, though the exact figure is set by your local wiring regulations.