Why voltage drop, not just ampacity, sizes the wire
Every conductor has resistance, so some of the supply voltage is spent pushing current down the cable and never reaches the load. On a short branch circuit that loss is trivial; on a long feeder, a heavy load, or a low-voltage DC run it can be large enough to matter. Ampacity — the current a conductor can carry without overheating — is the first limit, and it is mandatory: it sets the minimum size a code will allow. But ampacity says nothing about how much voltage arrives at the far end. A 12 AWG copper conductor might comfortably carry 20 A, yet drop more than 6% over a 30 m run, and a light or a motor at that end will notice. This is why long runs are so often governed by voltage drop rather than by ampacity: past a certain distance you must size the wire up, not because it would overheat, but because too little voltage would reach the load. A proper design checks both limits and takes the larger conductor the two demand.
The voltage-drop formula and the numbers that drive it
Voltage drop is Ohm's law applied over the whole path the current travels. Multiply the load current by the conductor's resistance for the round trip and you get the volts lost; divide by the supply voltage and you have the drop as a percentage. Four things move that number: the current (double the load, double the drop), the resistance per unit length (a thinner wire has a higher resistance and drops more), the length of the run (drop is proportional to distance), and a phase factor that accounts for how many conductors carry the round trip — 2 for single-phase and DC, and √3 (about 1.732) for three-phase line-to-line. Because current flows out to the load and back, the calculation always uses the round-trip path even though you enter the one-way length. Resistance itself comes from published tables: AWG conductors in ohms per 1000 ft (NEC Chapter 9, Table 8, at 75 °C) and metric conductors in ohms per kilometre (IEC 60228, at 20 °C). Aluminium sits above copper at every size, which is why an aluminium conductor usually has to be a gauge or two larger than copper for the same run and the same drop.
How much drop is acceptable, and who says so
The limits a voltage-drop check measures against are recommendations first and enforceable rules only where a local jurisdiction adopts them. The most common targets are 3% on a branch or lighting circuit and 5% for a feeder or the combined feeder-plus-branch total. In North America these come from the informational notes at NEC 210.19 and 215.2 — informative, not mandatory, unless your authority having jurisdiction has made them binding. In IEC/BS 7671 practice the same 3% lighting and 5% other-uses figures appear in the informative annex of IEC 60364-5-52 and in BS 7671, with a more generous 6%/8% allowed for installations fed from their own transformer or generator rather than the public supply. The practical takeaway is that these are engineering targets you design to, not a licence to ignore your local code: keep a branch circuit under 3% and the whole run under 5% and you will satisfy the great majority of installations, but always confirm the figure your jurisdiction actually enforces before you finalise a design.
Copper or aluminium, AWG or mm², and where the two standards meet
Conductor sizing lives in two parallel worlds that do not convert cleanly into each other. North American practice uses AWG (American Wire Gauge) for smaller conductors and kcmil above 4/0, where a smaller gauge number means a larger conductor; the rest of the world uses cross-sectional area in square millimetres, where a bigger number simply means a bigger wire. There is no exact one-to-one map between them — 12 AWG is close to 4 mm², 10 AWG near 6 mm² — so switching a calculator between imperial and metric does not convert a value, it swaps the whole catalogue of sizes. Material is the other big lever. Aluminium is lighter and cheaper per amp than copper, which is why it dominates larger feeders and service entrances, but its higher resistivity means it drops more voltage and generally has to be sized up relative to copper; it also has termination and connection requirements copper does not. Whichever system and material you use, remember the calculator is answering the voltage-drop half of the question only — it does not check ampacity, overcurrent protection, derating for temperature or conduit fill, or anything else your code requires, so treat its output as a planning figure to be confirmed by a qualified electrician against the code that governs your installation.
