Voltage drop table — mV/A/m for every copper cable size

The voltage drop formula

Voltage drop in a copper cable is its mV/A/m figure multiplied by the current in amps and by the one-way run in metres, divided by 1000 to give volts; for a three-phase circuit the result is multiplied by 0.866. The mV/A/m figures published here are BS 7671 (IEC 60364-5-52) values for PVC-insulated copper conductors at their 70°C operating temperature, reference method C, clipped direct, 30°C ambient, and Pilone Cables sizes against a 2.5% design target — 5.75 V on a 230 V single-phase supply, 10.00 V on a 400 V three-phase supply.

Two things the formula does not ask for, and both catch people out. It does not ask for the return length — the mV/A/m figure already counts the current going out and coming back, so measure one way only. And it does not ask for the breaker size. A 63 A breaker on a circuit that draws 20 A drops the volts of 20 A, not 63 A.

Voltage drop table: mV/A/m by copper size

This is the reference table. The single-phase column is the published figure; the three-phase column is that figure × 0.866. The ready-reckoner column is there for a quick sanity check in the field: it is the volts this size loses at 10 A over 10 m.

The capacity columns are on the table for one reason: a run that passes on volt drop can still be over the conductor. Both checks have to pass, and on short runs it is capacity that decides the size while on long runs it is volt drop. 0.75 mm² and 1 mm² are flexible-cord sizes and IEC 60228 class 2 solid or stranded resistance is not published for them here, so those two rows carry a dash. The capacity figures here are the base method C values before any derating; the corrected ones, for conduit, bunching and Pakistani summer ambient, are on the cable current capacity chart.

Worked example 1 — single phase, 230 V

An air conditioner point on a house circuit. 4 mm² copper, 8 A running current, 25 m from the distribution board to the isolator.

Because it passes with room to spare, the useful next question is how far that size could go before the budget runs out: 5750 ÷ (11 × 8) = 65 m at this current. Below 4 mm² the arithmetic tightens fast. The same 8 A over the same 25 m on 2.5 mm² gives 18 × 8 × 25 ÷ 1000 = 3.6 V, which still passes, but its ceiling is 5750 ÷ (18 × 8) = 40 m. On 1.5 mm² it is 29 × 8 × 25 ÷ 1000 = 5.8 V — over budget at 25 m, and that size is under the 20 A breaker such a circuit usually carries anyway. The 1.5 ton AC cable guide works the same circuit through in full.

Worked example 2 — three phase, 400 V, and the 0.866 factor

A three-phase submain. 25 mm² four-core copper, 60 A per phase, 45 m from the main switch to the sub-board.

Where does 0.866 come from? On a balanced three-phase circuit the drop that matters is line to line, and the phase-to-phase relationship puts a factor of √3 into the numerator and 2 into the denominator: √3 ÷ 2 = 0.866. In plain terms, three-phase distribution moves the same power with less volt drop than single phase, which is why long factory and farm runs go three-phase. Check the capacity as well as the drop: at 60 A this cable is inside the 96 A three-loaded-conductor rating before derating, and the run could reach 10000 ÷ (1.52 × 60) = 110 m before the budget is spent. The cable itself is our 25mm 4 core standard cable; the sizing context sits on 3 phase cable size.

What the 2.5% budget means in volts

A percentage is not a number you can check a cable against. Convert it to volts first, then the arithmetic above either fits inside it or does not.

The budget is spent once, across the whole path from the meter to the appliance, not once per cable. If the submain from the meter to the distribution board has already used 2 V, the final circuit has 3.75 V left of a 5.75 V allowance, not the full amount. On a long plot — a kanal house with the meter at the boundary wall and the board inside — the submain can eat most of it before a single room is wired, which is why the meter to distribution board cable is sized on drop rather than on load.

Two consequences worth stating plainly. Lighting shows the drop first: filament and older fittings dim visibly, and an LED driver may hum or flicker. Motors show it worst: a motor at reduced voltage draws more current to hold its torque, gets hotter, and the extra current makes the drop worse still. That feedback loop is what burns out a water pump at the far end of a long run, and it is covered on voltage drop problems.

Longest single-phase run each size can hold

The formula rearranged. Longest run in metres = 5750 ÷ (mV/A/m × amps). Find your current across the top, your size down the side, and read the metres where they meet.

Read the 2.5 mm² row honestly and a common Pakistani wiring habit falls apart. At 20 A it holds 16 m. A socket ring or a geyser point at the far corner of a 10 marla house is past that before the cable leaves the corridor, and the answer is 4 mm² or 6 mm², not a bigger breaker. A bigger breaker protects nothing about volt drop; it only lets the same thin conductor carry more current and lose more volts.

Longest three-phase run at the 10 V budget

Same rearrangement, three-phase figures: longest run = 10000 ÷ (mV/A/m × 0.866 × amps). Currents are per phase.

Sizes below 4 mm² are left off this table because a three-phase circuit rarely runs that small and the rows would be dashes. Note what the two tables say side by side: 25 mm² at 63 A reaches 52 m single-phase and 105 m three-phase. Same copper, double the reach, because the drop factor and the supply voltage both work in your favour.

Check these figures against IEC 60228 yourself

A volt-drop table with no derivation is a table you have to trust. This one you can audit. Take the IEC 60228 class 2 DC resistance at 20°C, double it because the current travels out and back, then multiply by about 1.20 because a PVC cable at its 70°C rating has around 20% more resistance than the same copper at 20°C. The result lands on the published mV/A/m figure across the whole ladder.

That is the whole reason the mV/A/m figure is not simply twice the resistance you find on a datasheet. A cold conductor and a working conductor are not the same conductor. It is also the check to run on any volt-drop chart you are handed: if its figures come out near twice the 20°C resistance with no temperature allowance, it is under-reporting the drop by about a fifth. Conductor resistance and stranding for our own goods are published on every product page — for instance 10mm 3 core standard cable and 16mm 4 core standard cable.

The supply is 230 V on paper and less on the street

The budget column is worked at 230 V because that is the declared single-phase voltage the tables are built on. Pakistani supply is not always that. On a 220 V measurement the same 2.5% is 5.50 V, and a 5.75 V drop that was exactly 2.5% becomes 2.61%. When WAPDA or K-Electric supply sags to 190 V at peak, that same 5.75 V is 3.03% of what the appliance actually sees.

There is a second effect, and it is the one that matters. A constant-power load draws more current at lower voltage: 1800 W at 230 V is 7.8 A, at 190 V it is 9.5 A. More current through the same cable means more drop, on top of the sag that started it. So a sagging supply and a thin cable compound each other. What that does not mean is that thicker copper fixes a bad supply. Take that same 9.5 A over a 20 m run: 2.5 mm² drops 18 × 9.5 × 20 ÷ 1000 = 3.42 V and 4 mm² drops 11 × 9.5 × 20 ÷ 1000 = 2.09 V, so the bigger cable hands back 1.33 V at the terminals against the 40 V the street took. Size the cable properly for the run, and treat street voltage as a separate problem with a separate solution.

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