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Copper, or aluminium — the conductor metal on the cable's data sheet.
Aluminium carries about 1.6 times copper's resistance for the same area, and IEC 60228 lists aluminium conductors only from 10 mm²; it is met mainly on larger submains and supply cables.
The cross-section of one conductor, as printed on the cable.
The area of a single conductor, not the whole cable: a cable marked 3 × 2.5 mm² has three conductors of 2.5 mm² each. For aluminium, IEC 60228 starts at 10 mm².
The cable's maximum operating temperature, from its data sheet.
A conductor's resistance rises with its temperature — about 0.4% per °C (0.22% per °F) for copper — and the published voltage-drop tables are worked at the cable's maximum operating temperature, so that is the figure to choose. Prysmian's 6242Y twin and earth is 70 °C (158 °F); thermosetting cables run to 90 °C (194 °F).
A single-phase or DC circuit, or a balanced three-phase one.
A single-phase circuit loses voltage in both the line and the neutral, so the resistance counts twice. A balanced three-phase circuit uses √3 in place of the 2, and the drop is between lines.
The route length from the origin of the circuit to the load, one way.
Measure the route the cable takes, including rises and drops, not the straight line. The return conductor is already counted by the 2 for single phase.
The circuit's design current, in amps.
Use the current the wiring rules tell you to calculate with — usually the design current of the load. For three phase, the current in one line.
The nominal supply voltage — line to line for three phase.
The voltage does not change the volts lost; it is what the percentage is taken of. For a DC circuit or any other supply, choose another voltage and type it.
Voltage drop
7.251 V
Worked from the conductor's IEC 60228 resistance at the chosen temperature: the same quantity the published mV/A/m tables give for a cable this size, where reactance is negligible. Compare the percentage with the limit your wiring rules set.
- Voltage drop as a share of the supply
- 3.15 %
- Voltage at the load
- 222.75 V
- Conductor size in mm²
- 2.5 mm²
- Voltage drop per amp per metre (mV/A/m)
- 18.02
- Conductor resistance at the chosen temperature (Ω/km)
- 9.01
- IEC 60228 resistance at 20 °C (68 °F), Ω/km
- 7.41
They open the calculator with your figures already in it
Cable Voltage Drop Calculator (mm², Copper or Aluminium): 7.25 V — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Bayerische Kabelwerke AG (Bayka), Conductor resistance at 20 °C (68 °F) acc. to EN 60228: maximum DC resistance of plain copper and aluminium conductors, class 1 and 2 — 2.5 mm² copper 7.41 Ω/km (2.26 Ω per 1,000 ft), 16 mm² copper 1.15 (0.351), 25 mm² aluminium 1.20 (0.366) — and of flexible (class 5 and 6) and tinned copper, mostly higher: flexible 2.5 mm² copper 7.98 (2.43), tinned and flexible 8.21 (2.50)
- Prysmian 6242Y data sheet (BS 6004 flat twin and earth): maximum conductor temperature 70 °C (158 °F); conductor resistance at 20 °C (68 °F) 18.1, 12.1, 7.41, 4.61, 3.08, 1.83 and 1.15 Ω/km for 1 to 16 mm² (5.52, 3.69, 2.26, 1.41, 0.939, 0.558 and 0.351 Ω per 1,000 ft); current ratings referred to BS 7671 Table 4D5
- NBS Handbook 100, Copper Wire Tables (National Bureau of Standards): the annealed copper standard, 0.017241 Ω·mm²/m (10.371 Ω·cmil/ft) at 20 °C (68 °F), with a temperature coefficient of 0.00393 per °C (0.00218 per °F) at 20 °C
- NBS Handbook 109, Aluminum Wire Tables (National Bureau of Standards with The Aluminum Association), Table 1: EC-H19 aluminium at 61.0% IACS, 0.028264 Ω·mm²/m (17.002 Ω·cmil/ft) at 20 °C (68 °F), temperature coefficient 0.00403 per °C (0.00224 per °F) at 20 °C
- Voltage drop from resistance: a single-phase or DC circuit loses I × R on the way out and again on the way back, 2 × R per metre; a balanced three-phase circuit loses √3 × R per metre between lines. Ω/km is numerically mV/A per metre, and 0.3048 times it is mV/A per foot
Inputs used
- Conductor
- Copper
- Conductor size (mm²)
- 2.5 mm²
- Conductor temperature
- 75 °C (167 °F) — cable whose insulation carries that rating
- Circuit
- Single phase or DC, out and back
- Length of run (one way)
- 66 ft
- Current (A)
- 20
- Supply voltage
- 230 V single phase
- Voltage you are working at (V)
- 48
Intermediate steps
- Voltage drop as a share of the supply
- 3.15 %
- Voltage at the load
- 222.75 V
- Conductor size in mm²
- 2.5 mm²
- Voltage drop per amp per metre (mV/A/m)
- 18.02
- Conductor resistance at the chosen temperature (Ω/km)
- 9.01
- IEC 60228 resistance at 20 °C (68 °F), Ω/km
- 7.41
Confidence note: Worked from the conductor's IEC 60228 resistance at the chosen temperature: the same quantity the published mV/A/m tables give for a cable this size, where reactance is negligible. Compare the percentage with the limit your wiring rules set.
What this calculation does not cover
- Resistance only. BS 7671's Appendix 4 and AS/NZS 3008.1.1 tabulate an impedance for the larger cables, which adds the conductor's reactance to its resistance; from about 25 mm² that reactive part is a growing share of the drop and this page leaves it out, so for a large cable the published table's figure is the one to use. Neither table is reproduced here.
- The conductor temperature is the one chosen, held along the whole run. The published tables assume the cable is at its maximum operating temperature; a lightly loaded cable runs cooler and drops a little less, and one bunched or in hot surroundings may sit at its limit sooner than the current suggests.
- The √3 for three phase holds only while the three line currents are equal and the neutral carries nothing. A load fed from one line and the neutral of a 400 V supply is a 230 V single-phase circuit, and should be entered as one; an unbalanced three-phase load needs each line worked on its own.
- The current used is the one entered. The wiring rules say which current a calculation should use — the design current, or for a circuit whose load is spread along it, a figure they set — and which length; this page takes both as typed.
- Voltage drop is not current-carrying capacity. A cable inside the voltage-drop limit can still be too small for the current once its installation method, grouping and surroundings are applied, and that is a separate selection under the wiring rules, made by the electrician who designs and certifies the circuit.
- The limit the drop is judged against belongs to the wiring rules in force — BS 7671's Appendix 4 in the UK and, in Australia and New Zealand, AS/NZS 3000 — and to any tighter figure an equipment maker sets. The page reports the percentage and does not judge it.
- The resistances are IEC 60228's for plain conductors of class 1 and 2, the solid and stranded conductors of fixed wiring. Bayka lists flexible conductors (classes 5 and 6) and tinned ones at a higher resistance for most sizes — flexible 2.5 mm² copper 7.7% above the plain figure, flexible 1.5 mm² 9.9%, tinned flexible 2.5 mm² 10.8% — so a flexible cord or an extension lead drops more than this page shows.
Add the equipment this sizes
This result is a specification — 7.251 V — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations5
- Bayerische Kabelwerke AG (Bayka), Conductor resistance at 20 °C (68 °F) acc. to EN 60228: maximum DC resistance of plain copper and aluminium conductors, class 1 and 2 — 2.5 mm² copper 7.41 Ω/km (2.26 Ω per 1,000 ft), 16 mm² copper 1.15 (0.351), 25 mm² aluminium 1.20 (0.366) — and of flexible (class 5 and 6) and tinned copper, mostly higher: flexible 2.5 mm² copper 7.98 (2.43), tinned and flexible 8.21 (2.50)
- Prysmian 6242Y data sheet (BS 6004 flat twin and earth): maximum conductor temperature 70 °C (158 °F); conductor resistance at 20 °C (68 °F) 18.1, 12.1, 7.41, 4.61, 3.08, 1.83 and 1.15 Ω/km for 1 to 16 mm² (5.52, 3.69, 2.26, 1.41, 0.939, 0.558 and 0.351 Ω per 1,000 ft); current ratings referred to BS 7671 Table 4D5
- NBS Handbook 100, Copper Wire Tables (National Bureau of Standards): the annealed copper standard, 0.017241 Ω·mm²/m (10.371 Ω·cmil/ft) at 20 °C (68 °F), with a temperature coefficient of 0.00393 per °C (0.00218 per °F) at 20 °C
- NBS Handbook 109, Aluminum Wire Tables (National Bureau of Standards with The Aluminum Association), Table 1: EC-H19 aluminium at 61.0% IACS, 0.028264 Ω·mm²/m (17.002 Ω·cmil/ft) at 20 °C (68 °F), temperature coefficient 0.00403 per °C (0.00224 per °F) at 20 °C
- Voltage drop from resistance: a single-phase or DC circuit loses I × R on the way out and again on the way back, 2 × R per metre; a balanced three-phase circuit loses √3 × R per metre between lines. Ω/km is numerically mV/A per metre, and 0.3048 times it is mV/A per foot
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Protection this work requires
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Tools and safety for this job
To first-fix electrical. Generic types, no brands, no prices.
- Assume every cable is live until proved dead at the point you will touch, with a two-pole tester you have just proved on a known source.
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