Voltage drop is Vd = I × R′₂₀ × (1 + 0.00393 × (T − 20)) × L, with R′₂₀ the wire's resistance per metre at 20 °C, T the conductor temperature and L the loop length (feed plus return). 10 A over 5 m of 1.5 mm² FLRY-B with a wire return drops 1.27 V at 20 °C and 1.47 V at 60 °C — about 10 % of 14 V.
The formula
- R′₂₀: the wire type's maximum conductor resistance per metre at 20 °C, from its datasheet — for FLRY-B 1.5 mm², 12.7 mΩ/m bare copper (Prysmian).
- Temperature: copper resistance rises by 0.393 % per kelvin (NBS Handbook 100). Using the insulation's rated temperature is the conservative choice.
- Loop length: with a wire return, add feed and return. With a chassis return, use the feed length plus the measured return resistance; FAA guidance treats structure return as negligible only when bonding is adequate and checked.
- Series parts: a fuse, contacts and splices add resistance too — a 10 A ATOF blade fuse alone drops about 0.11 V at its rated current (Littelfuse: 109 mV typical; cold resistance 7.70 mΩ).
Worked example
| 1.5 mm² FLRY-B | 2.5 mm² FLRY-B | |
|---|---|---|
| R′₂₀ | 12.7 mΩ/m | 7.6 mΩ/m |
| Drop at 20 °C | 1.27 V | 0.76 V |
| Drop at 60 °C (× 1.157) | 1.47 V | 0.88 V |
| Share of 14 V at 60 °C | ≈ 10.5 % | ≈ 6.3 % |
What limit to use
| Sector | Limit | Source |
|---|---|---|
| Aircraft, 14 V system | 0.5 V continuous, 1 V intermittent | FAA AC 43.13-1B, Table 11-6 |
| Aircraft, 28 V system | 1 V continuous, 2 V intermittent | same |
| Marine | 3 % for critical loads, 10 % for others | BoatTEST (third party; ABYC page not reachable) |
| Buildings (IEC, public supply) | 3 % lighting, 5 % other uses | IEC 60364-5-52 Annex G via etoolsbox (third party) |
| Starter cranking circuit | 0.5 V in total | Delco Remy |
| Automotive, general | No public percentage found: check the voltage at the load against the device's minimum operating voltage (ISO 16750-2 codes, e.g. 9 V or 10.5 V for code C or D in 12 V systems) | ISO 16750-2:2003 preview |
Use the limit your customer or the applicable standard specifies. Where there is none, a percentage you choose is a design decision, not a fact — write it on the drawing.
In Zorowire
The engineering checks in the Workspace compute the voltage at each load from the harness's real wire lengths, gauges and series parts, and show the source of every value — labelled as checks to verify, never as approvals.
Questions
How much voltage drop is acceptable in a 12 V harness?
There is no single public automotive percentage. Check that the voltage at the load stays above the device's minimum operating voltage; aircraft guidance gives 0.5 V continuous for 14 V systems.
Do I count the return wire?
Yes. With a wire return the loop is feed plus return. With a chassis return, use a measured return resistance rather than assuming zero.
Why does temperature matter?
Copper's resistance rises about 0.39 % per kelvin, so a wire at 60 °C has about 16 % more resistance than at 20 °C.
Sources
- NBS Handbook 100, Copper wire tables (resistivity, temperature coefficient) · retrieved 2026-10-30
- Prysmian FLRY-B datasheet DF 049-2000 index V (2020-10-06) · retrieved 2026-10-30
- FAA AC 43.13-1B, chapter 11 (voltage-drop table 11-6, bundle derating) · retrieved 2026-10-30
- BoatTEST, boat DC electrics part 3 (3 % / 10 % marine voltage drop) · retrieved 2026-10-30
- etoolsbox, IEC 60364-5-52 Annex G voltage-drop limits (3 % / 5 %) · retrieved 2026-10-30
- Delco Remy tech tip, diagnosing starter cranking problems (0.5 V) · retrieved 2026-10-30
- ISO 16750-2:2003 free preview (supply voltage codes A–G) · retrieved 2026-10-30
- Littelfuse ATOF 287 blade fuse datasheet (opening times, cold resistance, I²t, temperature table; distributor copy) · retrieved 2026-10-30
- Zorowire electrical research and published test vectors — Zorowire test or research, 2026-10-30
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