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Current and voltage-drop checker

Wire size, insulation, ambient, bundle and current in; current capacity, conductor temperature, voltage drop and fuse bounds out — with every table and assumption shown.

No account. Nothing is stored. · Last updated

In short

A wire's current capacity is its rating from a published table for its size and insulation temperature, reduced for a hotter ambient (∝ √(Tc − Ta)) and for the other loaded wires in the bundle. Voltage drop is current × resistance per metre × the loop length (feed and return), with the resistance taken at the insulation's rated temperature to stay on the cautious side. A fuse should carry the load continuously at its own ambient and must not be larger than the smallest current capacity of the wire it protects.

Engineering check — verify against the manufacturer's data and the applicable standard.

Worked example

10 A through 5 m of FLRY-B 1.5 mm² (105 °C insulation) with a 5 m return wire, at 40 °C ambient, in a bundle of 4 loaded wires: current capacity 15.46 A, conductor about 64 °C, voltage drop 1.69 V (14.1 % of a 12 V supply). Littelfuse ATOF blade fuses that pass both bounds: 15 A.

The wire
The circuit
Fuse

Results

OK up to 80 % of the limit, warning up to 100 %, error above. A result built on an estimate never reads OK.

Current capacityOK
15.46 A65 % usedContinuous current 10 A · Tc 105 °C · Ta 40 °C · k 0.55 (4)Engineering check — verify against the manufacturer's data and the applicable standard.
Conductor temperatureOK
63.9 °Climit 105 °CEngineering check — verify against the manufacturer's data and the applicable standard.
Voltage drop
1.694 V14.1 % · 12 VEngineering check — verify against the manufacturer's data and the applicable standard.
Voltage at the loadError
10.31 Vlimit 10.5 V · 113 % usedEngineering check — verify against the manufacturer's data and the applicable standard.
Fuse carries the loadOK
13.11 A76 % usedEngineering check — verify against the manufacturer's data and the applicable standard.
Fuse protects the wireOK
15 Alimit 15.46 AEngineering check — verify against the manufacturer's data and the applicable standard.
Short circuit (I²t)WarningEstimate
510 A²slimit 17,187.2 A²sEngineering check — verify against the manufacturer's data and the applicable standard.

Ratings in this family that pass both bounds: 15 A

All ratings of the family
RatingContinuousProtects wireShort circuit
1 AErrorOKWarning
2 AErrorOKWarning
3 AErrorOKWarning
4 AErrorOKWarning
5 AErrorOKWarning
7.5 AErrorOKWarning
10 AErrorOKWarning
15 AOKOKWarning
20 AOKErrorWarning
25 AOKErrorWarning
30 AOKErrorWarning
35 AOKErrorWarning
40 AOKErrorWarning

Estimates in this result

  • Estimate — based on short-circuit k 87.4 computed for PVC 105 °C → 160 °C (assumed final temperature). Replace with the manufacturer's rating to confirm.

Values from

  • Automotive cables, single cable isolated in free air, 40 C ambient — objects.eanixter.com (2026-10-30)
  • FLRY-B (ISO 6722-1 conductor), max conductor resistance at 20 C — na.prysmian.com (2026-10-30)
  • Grouped and touching automotive cables: rating factor — objects.eanixter.com (2026-10-30)
  • Value from Rating factor for bundling of loaded wires (H05V-K family, per EUPEN) as published by eupen.com; not taken from the standard itself. — eupen.com (2026-10-30)
  • Max allowed continuous load by ambient (typical derating, 20 % temperature security margin) — static.chipdip.ru (2026-10-30)
  • Littelfuse ATOF / ATO Ag 287 — static.chipdip.ru (2026-10-30)
  • Per-rating electrical data — static.chipdip.ru (2026-10-30)
  • What the fuse is for — infinitybox.com (2026-10-30)
  • Adiabatic short-circuit k factors (I2t <= k2 S2, t <= 5 s) — mycableengineering.com (2026-10-30)
  • DC voltage drop — nvlpubs.nist.gov (2026-10-30)
Assumptions
  • profile automotive (automotive_lv)
  • plain copper, max value
  • lower of Anixter (2–8 wires) and VDE 0298-4 (EUPEN excerpt); beyond 8 wires only the VDE list (H05V-K family) applies
  • interpolated between 20 °C (14 A) and 65 °C (12 A)
  • typical melting I²t (not total clearing)

These results are calculations from the data and assumptions listed in this report. They are not a certification, approval or statement of compliance with any standard or regulation (including ISO, SAE, IEC, UL, NFPA, ABYC, UN R100, FMVSS or IPC/WHMA-A-620). Ratings depend on the actual parts, installation, ambient temperature, bundling and duty cycle. The designer remains responsible for the design, for checking every value against the current manufacturer data and applicable standards, and for testing the finished harness.

How it works

Base rating
Automotive: Anixter's free-air table at 40 °C for 90 °C and 105 °C conductors; 125 °C (TXL, GXL, SXL) is scaled from the 105 °C column and marked as an estimate. Aircraft and drones: FAA AC 43.13-1B Table 11-9 (already a worst-case bundle). Machine panels: an EN 60204-1 excerpt. Marine: an ABYC E-11 excerpt.
Ambient and bundle
The rating is multiplied by √((Tc − Ta)/(Tc − Tref)) for the ambient, and by the bundle factor for the number of loaded wires (the lower of Anixter's and VDE 0298-4's factors for vehicles).
Conductor temperature
From the heat balance at the rated point, with resistance rising with temperature; a result above the insulation's rating is an error.
Voltage drop
Resistance per metre from the wire maker (Prysmian FLRY-B, Tensility TXL, FAA aerospace copper), else the IEC 60228 class 5 maximum, else solid-copper AWG values marked as an estimate.
Fuse
Continuous: the load against the fuse's allowed current at its ambient (the maker's temperature table). Wire protection: rating ≤ the wire's current capacity. Short circuit: the fuse's melting I²t × 1.5 against k²S² of the wire.

These results are calculations from the data and assumptions listed in this report. They are not a certification, approval or statement of compliance with any standard or regulation (including ISO, SAE, IEC, UL, NFPA, ABYC, UN R100, FMVSS or IPC/WHMA-A-620). Ratings depend on the actual parts, installation, ambient temperature, bundling and duty cycle. The designer remains responsible for the design, for checking every value against the current manufacturer data and applicable standards, and for testing the finished harness.

Questions

Can I use the result as proof that the harness meets a standard?
No. It is a calculation aid built on published tables and the assumptions listed with each result. The designer stays responsible for the design and for checking it against the manufacturer's data and the applicable standard.
Why does an estimate never show OK?
Because an estimated input (a scaled rating, a converted size, a solid-copper resistance) can be wrong in either direction. The result says which input is estimated, and replacing it with the maker's value clears the flag.
Which voltage-drop limit should I use?
It depends on the load. For vehicles no general percentage is published; the checker compares the voltage left at the load with the lowest voltage the device accepts, which you enter from its datasheet.
Why is the return wire not in the bundle?
The checker routes it on its own. If it runs in the same bundle, count it in the loaded wires.
What does the fuse range mean?
The ratings of the chosen fuse family that carry your load continuously at the fuse's ambient and are not larger than the wire's current capacity (and pass the short-circuit check where its data exists). Outside that range the fuse either opens in normal use or does not protect the wire.

Sources

Last updated

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