Calculate conductor current capacity from voltage-drop and corrected ampacity limits, or size metric conductor area for a load with copper/aluminum and phase options.
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Current Capacity and Voltage-Drop Formulas
This tool has two modes: estimate current for a chosen conductor, or calculate minimum conductor area for voltage drop. It uses a uniform conductor at 20°C. AC assumes unity power factor and negligible reactance.
Here A is conductor area in mm², ρ is resistivity in Ω·mm²/m, L is one-way length in meters and I is current in amperes. k is 2 for a two-wire DC/single-phase circuit and √3 for balanced three-phase. For three-phase, enter line-to-line circuit voltage. ΔV_limit is circuit voltage times the entered drop percentage divided by 100.
Corrected candidate ampacity = applicable table/manufacturer ampacity × temperature factor/100 × bundling factor/100. Current mode returns the lower of that value and the voltage-drop limit. Ampacity must describe the same chosen conductor, insulation and installation; no rating is inferred from area.
Area mode addresses voltage drop only. Select an actual conductor at least this large, then verify its own ampacity, terminals, protection and installation requirements. The entered candidate comparison does not certify the computed area as safe. The ampacity temperature factor does not increase the resistance used in this 20°C model.
Common Resistivity Values for Conductors
Resistivity depends on material and temperature. The values below are typical approximate values near 20°C.
| Material | Typical resistivity, Ω·m | Conductivity note |
|---|---|---|
| Silver | 1.59 × 10-8 | Very high conductivity |
| Copper | 1.7241 × 10-8 | Common wire conductor |
| Aluminum | 2.8264 × 10-8 | Common lightweight conductor |
| Tungsten | 5.60 × 10-8 | Higher resistance than copper |
| Nichrome | 1.10 × 10-6 | Often used as a resistance wire |
Reference Unit Conversions
The calculator accepts V, mm², m and A as labeled. The table below provides independent reference conversions; it does not describe additional input controls.
| Quantity | Unit | Base-unit conversion |
|---|---|---|
| Voltage | mV | 1 mV = 0.001 V |
| Voltage | kV | 1 kV = 1000 V |
| Area | ft² | 1 ft² = 0.092903 m² |
| Area | yd² | 1 yd² = 0.836127 m² |
| Length | ft | 1 ft = 0.3048 m |
| Length | yd | 1 yd = 0.9144 m |
| Current | mA | 1 mA = 0.001 A |
| Current | kA | 1 kA = 1000 A |
Example
Example 1: Current for a chosen conductor. Use copper at 20°C, a two-wire circuit, 240 V, 30 m one-way length, 3% allowed drop and 10 mm² area. The voltage-drop limit is 7.2 × 10 / (2 × 0.017241 × 30) = 69.6015 A. If the applicable ampacity is 50 A with both correction factors at 100%, the planning result is the lower value, 50 A. Voltage drop at 50 A is 5.1723 V (2.1551%). These are illustrative inputs, not a recommended installation rating.
Example 2: Area for voltage drop. With the same circuit, a design current of 50 A requires 2 × 0.017241 × 30 × 50 / 7.2 = 7.18375 mm² for this 20°C resistive drop limit. Choose an actual area at least this large; its own ampacity and other installation requirements must be verified separately.
FAQ
Why does current decrease when the conductor is longer?
At a fixed allowed voltage drop, area and resistivity, a longer conductor increases resistance and lowers the voltage-drop current limit. The entered corrected ampacity is an independent limit.
Why does a larger cross-sectional area increase current?
A larger area lowers resistance and increases the voltage-drop current limit for the same path and allowed drop. It does not establish ampacity without an applicable rating for the actual conductor and installation.
Can this calculation be used for any wire?
No. This is a 20°C resistive planning approximation using standard annealed copper or electrical-grade aluminum. Operating-temperature resistance can be higher; other alloys, AC reactance, power factor, insulation, installation, protection, terminals and applicable electrical rules need separate assessment. It is not a code-compliance or safe-current certification.
