Voltage Drop Calculator for Harness Wire Sizing
Calculate voltage drop, power loss, and recommended wire size for harness wire sizing, long runs, and release decisions.
How do you calculate voltage drop in a wire?
Voltage drop equals current multiplied by total wire resistance: V_drop = I × R. Wire resistance depends on material (copper or aluminum), length, cross-section area, and temperature. For DC 2-wire circuits, multiply one-way resistance by 2. NEC recommends maximum 3% drop for branch circuits.
Based on NEC Chapter 9, Table 8
Circuit Configuration
Includes supply + return wire (×2)
Typical crimp: 5-10 mΩ per connection
NEC recommends 3% branch, 5% total
Results
Configure your circuit and click Calculate
Validated the sizing?
Carry the sizing decision into the harness workflow so BOMs, cutlists, and release documents stay tied to the same design state.
Validated the wire size? Move into the harness workflow →Next Steps
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📚 Related Terms
How to Calculate Voltage Drop
Voltage drop is the reduction in voltage as electrical current flows through a conductor. Excessive voltage drop can cause equipment malfunction, dim lights, and reduced motor performance.
The Formula
This calculator uses Ohm's Law with temperature-corrected resistivity:
- Calculate resistivity at operating temperature: ρ(T) = ρ₂₀ × [1 + α × (T - 20)]
- Calculate wire resistance: R = (ρ × L × multiplier) / A
- Apply stranding factor (×1.02) for real-world wires
- Calculate drop: V_drop = I × R_total
Acceptable Voltage Drop Limits
- NEC (Residential/Commercial): 3% for branch circuits, 5% total including feeders
- Automotive (12V): Critical circuits often require <0.5V drop (~4%)
- Aerospace (28V): Typically must deliver 24V minimum at load
Circuit Type Multipliers
- 2-Wire (DC/AC Single Phase): ×2 - current travels through both supply and return conductors
- Chassis Return: ×1 - vehicle frame provides low-resistance return path
- 3-Phase: ×√3 (1.732) - line-to-line voltage calculation