Voltage Drop Calculator
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Simple DC drop in copper wire — round-trip length, not electrical code.
Copper wire
Voltage drop
3.36 V
Wire resistance ≈ 0.336 Ω
- Resistance (round trip)
- 0.336 Ω
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Wire voltage drop
Voltage drop is the loss across wire resistance: V = I × R. For DC copper wire, resistance depends on length (round trip), cross-sectional area, and resistivity (ρ ≈ 1.68×10⁻⁸ Ω·m for copper at room temperature).
Long runs, thin wire, and high current increase drop — motors and inverters may underperform if supply voltage sags. National electrical codes specify maximum drops for branch circuits; this is a quick estimate, not a NEC compliance check.
Example: 10 A through 25 m one-way (50 m loop) of 2.5 mm² copper drops about 3.4 V. Size wire for your acceptable drop and local code.
Common questions
Is this AC or DC?
DC resistive estimate. AC lines also have reactance; use this for low-voltage DC, solar, or bench wiring sanity checks only.
Does this replace an electrician?
No. Verify wire sizing with local electrical code and licensed professionals for permanent installations.