DC voltage drop equation
For two-wire DC circuits, the voltage drop is calculated across both the outgoing and return conductors.
Vd = 2 × I × Rft × LVload = Vsource - VdVd% = Vd / Vsource × 100Rft = R20 × M × (1 + alpha × (T - 20))where:
- Vd
- DC voltage dropV
- I
- Load currentA
- Rft
- Adjusted conductor resistance per footohm/ft
- L
- One-way conductor lengthft
- Vload
- Estimated voltage available at the loadV
- Vsource
- Source voltageV
- R20
- Copper conductor resistance at 20 C
- M
- Material multiplier for copper, aluminum, or CCA
- alpha
- Copper temperature coefficient used for approximation
- T
- Ambient temperature converted to CelsiusC
Low-voltage DC circuits are sensitive to small absolute voltage losses, so equipment minimum input voltage and fuse protection should be checked after the calculation.
Assumptions
- Two-wire DC circuit with one positive and one return conductor
- AWG resistance references are based on copper at 20 C with temperature correction
- CCA is modeled as a high-resistance estimate, not as a recommended conductor
Important Warnings
- Always check equipment minimum input voltage, fuse protection, terminal ratings, and installation temperature.
- Do not use copper-clad aluminum for critical high-current DC circuits.
FAQ
Why does 24 V fail sooner than 230 V?
The same absolute voltage loss is a much larger percentage of a low DC source voltage.
Should I enter round-trip distance?
No. Enter one-way physical length; the calculator multiplies by two for the outbound and return conductors.