DC voltage drop equation
For two-wire DC circuits, the voltage drop is calculated across both the outgoing and return conductors.
where:
- Vd
- DC voltage drop[V]
- I
- Load current[A]
- Rft
- Adjusted conductor resistance per foot[ohm/ft]
- L
- One-way conductor length[ft]
- Vload
- Estimated voltage available at the load[V]
- Vsource
- Source voltage[V]
- 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 Celsius[C]
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.
How to calculate DC voltage drop
A two-wire DC circuit loses voltage in both the positive and return conductors. Enter the physical one-way distance; the calculator doubles that length internally, adjusts conductor resistance for material and temperature, and compares the loss with source voltage.
- Enter source voltage and load current.
- Enter the one-way cable length.
- Select AWG size and conductor material.
- Enter ambient temperature and the project drop limit.
- Check load-end voltage as well as percentage drop.
DC voltage drop example
For a 24 V DC load drawing 3 A through 100 ft of 14 AWG copper at 75°F, the simplified calculation gives about 1.54 V drop, or approximately 6.4%. The estimated voltage at the load is about 22.5 V.
| Input or result | Value |
|---|---|
| Source voltage | 24 V DC |
| Load current | 3 A |
| One-way length | 100 ft |
| Conductor | 14 AWG copper |
| Voltage drop | About 1.54 V / 6.4% |
| Load voltage | About 22.5 V |
Why low-voltage DC circuits are sensitive to cable loss
The same absolute loss is a much larger percentage of a 12 V or 24 V supply than of a 230 V circuit. Battery systems, controls, LED loads, solar circuits, and communications equipment can stop operating correctly even when conductor ampacity is adequate.
Copper vs. aluminum vs. CCA wire
Copper provides the lowest resistance among the options in this simplified model. Aluminum and copper-clad aluminum require additional attention to conductor size, terminals, joining methods, mechanical strength, and product approval. CCA should not be treated as a direct copper substitute for critical high-current circuits.
How to reduce DC voltage drop
Improve load-end voltage by reducing total circuit resistance or current.
- Increase wire size.
- Shorten the positive and return path.
- Raise system voltage when the equipment permits it.
- Use parallel conductors only where the installation rules allow.
- Check fuse, terminals, connectors, and joints for additional resistance.
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.