For a simple two-wire DC circuit, voltage drop includes resistance in both the outgoing conductor and the return conductor. The correct length to enter depends on the calculator: some ask for total circuit length, while others ask for one-way length and multiply it by two internally.
The Power Infra Lab DC voltage drop calculator uses one-way length. Its model assumes equal outgoing and return conductors. Entering the out-and-back distance in that field counts the return path twice and doubles the calculated drop.
Read the length convention before the number
Suppose a load is connected by a 20 m outgoing conductor and an equal 20 m return conductor. One-way length is 20 m; total conductor length around the circuit is 40 m. The route drawn on a floor plan may show only the first distance, while a cable schedule may list both.
Manufacturer resources do not all present the input in the same form. Victron’s DC wiring guide states that its recommended battery-cable table uses the sum of positive and negative cable lengths. Blue Sea Systems’ voltage-drop chart instructions likewise ask for the distance from the source to the load and back. Those conventions are compatible with the same circuit physics; they simply differ from a one-way input field.
A 48 V worked example
Use a hypothetical constant-current DC circuit with a 48 V source, 40 A load current, 20 m one-way length and conductor resistance of 0.5 Ω/km for each conductor. Treat the resistance as a selected example value at the intended operating temperature, not as a cable-size recommendation.
- Total conductor length = 2 × 20 = 40 m.
- Loop resistance = 40 × 0.5 ÷ 1,000 = 0.020 Ω.
- Voltage drop = 40 × 0.020 = 0.80 V.
- Percentage drop = 0.80 ÷ 48 × 100 = 1.67%.
- Estimated receiving voltage = 48 − 0.80 = 47.20 V.
In the Power Infra Lab tool, enter 20 in the one-way-length field. The formula already includes the factor of two. Do not enter 40 merely because the physical circuit contains 40 m of conductor.
Two opposite factor-of-two mistakes
| Calculation approach | Resistance counted | Calculated drop | Meaning |
|---|---|---|---|
| Correct: 20 m one-way, doubled once | 0.020 Ω | 0.80 V | Both 20 m conductors included |
| Wrong: 40 m entered as one-way | 0.040 Ω | 1.60 V | Return path counted twice |
| Wrong: only one 20 m conductor counted | 0.010 Ω | 0.40 V | Return path omitted |
The second result is 100% too high relative to the correct 0.80 V, while the third is 50% too low. These errors can make two calculators appear inconsistent even when both are applying Ohm’s law correctly. Compare the field definitions before comparing their results.
Check resistance units as carefully as length
The tool expects the resistance of one conductor in ohms per kilometre. The example’s 0.5 Ω/km is also 0.0005 Ω/m or 0.5 mΩ/m. Entering 0.0005 into an Ω/km field would make the conductor resistance one thousand times too small.
Keep the material, conductor construction and temperature basis attached to any resistance taken from a manufacturer’s data sheet. If a value is already a measured total loop resistance in ohms, it is not an Ω/km input and must not be multiplied by length again. Such a measurement belongs in a direct V = I × R calculation with a clearly defined circuit boundary.
What if the outgoing and return paths differ?
The shortcut 2 × one-way length × conductor resistance assumes equal paths. For a separate hypothetical circuit with a 20 m outgoing conductor and a 30 m return conductor, both at 0.5 Ω/km, loop resistance is (20 + 30) × 0.5 ÷ 1,000 = 0.025 Ω. At 40 A, drop is 1.00 V.
If the two conductors also have different resistance per unit length, calculate their resistances separately and add them. Contact resistance, fuses, busbars and other series components require their own treatment. The equal-conductor calculator does not model those details, a chassis return, parallel conductors or AC reactance.
A low percentage is not a cable-safety approval
The example has not established an allowable voltage drop, conductor ampacity, fuse rating or installation method. Blue Sea’s marine chart treats current-carrying capacity and voltage drop as separate checks; its application-specific percentage choices should not be copied into every DC project. Use the equipment requirements and applicable project standards, with qualified review.
The calculation also holds current constant. A load whose current changes as terminal voltage changes needs a more detailed model. A nominal 48 V label does not mean that source voltage stays at 48 V throughout battery discharge.
Before saving a result
- Label the distance as one-way or total loop length.
- State whether the outgoing and return paths are equal.
- Record resistance units and temperature basis.
- Record source voltage and the load-current assumption.
- List omitted connections and other resistance, then keep ampacity and protection checks separate.
Once those definitions are clear, rerun the DC calculator and export the assumptions with the result. A reproducible calculation begins with unambiguous inputs, not with choosing a reassuring percentage.
Sources & further reading
- Victron Wiring Unlimited: DC wiring ↗
- Blue Sea Systems: Voltage Drop in Conductor — Wire Sizing Chart ↗