Example Data Table
| Source |
Current |
Length |
Conductor |
Material |
Expected Use |
| 12 V |
10 A |
10 m |
10 AWG |
Copper |
Low voltage lighting |
| 24 V |
18 A |
35 ft |
8 AWG |
Copper |
Solar controller output |
| 48 V |
30 A |
20 m |
16 mm² |
Aluminum |
Battery inverter feed |
Formula Used
Conductor resistance: R = ρ × L ÷ A
Temperature correction: RT = R20 × [1 + α × (T - 20)]
DC loop resistance: Rloop = (2 × Rconductor ÷ parallel paths) + connection resistance
Voltage drop: Vdrop = I × Rloop
Drop percentage: Drop % = Vdrop ÷ source voltage × 100
Power loss: Ploss = I² × Rloop
The factor of two is used because a DC circuit needs outgoing and return conductors.
How to Use This Calculator
Enter the source voltage and the full load current.
Enter the one way cable length, not the loop length.
Select AWG, area, or custom resistance mode.
Choose copper, aluminum, or a custom conductor value.
Add conductor temperature and terminal resistance if known.
Set your acceptable voltage drop limit.
Press the calculate button and review the result table.
Use the download buttons to save the result.
DC Voltage Drop Planning
DC voltage drop matters because current travels out and back. A long conductor acts like a resistor. That resistance subtracts voltage from the load. Motors may start weakly. Lights may dim. Chargers may run hotter. Control devices may reset during surge current. Good design checks the drop before wire is installed.
Key Design Factors
The main inputs are source voltage, load current, run length, conductor size, conductor material, and operating temperature. Copper has lower resistance than aluminum. Larger conductors have lower resistance than smaller conductors. Higher temperature increases conductor resistance. Parallel conductors reduce effective resistance when each path shares current correctly. Extra terminal resistance should also be counted. Small connections can matter in low voltage systems.
Interpreting Results
A low percentage drop means the load receives nearly the full supply voltage. A high percentage drop means energy is being lost as heat in the cable. The calculator also estimates cable power loss, load voltage, circuit resistance, and maximum one way length for the selected limit. These values help compare several cable sizes before buying material.
Practical Electrical Use
For battery banks, solar circuits, LED strips, RV wiring, marine loads, and control circuits, voltage margin is often limited. A three percent limit is common for sensitive loads. Five percent may be acceptable for some general loads. Starting loads can need a lower drop, because inrush current creates a brief larger loss. Always check the actual equipment manual and local electrical rules.
Field Checks
Measure voltage at the source and load during real operation. Use the highest normal current for testing. Inspect terminals for looseness, corrosion, and heat marks. Recheck after the first heavy duty cycle. Cable routes can change over time. Added accessories can raise current. A design that passed once may fail later.
Better Cable Decisions
Use the calculated result as a design guide, not as a final safety approval. Confirm insulation rating, ampacity, conduit fill, protection device size, short circuit current, and installation conditions. Keep return paths equal in length. Tighten terminals properly. Protect cables from abrasion and heat. When results are near the limit, choose a larger conductor. The extra cost may prevent wasted energy and unstable performance. Document assumptions for future service work.
FAQs
What is DC voltage drop?
It is the voltage lost in a cable because the conductor has resistance. In DC circuits, current flows through the supply conductor and returns through another path, so both paths must be counted.
Should I enter one way or round trip length?
Enter the one way length only. The calculator multiplies conductor resistance by two for the outgoing and return path used in normal DC wiring.
Why does temperature affect the result?
Most metal conductors have higher resistance when hot. Higher resistance creates more voltage drop and more cable heat under the same current.
What voltage drop percentage is acceptable?
Many designers target three percent for sensitive loads and five percent for general loads. Equipment manuals and local rules should guide the final choice.
Can this calculator size a fuse?
No. It estimates voltage drop and loss. Fuse or breaker sizing also needs ampacity, fault current, insulation rating, installation method, and code rules.
Why include connection resistance?
Terminals, lugs, switches, and joints can add small resistance. In low voltage systems, small resistance can create noticeable drop and heating.
What does parallel conductors mean?
It means more than one conductor shares current for each polarity. The calculator assumes equal length, equal size, and balanced current sharing.
Can I use custom resistance values?
Yes. Select custom resistance mode when you already know cable resistance from a datasheet. Choose ohms per meter, kilometer, or 1000 feet.