DC wiring planning
RV DC wire size and voltage drop calculator
Estimate copper wire size for solar, DC-DC chargers, inverter feeds, and other RV DC circuits before voltage drop turns into heat and wasted charging.
DC wire size calculator
Size copper DC wire by current, run length, and voltage drop.
Enter the one-way cable distance, system voltage, load current, and acceptable voltage drop. The calculator checks both voltage drop and planning ampacity so long 12V runs do not look safer than they are.
Wire size estimate
The wire size works, but the margin is tight
For a 12V DC-DC charger at 40A over 15 ft one-way, the calculator recommends 4 AWG. Estimated voltage drop is 0.3V (2.5%).
Recommended wire
4 AWG
70A planning ampacity
Voltage drop
2.5%
0.30V and 12W lost as heat
Design current
50A
40A load with 125% factor
Fuse floor
50A
Planning floor only; do not exceed device or wire limits
Circuit checks
Round-trip length
30 ft
15 ft one-way cable path
Ampacity margin
20A
good planning status
Resistance
0.00746 ohm
Estimated copper conductor resistance for the round trip
Parallel runs
1
Single run estimate
Watch-outs
This is a planning calculator for copper DC conductors. Final wire, fuse, insulation temperature, routing, and installation details should be checked against applicable codes and manufacturer instructions.
Voltage drop uses round-trip circuit length. Enter the one-way distance from source to load; the calculator doubles it internally.
Recommended next move
Consider upsizing one more gauge or shortening the run if this circuit is mission-critical or heat-prone.
Why this exists
DC wire size is where cheap RV electrical plans get expensive.
A 12V circuit can need surprisingly large copper once amps and distance climb. This calculator makes voltage drop visible before you buy cable, lugs, fuses, or a charger that belongs closer to the battery bank.
Use this with
RV electrical 101
Use this when the reader needs the difference between volts, amps, watts, fuses, and wire size first.
Open next stepRecharge time calculator
Use this when wire size affects whether a charger can actually deliver the current you planned.
Open next stepSolar wiring guide
Use this when array voltage and current affect both charge-controller choice and wire size.
Open next stepBattery calculator
Use this when inverter or DC-load current starts with the battery bank and daily load plan.
Open next stepTool notes
What the wire-size estimate is actually saying
This output is a planning estimate for copper RV DC circuits. It does not replace code review, device manuals, fuse coordination, terminal temperature limits, or qualified electrical work.
Round-trip circuit length
Voltage drop happens on the outbound and return conductors, so the calculator doubles the one-way cable path.
Voltage drop
Load current is multiplied by copper conductor resistance for the round trip, then compared against system voltage.
Design current
The entered load current is multiplied by the continuous-load factor so charging circuits and long-running loads get extra margin.
Avoid these traps
Common mistakes before buying
Entering round-trip length
Enter the one-way cable path. The calculator doubles it internally, because current has to travel out and back.
Sizing only by ampacity
A wire can technically carry the current and still waste too much voltage on a long 12V run.
Treating fuse size as a shopping answer
The fuse output is a planning floor, not installation approval. Device manuals, wire insulation, routing, and code rules still matter.
Treat the calculator result as a planning range, then verify wiring, clearances, fusing, ventilation, and manufacturer limits before installation.See assumptions
Frequently asked
Questions RVers usually ask next.
Is this a code-compliant wire-size calculator?
No. It is a planning tool for copper RV DC circuits. Final wire, fuse, insulation temperature, routing, conduit, and termination choices need to be checked against applicable codes and manufacturer instructions.
Why does 12V need such large cable?
Low voltage means more current for the same wattage. More current creates more voltage drop and heat, so long 12V inverter or charger runs get expensive quickly.
Should solar wiring use a lower voltage-drop target?
Usually, yes. Voltage drop in a solar charging path can reduce harvest, so many installers target around 2% to 3% on important charging circuits.
Can I use aluminum wire with this calculator?
No. This calculator is built around copper conductor resistance and planning ampacity. Aluminum requires different sizing, compatible terminals, and extra installation care.