DC wiring planning
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.
Quick wire read
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%).
Planning wire
4 AWG
70A planning ampacity.
Voltage drop
2.5%
0.30V and 12W lost.
Round trip
30 ft
15 ft one-way path doubled internally.
Start over
This calculator stores inputs locally in this browser. Clear saved inputs when stale values are getting in the way.
DC wire size calculator
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
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
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.
Read RV electrical basicsRecharge time calculator
Use this when wire size affects whether a charger can actually deliver the current you planned.
Run the recharge time calculatorSolar wiring guide
Use this when array voltage and current affect both charge-controller choice and wire size.
Read the solar wiring guideBattery calculator
Use this when inverter or DC-load current starts with the battery bank and daily load plan.
Run the battery bank calculatorTool notes
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.
Voltage drop happens on the outbound and return conductors, so the calculator doubles the one-way cable path.
Load current is multiplied by copper conductor resistance for the round trip, then compared against system voltage.
The entered load current is multiplied by the continuous-load factor so charging circuits and long-running loads get extra margin.
Avoid these traps
Enter the one-way cable path. The calculator doubles it internally, because current has to travel out and back.
A wire can technically carry the current and still waste too much voltage on a long 12V run.
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 the relevant manufacturer guidance, safety limits, installation requirements, and local rules before changing the rig.See assumptions
Gear to compare after the math
These handoffs match the calculator family, not a one-click prescription. Verify fit, specs, clearances, and install limits before buying.
SOK SK12V100P 12V 100Ah LiFePO4 Battery
Best for
Budget lithium builds after a smaller bank result
A common 100Ah building block when the calculator result still fits a straightforward 12V lithium bank.
Current listing
SOK SK12V100P 12V 100Ah LiFePO4 Battery at SOK.
Victron SmartShunt 500A/50mV
Best for
Making battery runtime visible instead of guessed
A shunt-based monitor is the usual next buy when the calculator result depends on real state-of-charge tracking.
Current listing
Victron SmartShunt 500A/50mV at Victron.
Victron Orion XS 12/12-50A DC-DC Battery Charger
Best for
Improving recharge support for larger daily loads
Use this class of DC-DC charger when the battery answer only works if drive-day recovery is part of the plan.
Current listing
Victron Orion XS 12/12-50A DC-DC Battery Charger at Victron.
Frequently asked
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.
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.
Usually, yes. Voltage drop in a solar charging path can reduce harvest, so many installers target around 2% to 3% on important charging circuits.
No. This calculator is built around copper conductor resistance and planning ampacity. Aluminum requires different sizing, compatible terminals, and extra installation care.