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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.

Quick wire read

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%).

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.

This is planning math for copper DC conductors only. Final wire, fuse, breaker, disconnect, insulation-temperature, terminal, routing, and code decisions still need the component manuals and qualified electrical review.

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

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.

Start from a common RV DC circuit
Circuit inputs and voltage-drop target

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.

Tool 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 the relevant manufacturer guidance, safety limits, installation requirements, and local rules before changing the rig.See assumptions

Gear to compare after the math

Spec-checked products 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.

Checked model
SK12V100P
Spec fit
A 100Ah LiFePO4 building block for smaller 12V banks when the calculator result is framed around modular battery count and charger compatibility.
Check battery priceMerchant link. Direct merchant or retailer listing.

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.

Checked model
SmartShunt 500A/50mV
Spec fit
A shunt-based monitor handoff when calculator results depend on real state-of-charge and consumed amp-hours instead of voltage guessing.
Check monitor priceMerchant link. Direct merchant or retailer listing.

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.

Checked model
Orion XS 12/12-50A
Spec fit
A 50A DC-to-DC charger handoff when route-day charging is part of the battery recovery plan and alternator reserve is verified.
Check charger priceMerchant link. Direct merchant or retailer listing.

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.