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DC Wire Size Calculator

Ready to calculate
12V / 24V / 48V.
Solar / RV / Marine.
NEC + ABYC Ref.
100% Free.
Privacy Secure.

How it Works

01DC System

12V / 24V / 48V battery banks, solar, RV, marine, golf carts.

02Load Amps

DC loads pull higher amps than AC — wire sizing is critical.

03Run Length

DC voltage drop over distance is dramatic. Short runs whenever possible.

04AWG Result

Recommended AWG with voltage drop and ampacity — usually 3% max for DC.

What is a DC Wire Size Calculator?

A DC wire size calculator sizes conductors for low-voltage DC systems — solar arrays, RV battery banks, marine dual-battery, golf carts, and industrial 24V/48V controls. DC wire sizing is unforgiving because low-voltage systems draw high amps: a 120W device on 12V pulls 10A (vs 1A on 120V AC), and every foot of wire bleeds voltage.

Our calculator applies the same core voltage drop formula as AC (factor-of-2 round trip) but with presets tuned for 12V / 24V / 48V DC systems and ABYC-reference-level drop targets (typically 3% max for critical loads).

How the DC Wire Size Calculator Works

Pick DC system voltage: 12V (automotive/RV), 24V (solar/marine), 48V (golf cart/industrial).
Enter load amps: Remember: 120W at 12V = 10A, but 120W at 48V = 2.5A.
Enter one-way length: DC runs are usually shorter — 10-30 ft in RVs, 50-100 ft for solar.
Pick material: Copper (marine-grade tinned copper for boats). Aluminum rarely used in DC.
Get AWG: Recommended size with voltage drop ≤ 3% and ampacity headroom.

DC Voltage Drop Formula

DC uses the same formula as single-phase AC:

Voltage Drop (V) = (2 × K × L × I) ÷ Circular Mils

Factor of 2 accounts for round-trip — DC current flows out the positive wire and returns on the negative. On 12V systems, a 3% drop is just 0.36V — critical for lights, inverters, and battery charge efficiency.

Real-World Example

Example: 30A RV Load, 20 ft Run, 12V

RV fridge / air-conditioner: 30A at 12V. Distance 20 ft one-way (40 ft round trip). Copper. 3% max = 0.36V.

  • Min cmils = (2 × 12.9 × 20 × 30) ÷ 0.36 = 43,000 cmils
  • 8 AWG = 16,510 cmils → too small
  • 6 AWG = 26,240 cmils → still too small
  • 4 AWG = 41,740 cmils → just at the edge (still technically 0.41V drop ≈ 3.4%)
  • For a comfortable 3%, use 3 AWG (52,620 cmils) or 2 AWG.

Lesson: 12V systems need massive wire for long runs — consider upgrading to 24V or 48V.

Who uses a DC wire size calculator?

1
Solar installers sizing PV string cables, battery bank jumpers, and inverter feeders.
2
RV owners wiring 12V/24V house batteries, inverters, and solar panels.
3
Boat owners running 12V or 24V wire per ABYC E-11 for instruments and accessories.
4
Golf cart and e-bike builders on 36V/48V DC systems.
5
Industrial control engineers on 24V DC panels.

DC Wire Size Quick Reference

Load12V 20 ft12V 40 ft24V 20 ft48V 20 ft
10A10 AWG8 AWG14 AWG16 AWG
20A6 AWG4 AWG10 AWG14 AWG
30A4 AWG2 AWG8 AWG12 AWG
50A2 AWG1/06 AWG10 AWG
100A1/04/02 AWG6 AWG

Key Takeaways

Low-voltage DC wire sizing is where careful math pays off — oversize wire means less battery life lost to resistance and a safer system. Use this DC wire size calculator for every 12V, 24V, or 48V run.

DC Wire Size FAQs

What size wire for 12V system?
Depends on amps and distance. Rule of thumb: 20A at 10 ft needs 10 AWG; at 20 ft needs 6 AWG; at 40 ft needs 4 AWG. 12V is the most wire-intensive common voltage — use this calculator for every run.
Why does 12V need bigger wire than 120V?
For the same watts, 12V pulls 10× the amps. A 120W load draws 1A on 120V but 10A on 12V. More amps mean more voltage drop and heat, so wire must be bigger. Many solar/RV systems go to 24V or 48V specifically to cut wire costs in half or quarter.
Do I size DC wire for 2% or 3% voltage drop?
ABYC marine standard is 3% max for critical circuits (navigation lights, bilge pumps) and 10% for non-critical. Solar industry targets 3% or lower. Automotive OEM practice is often 5%. For safety, use 3% for most DC runs.
Can I use automotive wire for solar?
Yes, if it is properly rated. Marine-grade tinned copper (THWN or SAE J1127 battery cable) is best for solar — UV-resistant, corrosion-resistant, and uses higher strand counts for flexibility. Standard THHN building wire works for dry interior runs.
What size wire for 100A 12V inverter?
A 1000W 12V inverter typically peaks at 80-100A continuous. For 10-foot battery-to-inverter run: 2/0 AWG minimum, preferably 4/0 for safety and low drop. Always fuse within 18 inches of the battery per ABYC.
What is the best wire for 24V solar system?
10 AWG or 8 AWG copper THHN or PV wire handles most 24V residential solar panels (30-40A typical). For battery bank jumpers in a 24V system, use 4 AWG or 2 AWG for 100A+ inverters. Always UV-rated for outdoor PV runs.
Should I use copper or aluminum for DC?
Copper is almost always preferred for DC — aluminum adds ~28% more cross-section for the same ampacity and is rarely used in the small-to-medium DC runs typical of RVs, boats, and residential solar.
Why is DC voltage drop so critical?
DC systems often have no voltage regulation — every 0.5V lost in the wire is 0.5V less at the load. Inverters may shut off under 11V. LED lights dim and flicker. Battery chargers undercharge. Solar panels lose harvest. Correct wire sizing is the single highest-leverage improvement for most DC systems.
How do I calculate DC wire size for solar?
Take panel short-circuit current × 1.25 safety factor × 1.25 NEC PV factor (so × 1.56 total). Apply the factor-of-2 voltage drop formula for round-trip. Target 3% max drop. This calculator automates every step.
Are fuses required on DC wire?
Yes — NEC 240 and ABYC E-11 both require overcurrent protection on every DC conductor, sized to protect the wire (not the load). Fuse should be within 7 inches of the battery in marine, 18 inches NEC. Always fuse before any switch.

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Disclaimer

Educational reference. Marine and solar installations may require ABYC E-11 / NEC 690 compliance. Verify with installer.