Article — DC Wire Size Calculator
DC Wire Size: Picking the Right AWG for Your Circuit
DC wire size is set by two limits: voltage drop and ampacity. The minimum cross-section is A = 2 × I × ρ × L / V_drop, where I is current, L is one-way length, ρ is resistivity (0.0172 Ω·mm²/m for copper), and V_drop is the tolerable drop. After computing the area, round up to the next standard AWG gauge that also handles the current safely.
Undersized DC wire is a quiet failure mode. The circuit works, but the load sees less voltage than it should, performance suffers, and the wire runs warm. Oversized wire is wasted money. This guide covers the math, the practical limits, and the rules of thumb from NEC, ABYC, and the solar industry.
Why DC wire size matters
Every wire has resistance, and resistance times current is voltage drop. On low-voltage DC systems, this drop is a much bigger percentage of the source voltage than on 120 V or 240 V AC. A 0.5 V drop is 4% of 12 V but only 0.4% of 120 V.
Voltage drop has three direct consequences. Loads run weaker — a "12V" pump getting 11.5 V pumps about 8% less water. Heat dissipation in the wire eats into the energy you paid for at the battery. Long-term degradation accelerates: hot terminations corrode and oxidise faster than cool ones.
American Wire Gauge (AWG) numbers go down as wire gets thicker. AWG 14 is thin household wire; AWG 4/0 (pronounced "four ought") is the largest standard cable, used in main service entries. Each step of 3 AWG numbers doubles the cross-section (so 6 numbers quadruple the area).
The DC wire sizing formula
Two formulas, depending on what you're solving for. To check whether a wire is large enough, calculate the voltage drop. To pick a wire, calculate the minimum cross-section needed.
V drop V_drop = 2 × I × ρ × L / AMin area A = 2 × I × ρ × L / V_dropCopper ρ 0.0172 Ω·mm²/mAluminum ρ 0.0285 Ω·mm²/mThe factor of 2 covers the loop: current goes out through one conductor and back through the other. Both contribute to drop.
Voltage drop limits
Codes specify maximum allowable drops. The National Electrical Code (NEC) Section 210.19 recommends 3% on a branch circuit and 5% total for the combined feeder and branch. The American Boat and Yacht Council (ABYC) standard E-11 matches this for general loads on boats.
- 2% feeder = NEC recommendation for the main feeder section
- 3% branch = NEC recommendation for the branch circuit
- 5% total = NEC absolute limit for the combined run
- 2% solar PV = NEC 690 requires 2% on the array side
- 3% engine cranking = ABYC for starter motor circuits
- 10% nav lighting = ABYC allows higher drop on non-critical loads
DC wire size for 12V systems
12V systems are notorious for needing huge wire on short runs. The reason is the small absolute voltage — 3% of 12 V is only 0.36 V, and ohm's law cannot make this any smaller with given current and resistance.
A practical example. A 20 A 12V load 5 metres from the battery (10 m round trip): A = 2 × 20 × 0.0172 × 5 / 0.36 = 9.56 mm². That's AWG 8 minimum. Bump to AWG 6 for margin if the run is longer or the load draws steady current.
Copper vs aluminum DC wire
Copper is the default in most DC systems. Its resistivity (0.0172 Ω·mm²/m at 20°C) is the lowest of any common wire metal. Aluminum is 65% more resistive, so an aluminum wire of the same gauge has 65% more drop.
To compensate, aluminum needs roughly 1.65× the cross-section, which usually means going up one AWG step. Aluminum saves cost on very long runs (main service entries, large solar farms) but adds complexity: terminations need antioxidant joint compound, special crimps, and re-torquing after the first thermal cycle to prevent loosening.
Never mix copper and aluminum at a termination without a CO/ALR-rated connector. The two metals form a galvanic cell in the presence of moisture and the connection will corrode quickly.
DC wire sizing by application
Different DC applications have their own conventions and code references.
Solar PV. NEC Article 690 governs DC wire on the array side. The 2% drop limit is strict, and 25%-margined ampacity per NEC 690.8 means a 20 A panel string needs wire rated for 25 A continuous. Heat from sun-exposed conduits compounds the rating problem.
Automotive 12V/24V. Battery cables in cars are sized for cranking surge (200–600 A momentarily) rather than steady current. SAE J1127 specifies the cable types. Conventional battery cable is AWG 1/0 to 4/0 for the main run.
Marine 12V/24V. ABYC E-11 governs all DC wiring on US boats. Stricter than NEC because of corrosion: tinned copper is preferred, and crimp lug connections are mandatory (no soldered terminations on a moving boat).
RV and camper. Combines automotive and residential practice. RVIA Standard 119 applies; ABYC tables are often used as a reference. Typical 12V house battery runs use AWG 4–6 to the breaker panel.
Temperature and bundling effects
Resistivity rises with temperature: copper gains about 0.4% per °C above 20°C. In a hot engine bay or attic at 60°C, resistance is 15% higher, and voltage drop scales the same way.
When 4 or more current-carrying conductors share a conduit, NEC Article 310 derates ampacity by 20–50%. The same applies to bundled wires in tight engine compartments. Calculator results assume single conductor in free air — apply corrections for bundled runs.
Common DC wire sizing mistakes
Field-tested list of what goes wrong.
- Using one-way distance — the formula needs round-trip length (×2).
- Ignoring the voltage drop limit — ampacity alone permits much smaller wire than is acceptable in practice.
- Treating aluminum as if it were copper — needs 1.65× more cross-section for the same drop.
- Skipping the temperature correction — hot environments reduce ampacity significantly.
- Forgetting connection resistance — each terminal adds 0.001–0.01 Ω, equivalent to a few metres of wire.
- Using AC tables for DC — AC ampacity tables include skin-effect derating that doesn't apply to DC.
One additional consideration that grows in importance for very long DC runs: the cost of conductor material. Aluminum is roughly one third the price of copper per kg and even cheaper per metre for equivalent ampacity (despite needing more cross-section). For 50+ metre runs in solar arrays or remote pumping installations, the material savings often justify the extra labour of aluminum termination. For runs under 10 metres, copper's ease of installation and termination usually wins.