Skip to main content
Solar & Renewables Free Pro Features Available

DC Wire Sizing Calculator — Gauge, Voltage Drop & NEC Ampacity

Size Copper and Aluminum Conductors for 12V, 24V, and 48V Solar, Battery, and DC Distribution Circuits

Free DC wire sizing calculator for solar installers, marine electricians, and off-grid system builders. Enter the circuit current, one-way wire run distance, and system voltage, and the calculator determines the minimum wire gauge (AWG) that satisfies both NEC ampacity requirements and your target voltage drop percentage. Supports copper and aluminum conductors, 12V through 48V systems, and all standard conduit fill and temperature correction factors per NEC Article 310. Displays actual voltage drop in volts and percent, power loss in watts, and the NEC ampacity table reference for the recommended conductor size.

Pro Tip: In DC systems, voltage drop is the dominant sizing constraint, not ampacity. A circuit carrying 30A at 12V is only 360W, but the current is enormous relative to the voltage. A 50-foot run of 10 AWG copper at 30A drops 1.58V, which is 13.2% at 12V but only 3.3% at 48V. This is why serious off-grid systems use 48V: the same power at 48V draws one-quarter the current, allowing wire that is two sizes smaller and one-quarter the copper cost. If you are designing a new system and have a choice, always go 48V. The wire savings alone often pay for the voltage difference in equipment cost.

Size the solar array feeding these DC circuits

Solar Array Sizing Calculator →

Size the battery bank connected to your DC bus

Battery Bank Sizing Calculator →

Select the charge controller for your system

Charge Controller Sizing Calculator →

Check conduit fill for your DC runs

Conduit Fill Calculator →

Read the complete DC wiring guide

DC Wiring Guide →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

DC Wire Sizing Calculator
Pop Out

How It Works

  1. Enter Circuit Parameters

    Input the maximum continuous current in amps and the system voltage (12V, 24V, 48V, or custom). For solar panel circuits, use the short-circuit current (Isc) multiplied by 1.25 per NEC 690.8. For battery circuits, use the maximum inverter draw current. For DC loads, use the rated current of the device.

  2. Set Wire Run Distance

    Enter the one-way distance from source to load in feet. The calculator automatically accounts for the round-trip (source to load and return) in the voltage drop calculation. Measure the actual wire route, not the straight-line distance. Include vertical runs, routing around obstacles, and service loops.

  3. Select Conductor Material and Insulation

    Choose copper or aluminum conductor. Select the insulation type (THHN/THWN-2, USE-2, PV Wire, etc.) which determines the temperature rating and NEC ampacity column. Solar installations typically require USE-2 or PV Wire for exposed outdoor runs and THWN-2 for conduit runs.

  4. Set Voltage Drop Target

    Enter your maximum acceptable voltage drop as a percentage. NEC recommends 3% for branch circuits and 5% total for feeder plus branch circuit combined. For solar and battery circuits, many designers target 2% or less to maximize energy transfer efficiency. Lower voltage systems require tighter voltage drop targets in absolute terms.

  5. Review Wire Size Recommendation

    The calculator displays the minimum AWG that satisfies both the voltage drop target and NEC ampacity requirements, whichever is more restrictive. It shows the actual voltage drop in volts and percentage, power lost as heat in the conductors, NEC ampacity at the selected temperature rating, and whether voltage drop or ampacity was the controlling factor.

Built For

  • Solar installers sizing conductors from PV array to charge controller and charge controller to battery bank per NEC 690
  • Marine electricians sizing battery cables and DC distribution wiring on boats per ABYC E-11 standards
  • Off-grid system builders sizing the main DC bus wiring between battery bank, inverter, and charge controller
  • RV electricians sizing shore-to-battery charging cables and 12V distribution circuits in motorhomes and trailers
  • Automotive electricians sizing power cables for aftermarket accessories, winches, and auxiliary lighting on 12V and 24V systems
  • Telecom technicians sizing DC power feeds from rectifiers to equipment racks in -48V telecom installations

Features & Capabilities

Dual-Constraint Sizing

Evaluates both NEC ampacity (current-carrying capacity based on insulation temperature rating, conduit fill, and ambient temperature) and voltage drop (based on conductor resistance, current, and distance). Reports which constraint is controlling and recommends the larger gauge that satisfies both. In low-voltage DC systems, voltage drop almost always governs.

Copper and Aluminum Support

Sizes conductors in both copper and aluminum with accurate resistivity values at operating temperature. Aluminum has 61% the conductivity of copper and requires a wire two sizes larger for the same ampacity. The calculator shows both options side by side so you can compare gauge, weight, and cost tradeoffs.

NEC Article 310 Ampacity Tables

References NEC Table 310.16 (formerly 310.15(B)(16)) for ampacity at 60C, 75C, and 90C insulation ratings. Applies ambient temperature correction factors per Table 310.15(B)(1) and conduit fill adjustment factors per Table 310.15(C)(1). These corrections reduce allowable ampacity in hot environments or crowded conduits.

Voltage Drop Visualization

Displays voltage at the source, voltage at the load after drop, and the percentage loss. For battery charging circuits, shows how voltage drop reduces the effective charging voltage reaching the battery terminals. A 0.5V drop on a 48V circuit is negligible, but 0.5V on a 12V circuit can prevent a battery from reaching full charge.

Power Loss Calculation

Calculates the power dissipated as heat in the conductors (I2R losses). On high-current DC circuits, power loss can be substantial. A 100A circuit on 50 feet of 2 AWG copper loses approximately 65W as heat. This wasted energy reduces system efficiency and heats conduits. Upsizing the conductor by one gauge cuts the loss roughly in half.

Solar-Specific NEC Factors

Automatically applies NEC 690.8 continuous-duty 125% current multiplier for PV source and output circuits. Applies NEC 690.7 cold-temperature voltage correction to string Voc for conductor insulation voltage rating verification. These NEC-specific solar requirements are frequently missed by general-purpose wire sizing tools.

Frequently Asked Questions

For a 12V 30A circuit with a 25-foot one-way run and 3% maximum voltage drop, you need 4 AWG copper. Here is the math: round-trip distance is 50 feet, 12V times 3% = 0.36V allowable drop, required resistance is 0.36V / 30A = 0.012 ohms, and 50 feet of 4 AWG copper has a resistance of about 0.012 ohms. If you target 2% drop, you would need 2 AWG. This illustrates why 12V systems require massive wire for even moderate loads over modest distances. The same 360W load at 48V draws only 7.5A and needs only 10 AWG copper for the same run at 3% drop.
In AC residential systems at 120V or 240V, the NEC ampacity limit usually governs wire size because voltage drop is a small percentage of the high supply voltage. In DC systems at 12V or 24V, voltage drop becomes enormous relative to supply voltage even at modest currents. A wire that is perfectly safe from a heat/ampacity standpoint might drop 15% of the voltage, which is unacceptable for system performance. This is why DC wire sizing must always check both constraints and use the larger wire. Skipping the voltage drop check is the most common wiring mistake in DIY solar and marine installations.
Both are rated for exposed outdoor use in PV systems but have different construction. USE-2 (Underground Service Entrance) is rated for direct burial and wet locations at 90C. PV Wire (also called PV Cable or Photovoltaic Wire) is specifically listed for PV systems with a 90C wet/dry rating and superior UV resistance. PV Wire is typically more flexible with finer strand count, making it easier to route on rooftop arrays. NEC 690.31 requires conductors in PV source circuits to be rated for the application, and either USE-2 or listed PV Wire satisfies this requirement. Inside conduit, THWN-2 is also acceptable.
Yes. NEC 690.8(A) requires that PV circuit conductors be sized for 125% of the maximum current. For a PV string with 10A Isc, the conductor must be rated for at least 12.5A after all derating factors. This 125% factor applies to PV source circuits (panel to combiner), PV output circuits (combiner to inverter/controller), and inverter output circuits. It does not apply to battery circuits in standalone systems. The calculator automatically applies this factor when you indicate a solar circuit type, but verify it is enabled if you are entering current manually.
Aluminum is allowed by NEC for solar circuits and is commonly used on large commercial arrays where conductor cost is significant. You must use AL-rated terminals, lugs, and disconnects because aluminum expands and contracts more than copper with temperature cycling, which can loosen connections and cause arcing. For residential rooftop arrays with relatively short DC runs, copper is standard because the cost difference is small and copper connections are more reliable. For ground-mount arrays with 200+ foot home runs, aluminum can save substantially on conductor cost. Always apply anti-oxidant compound to aluminum terminations and torque lugs to manufacturer specifications.
Disclaimer: This calculator provides DC wire sizing estimates based on NEC methodology and Ohm's law voltage drop calculations. Actual installation must comply with NEC Articles 310, 690, and all applicable local codes. Wire sizing depends on accurate current and distance measurements. Solar installations must be designed and installed by qualified personnel. Undersized wiring creates fire hazards. ToolGrit is not responsible for wiring design decisions or installation outcomes.

Learn More

Solar & Renewables

DC Wiring Best Practices for Solar Systems

DC wiring guide for solar installations. Covers voltage drop calculations, wire sizing, connectors, conduit, grounding, NEC 690 overcurrent protection, rapid shutdown, and labeling requirements.

Read Guide →

Related Tools

Solar & Renewables Live

Solar Array Sizing Calculator

Size your solar panel array from daily kWh load, peak sun hours by region, system losses, and tilt derating. Grid-tied and off-grid modes with monthly production estimates.

Launch Tool →
Solar & Renewables Live

Battery Bank Sizing Calculator

Size lead-acid or LiFePO4 battery banks for off-grid and backup solar systems. Accounts for depth of discharge, days of autonomy, temperature derating, and battery configuration.

Launch Tool →
Solar & Renewables Live

Charge Controller Sizing Calculator

Size MPPT and PWM solar charge controllers per NEC 690.7. Cold-weather Voc correction, string configuration, and common controller size recommendations.

Launch Tool →