DC Wire Sizing Calculator - Gauge and Voltage Drop Calculator

How It Works

1. Enter Circuit Parameters

   Input the current in amps and the system voltage (12V, 24V, 48V, or custom). Solar mode applies a simple 1.25 planning multiplier, but final PV source-circuit current, conductor sizing, and overcurrent protection must be checked against the adopted NEC edition, equipment instructions, and AHJ requirements.

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

   Choose copper or aluminum conductor for the local sizing table. The app does not select insulation type, terminal temperature rating, conductor listing, lug compatibility, conduit fill, bundling derate, or ambient correction factors.

4. Set Voltage Drop Target

   Enter your maximum acceptable voltage drop as a percentage. The app includes common planning targets such as 3% branch, 2% feeder, and 5% total feeder plus branch, but the final target can be set by equipment, owner standards, listing, design basis, or AHJ direction.

5. Review Wire Size Calculator

   The calculator displays the smallest local-table AWG/kcmil row that passes both the voltage drop target and the local ampacity calculator. It shows voltage drop in volts and percentage, power lost as heat in the conductors, and estimated wire cost. It is not a code compliance determination or installation approval.

Built For

• Solar installers sizing conductors from PV array to charge controller before NEC 690/AHJ review
• Marine electricians doing a preliminary low-voltage DC calculation before using the ABYC-specific calculator and current ABYC E-11
• 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.

Assumptions

• Conductor resistance values are from NEC Chapter 9, Table 8 at 75C operating temperature.
• Voltage drop is calculated using the round-trip (2x one-way) distance for DC circuits.
• NEC 690.8 125% continuous duty factor is applied to PV source and output circuits.
• Conduit fill and ambient temperature correction factors follow NEC Table 310.15(B).
• Target voltage drop is 2-3% per circuit segment (NEC recommendation, not a code maximum).

Limitations

• Does not account for terminal connection resistance or fuse/breaker voltage drops.
• Aluminum conductor sizing requires AL-rated terminations not verified by this tool.
• Parallel conductor runs require additional derating not fully modeled here.
• High-temperature environments above 40C may require additional ampacity derating beyond defaults.
• Does not verify conduit fill percentage - use a separate conduit fill calculator for that check.

References

• NEC Article 310 - Conductors for General Wiring (ampacity tables).
• NEC Article 690 - Solar Photovoltaic (PV) Systems.
• NEC Chapter 9, Table 8 - Conductor Properties (DC resistance per 1,000 ft).
• Copper Development Association - wire resistance data for copper and aluminum conductors.

Frequently Asked Questions