Solar Wire Sizing Calculator
Find the correct AWG wire gauge for your solar installation. Enter your system voltage, current, wire run distance, and acceptable voltage drop to get an NEC-compliant wire size recommendation - with both ampacity and voltage drop verified.
⚡ Wire Run Parameters
🔌 Wire Sizing Results
How Solar Wire Sizing Works
Proper wire sizing in a solar installation requires balancing two NEC requirements: voltage drop limits and conductor ampacity. Using undersized wire wastes energy as heat and can create fire hazards. Using oversized wire adds unnecessary cost. This calculator checks both requirements and recommends the smallest wire that satisfies both.
Voltage Drop and Voltage Rise
Voltage drop is the loss of voltage as current flows through wire resistance. In solar systems, the same phenomenon is called voltage rise when viewed from the grid side - panels push current that raises voltage at the source end. For a two-conductor circuit (positive and negative), the formula for both voltage drop and voltage rise is:
Voltage Drop (%) = (Voltage Drop / System Voltage) x 100
Power Loss (W) = Voltage Drop (V) x Current (A)
The factor of 2 accounts for both the outgoing and return conductors. Wire resistance values come from NEC Chapter 9, Table 8 at 75°C. Aluminum wire has approximately 1.61 times the resistance of copper wire of the same gauge.
NEC Voltage Drop Recommendations
While the NEC does not mandate a specific voltage drop limit, NEC 210.19(A) Informational Note No. 4 and 215.2(A)(4) recommend no more than 3% voltage drop for branch circuits and no more than 5% total for feeder plus branch circuit combined. Most solar installers design for 2-3% voltage drop on each circuit segment.
NEC Ampacity Requirements
NEC Table 310.16 lists the maximum allowable current for each wire size based on insulation temperature rating and conductor material. For solar PV circuits, NEC 690.8 requires conductors to be rated for at least 125% of the maximum circuit current because solar is a continuous load (operating for 3+ hours). For example, a 30A solar circuit needs wire rated for at least 37.5A.
Copper vs. Aluminum
Copper is the standard conductor for solar installations. It has lower resistance, better corrosion resistance, and is compatible with most terminal and connector types. Aluminum costs less per foot for equivalent ampacity but requires approximately 1.6 times the cross-sectional area for the same conductivity. Aluminum connections also require anti-oxidant compound and AL-rated terminals to prevent galvanic corrosion.
Step-by-Step Example
Scenario: 48V DC system, 30A, 50 ft one-way run, 3% max voltage drop, copper THHN wire.
Step 1 - Voltage drop budget: 48V x 3% = 1.44V maximum drop.
Step 2 - Check wire sizes (starting from smallest):
10 AWG: R = 1.24 ohms/1000ft. VD = 2 x 50 x 30 x 0.00124 = 3.72V (7.75%) - exceeds 3%.
8 AWG: R = 0.778 ohms/1000ft. VD = 2 x 50 x 30 x 0.000778 = 2.33V (4.86%) - exceeds 3%.
6 AWG: R = 0.491 ohms/1000ft. VD = 2 x 50 x 30 x 0.000491 = 1.47V (3.07%) - exceeds 3%.
4 AWG: R = 0.308 ohms/1000ft. VD = 2 x 50 x 30 x 0.000308 = 0.92V (1.93%) - passes!
Step 3 - Ampacity check: 30A x 125% = 37.5A. 8 AWG copper is rated 50A, which meets this requirement.
Step 4 - Final result: Voltage drop requires 4 AWG. Ampacity requires 8 AWG. Use the larger wire: 4 AWG copper. Sizing basis: Voltage Drop.
Why Low-Voltage Systems Need Bigger Wire
A 12V system carrying the same wattage as a 48V system draws 4 times the current. Higher current means more voltage drop and requires larger (more expensive) wire. This is a key reason why 48V is the standard for off-grid solar systems - it significantly reduces wire costs compared to 12V or 24V.
Wire Type Selection Guide
PV Wire: Required for exposed outdoor runs between solar panels. UV-resistant, sunlight-rated, 90°C, and specifically listed for photovoltaic use.
USE-2: Required for direct burial or underground conduit. Moisture and sunlight resistant with a 90°C wet rating.
THHN/THWN: Standard for conduit runs inside buildings or between combiner boxes and inverters. THWN-2 adds wet location rating.
Frequently Asked Questions
The wire size depends on your system voltage, current, wire run distance, and acceptable voltage drop. For a typical 48V system carrying 30A over 50 feet with a 3% voltage drop limit, you would need 4 AWG copper wire. Lower voltage systems (12V, 24V) require much larger wire for the same current and distance because voltage drop is a bigger percentage of the total voltage.
Voltage drop represents energy lost as heat in the wires. Excessive voltage drop reduces system efficiency, can cause inverters and charge controllers to underperform or shut down, and wastes the power your solar panels generate. The industry standard is to keep voltage drop under 3% for branch circuits and under 5% total for feeder plus branch circuits combined.
Copper is preferred for most solar installations because it has lower resistance (about 61% less than aluminum), better corrosion resistance, and is easier to terminate. Aluminum is sometimes used for long runs where cost savings matter, but it requires larger wire sizes, anti-oxidant compound at connections, and AL-rated connectors. For residential solar, copper is the standard choice.
NEC Section 690.8 requires that conductors in solar PV circuits be sized for at least 125% of the maximum circuit current. Since solar panels produce power continuously during daylight hours, this is considered a continuous load. For example, a 30A solar circuit requires conductors rated for at least 37.5A. This safety margin accounts for sustained heating over extended periods.
Voltage drop is calculated using the formula: VD = 2 x Length (ft) x Current (A) x Wire Resistance (ohms per foot). The factor of 2 accounts for the round trip (positive and negative conductors). Wire resistance values come from NEC Chapter 9, Table 8. Divide the voltage drop by system voltage and multiply by 100 to get the percentage.
Use PV Wire or USE-2 rated cable for exposed outdoor runs between solar panels. PV Wire is specifically designed for solar with UV resistance, sunlight rating, and 90°C temperature rating. For conduit runs, THHN/THWN is standard. For underground runs, USE-2 is required. All wire in solar installations should be rated for at least 90°C and listed for the specific application.
Voltage rise is the increase in voltage at the connection point when solar panels push power back through the wiring. While voltage drop describes the loss from source to load, voltage rise is the same phenomenon viewed from the grid or inverter side - the panel end of the wire sits at a higher voltage than the connection end.
The calculation is identical to voltage drop: VR = 2 x Length x Current x Wire Resistance. The NEC limits combined voltage rise and drop to 5% total. In grid-tied systems, excessive voltage rise can trip inverter overvoltage protection and shut the system down. This calculator handles both scenarios since the math is the same - just enter your values and check the percentage stays under your limit.
🛒 Recommended Solar Wire
Affiliate links. We may earn a commissionResults are estimates based on NEC Chapter 9, Table 8 conductor resistance values and NEC Table 310.16 ampacity ratings at 75°C. Actual performance varies by installation conditions, ambient temperature, conduit fill, and conductor bundling. Temperature correction and conduit fill adjustment factors are not included in this calculator. Consult a certified electrician or solar installer for final wire sizing. Always follow local electrical codes and NEC requirements.