Wire Size Calculator
Find the correct AWG or kcmil wire size for any solar DC string, home run, or AC circuit based on ampacity and voltage drop limits per NEC — free, no signup.
Wire Size Calculator for Solar
Enter current, voltage, wire run length, and conductor parameters. Get minimum wire gauge, derated ampacity, voltage drop percentage, and NEC compliance status.
NEC multiplier is auto-set per circuit type. Override only if your AHJ requires different. OCPD is auto-calculated — adjust if you know the actual breaker/fuse rating.
- Base current—
- NEC multiplier—
- Required ampacity—
- Effective ambient—
- Temp correction factor—
- Conduit fill factor—
- Adjusted required ampacity—
- Final recommendation—
| Copper | Aluminum | |
|---|---|---|
| Wire Size | — | — |
| Voltage Drop | — | — |
| Est. Cost (total) | — | — |
NEC 310.16 Ampacity Table
Allowable ampacities of insulated conductors — 30°C ambient, ≤3 conductors in raceway
| Wire Size | 60°C TW, UF | 75°C THW, THWN | 90°C THWN-2, USE-2 |
|---|
The Two-Check Approach
NEC-compliant wire sizing requires checking both ampacity (can the wire carry the current without overheating?) and voltage drop (will adequate voltage reach the load?). The correct wire size is the LARGER of the two results.
Ampacity Sizing
Base ampacity from NEC Table 310.16 is adjusted for three factors:
1. NEC 690.8(A): PV circuits use Isc × 1.25 to account for irradiance above STC conditions.
2. Continuous load (NEC 210.20(A)): Another × 1.25 for circuits operating 3+ hours. Combined = × 1.56.
3. Environmental derating: Temperature correction per NEC 310.15(B)(1), rooftop adders per 310.15(B)(3)(c), and conduit fill per 310.15(C)(1).
Adjusted Required Ampacity = (Isc × 1.56) ÷ (Temp Factor × Conduit Factor) Voltage Drop
Even if a wire passes the ampacity check, long runs can lose too much voltage. NEC recommends ≤2% for DC branch circuits and ≤3% for AC, with ≤5% total system.
VD% = (2 × Length × Current × Resistance) ÷ (Voltage × 1000) × 100 For three-phase circuits, replace the factor of 2 with √3 (1.732).
Equipment Grounding Conductor
EGC is sized per NEC Table 250.122, based on the overcurrent protection device (fuse/breaker) rating protecting the circuit.
NEC Edition Compatibility
This tool uses the conservative Isc × 1.56 approach for PV circuits, which is accepted by all AHJs across NEC 2014, 2017, 2020, and 2023 editions.
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What This Tool Covers
The Wire Size Calculator determines the correct AWG conductor for DC and AC solar circuits per NEC requirements. It accounts for ampacity, temperature derating, conduit fill, ambient temperature correction, and voltage drop — returning the minimum AWG, derated ampacity, voltage drop percentage, recommended AWG, and NEC compliance status.
Inputs Accepted
- • Circuit type: DC or AC
- • Circuit current (amps)
- • System voltage (V)
- • Wire run length (feet, one-way)
- • Wire material: copper or aluminum
- • Conduit type: EMT, PVC, or free air
- • Ambient temperature (°C or °F)
- • Number of current-carrying conductors in conduit
Outputs Generated
- • Minimum AWG for ampacity (before derating)
- • Temperature correction factor
- • Conduit fill correction factor
- • Derated ampacity for selected AWG
- • Voltage drop percentage for selected run length
- • Recommended AWG (larger of ampacity vs. voltage drop)
- • NEC compliance status
Features
Full NEC Derating
Applies NEC Table 310.15(B)(1) temperature correction factors and Table 310.15(C)(1) conduit fill adjustment factors simultaneously — the same method an electrical inspector expects.
Dual-Check Sizing
Calculates the minimum AWG for both ampacity and voltage drop independently, then returns the larger of the two as the recommended size — so you never miss a code requirement by optimizing for only one parameter.
Copper and Aluminum Support
Switch between copper and aluminum to compare cost and size trade-offs. Aluminum requires 2 AWG sizes larger than copper for equivalent ampacity but is cheaper for long AC feeder runs.
How It Works
Enter Circuit Parameters
Select DC or AC circuit type, enter current in amps, system voltage, and one-way run length. Choose copper or aluminum and select the conduit type (EMT, PVC, or free air). Enter ambient temperature and conductor count.
Apply Ampacity Derating
The calculator looks up the base ampacity from NEC Table 310.16 (for conduit) or 310.17 (for free air). It then applies the temperature correction factor (for ambient above 30°C) and conduit fill factor (for 4+ conductors) to get derated ampacity.
Find Minimum AWG for Ampacity
The tool iterates through AWG sizes (starting from 14 AWG) until it finds the smallest size whose derated ampacity meets or exceeds the circuit current. This is the ampacity-based minimum AWG.
Check Voltage Drop
Using the ampacity-based minimum AWG, it calculates voltage drop for the specified run length. If drop exceeds 2% (DC) or 3% (AC), it steps up AWG sizes until the limit is met and reports the voltage-drop-based minimum AWG.
Return Final Recommendation
The larger of the ampacity-based and voltage-drop-based AWG becomes the recommended size. The output shows both checks so you understand which constraint drove the recommendation.
Use Cases
DC String Home-Run Wiring
Size the home-run conductors from each string to the inverter or combiner box. High-temperature conduit runs on rooftops require significant derating — this tool applies the correct factors automatically.
AC Output and Feeder Sizing
Size the AC output circuit from inverter to the main panel, including feeders for subpanels in detached structures. Long runs require both ampacity and voltage drop checks — both are covered here.
Permit and Inspection Preparation
Use the NEC compliance status and derating factors in permit documentation. AHJs increasingly require wire sizing calculations to be shown on single-line diagrams — this tool generates the supporting numbers.
Calculation Methodology
Derated Ampacity
Derated A = Base Ampacity × Temp Correction Factor × Conduit Fill Factor
Temperature Correction Factor
Cf = √((90°C - Ambient °C) ÷ (90°C - 30°C)) for THWN-2 conductors
Conduit Fill Factor
4-6 conductors: 0.80 · 7-9 conductors: 0.70 · 10+ conductors: 0.50 (NEC 310.15(C)(1))
Minimum Required Ampacity
For DC strings: Min A = 1.25 × Isc (NEC 690.8(A))
Pro Tips
Rooftop conduit gets hot. Conduit on a dark roof in direct sun can reach 70°C or higher ambient — well above the standard 30°C table value. Use 60°C or higher as the ambient temperature for rooftop DC conduit runs in sunny climates to apply the correct derating.
Apply 1.25 × Isc for DC strings before entering current. NEC 690.8(A) requires DC conductors to be sized at 125% of Isc. Multiply Isc from the panel datasheet by 1.25 and use that as the current input — this makes the output directly code-compliant without further adjustment.
Count all current-carrying conductors, not just the circuit wires. If multiple circuits share a conduit, the fill factor applies to all current-carrying conductors in that conduit — not just the ones you're sizing. Miscount the conductors and the derating factor is wrong.
Aluminum saves money on AC feeders, not DC strings. Aluminum is not permitted for conductors smaller than 8 AWG under NEC. For DC string conductors, which are typically 10–12 AWG, use copper. For longer AC feeders (8 AWG and larger), aluminum is a legitimate cost-reduction option.
Frequently Asked Questions
What NEC tables does this calculator use?
The calculator uses NEC Table 310.16 for conductors in conduit (or Table 310.17 for free air), Table 310.15(B)(1) for temperature correction factors, and Table 310.15(C)(1) for conduit fill adjustment factors. These are the standard tables used by electrical engineers for residential and commercial solar conductor sizing.
When should I use free air vs. conduit?
Use free air when conductors are not in conduit — for example, USE-2 wire on the roof between panels, or conductors in open cable trays. Use conduit (EMT or PVC) when wires run in enclosed raceways. Free air has higher base ampacity because heat dissipates more easily, but it may still require temperature correction for hot rooftop environments.
Why does the recommended AWG sometimes jump two sizes?
When temperature and conduit fill derating factors combine, the ampacity of a given AWG may fall significantly below its base rating. The calculator may need to jump two AWG sizes to find one whose derated ampacity still meets the circuit requirement. This is normal and correct — don't try to save money by using the ampacity-table size without derating.
What conductor insulation type should I assume?
For solar applications, THWN-2 or XHHW-2 (both rated 90°C) is standard. The 90°C column applies for temperature derating, but the termination temperature limit (typically 75°C for equipment) caps the usable ampacity. The calculator uses the 75°C base ampacity column with 90°C derating factors, which is the correct NEC approach for most installations.
Does this cover EGC (equipment grounding conductor) sizing?
No. EGC sizing follows NEC Table 250.122, which is based on the overcurrent device rating rather than the conductor ampacity. This tool covers current-carrying conductors only. Size EGCs separately using Table 250.122 based on your breaker or fuse rating.
Can I use this for battery bank wiring?
Yes. Enter the maximum charge or discharge current of your battery system as the circuit current. For lithium battery systems with high C-rate discharge (e.g., 200A from a 100Ah battery), the short runs between batteries and inverter often require 2/0 AWG or larger copper conductors to stay within voltage drop and ampacity limits.
Is this calculator valid for 2023 NEC?
The core ampacity tables (310.16, 310.15 factors) and DC conductor sizing rules (690.8) have been substantively consistent across recent NEC editions (2017, 2020, 2023). The methodology this calculator implements is valid under current NEC. Always verify with your local AHJ for any jurisdiction-specific amendments.
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