Solar Array to MPPT Cable Sizer

Sizes high-voltage DC string conductors from solar panels to an MPPT charge controller under NEC 690.8 continuous current rules (1.56x multiplier). For general 12V/24V low-voltage DC branch circuits, use the DC Wire Size Calculator.

⚡ Quick Engineering Answer: How Do You Size Solar PV Array Wire?

NEC 690.8 • UL 4703

Solar photovoltaic (PV) DC wire sizing is mandated by NEC Article 690.8 to carry 156% of array short-circuit current (1.56 × Isc), combining a 125% factor for edge-of-cloud irradiance surge with a 125% continuous duty factor. Conductor gauge must also restrict maximum one-way DC loop voltage drop under 2% to 3% of array maximum power voltage (Vmp) to preserve daily solar harvest efficiency. For example, a 400W solar array operating at Vmp = 36V and Imp = 11.1A over a 30-foot rooftop conduit run (60ft loop) requires 10 AWG UL 4703 XLPE PV wire to limit voltage drop to 1.63% (0.59V). For arrays mounted on dark roofs or engine compartments, NEC 310.15 mandates high ambient temperature correction adders, derating conductor ampacity by 18% to 42% in climates exceeding 40°C (104°F).

Array Electrical & Run Parameters

Array Max Power Voltage (Vmp)

String Presets:

Enter total string Vmp from panel spec sheet (sum of panels in series).

Array Max Current (Imp)

Array Imp (sum of parallel strings).

Total Array Power

Cross-calculates automatically: P = Vmp × Imp.

Cable Run Distance

Typical:

Run Measurement Basis

One-way distance accounts for both positive and negative PV conductors.

Allowable Voltage Drop Target

Standard Targets:

Solar arrays should target 1.0% to 2.0% to avoid unrecoverable daily harvest deficits.

Advanced: Peak Sun Hours, Rooftop Temperature & Existing Gauge Analysis

Peak Sun Hours (PSH / Day)

h/day

Solar Insolation:

Rooftop Ambient Temperature

°C

Roof Exposure:

Evaluate Existing Installed Conductor

Select an existing wire size to calculate exact voltage drop and transmission watt loss.

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Enter Parameters to Calculate

Enter the values above and click Calculate to view recommendations and engineering specifications.

Solar PV Gauge Trade-Off Analysis

Conductor Gauge	Metric Size	Voltage Drop	Drop %	Power Lost	Daily Harvest Lost	Status

Solar PV Conductor Engineering Principles

Deterministic & Standards-Based Photovoltaic Model

Calculation Assumptions

DC resistance based on uncoated annealed copper at 75°C (167°F) per NEC Chapter 9, Table 8.
Conductor ampacity based on ABYC Table 6B / NEC Table 310.16 (105°C insulation in free air).
Temperature correction factor applied from ABYC Table 6A to account for elevated solar rooftop thermal exposure.
Power loss calculated at Maximum Power Point ($P_{\text{loss}} = I_{\text{mp}}^2 \times R_{\text{loop}}$).

Engineering Limitations

Photovoltaic rooftop wiring requires sunlight-resistant, direct-burial, double-insulated cross-linked polyethylene (XLPE) PV wire (UL 4703). Regular automotive or marine primary wire degrades rapidly under direct UV exposure.
Does not account for temperature-dependent panel $V_{\text{mp}}$ shift (panels drop ~0.35%/°C in peak midday heat, increasing percentage drop slightly).

Engineering Insight: The Series Voltage Advantage

Resistive transmission loss is governed by Joule's Law: $P_{\text{loss}} = I^2 R$. By wiring four 200W panels in series (80V, 10A) rather than parallel (20V, 40A), the transmission current is reduced by a factor of 4. Because power loss scales with current squared ($4^2 = 16$), series stringing reduces transmission loss by 93.75% over the exact same wire run.

Engineering Notice: Calculated voltage drop uses nominal uncoated-copper conductor resistance at 75°C (167°F), based on NEC Chapter 9, Table 8. Conductor ampacity uses ABYC Table 6B and Table 6A thermal derating factors. Actual installations must account for local electrical codes (NEC Article 690), conduit fill derating, maximum string open-circuit voltage ($V_{\text{oc}}$), and manufacturer equipment limits. This calculator is provided for engineering planning and educational purposes.

Quick-Reference Solar PV Wire Sizing Chart — 2% Voltage Drop

UL 4703 Solar Photovoltaic Sizing Matrix (Copper, 75°C)

In solar photovoltaic circuits between panels and charge controllers, keeping total voltage drop under 2% maximizes daily watt-hour energy harvest. This static reference table outlines the minimum recommended copper PV wire gauge for common string configurations and operating voltages over one-way rooftop/ground-mount distances.

Array Configuration	Operating Voltage (Vmp)	20 ft Run (6m)	40 ft Run (12m)	60 ft Run (18m)	NEC 690.8 Rating
1 String (10A Imp)	36V DC (2 panels series)	12 AWG (4 mm²)	10 AWG (6 mm²)	8 AWG (10 mm²)	15.6A (12 AWG)
2 Strings (20A Imp)	36V DC (2S2P array)	10 AWG (6 mm²)	6 AWG (16 mm²)	4 AWG (25 mm²)	31.2A (8 AWG)
3 Strings (30A Imp)	36V DC (2S3P array)	8 AWG (10 mm²)	4 AWG (25 mm²)	2 AWG (35 mm²)	46.8A (6 AWG)
1 String (10A Imp)	72V DC (4 panels series)	14 AWG (2.5 mm²)	12 AWG (4 mm²)	10 AWG (6 mm²)	15.6A (14 AWG)
2 Strings (20A Imp)	72V DC (4S2P array)	12 AWG (4 mm²)	10 AWG (6 mm²)	6 AWG (16 mm²)	31.2A (10 AWG)
1 String (10A Imp)	150V DC (Commercial MPPT)	14 AWG (2.5 mm²)	14 AWG (2.5 mm²)	12 AWG (4 mm²)	15.6A (14 AWG)

← Swipe horizontally to view full table →

Notice: Higher voltage series strings dramatically reduce current, allowing smaller conductor sizes and virtually eliminating transmission losses over long wire runs.

Solar PV Array Wire Calculations & NEC 690 Rules

Photovoltaic Physics: Short-Circuit Sizing & String Losses

⚡ NEC 690.8 Continuous Current Safety Sizing

$$\begin{aligned} I_{\text{design}} &= I_{\text{sc}} \times 1.25 \times 1.25 \\[8pt] &= 1.56 \times I_{\text{sc}} \end{aligned}$$

The first 1.25 factor accounts for cloud-edge irradiance spikes exceeding 1000 W/m². The second 1.25 factor satisfies the NEC continuous duty requirement (>3 hours).

⚡ PV String Voltage Drop at Maximum Power Point (Vmp)

$$V_{\text{drop}} = I_{\text{mp}} \times R_{\text{loop}}$$ $$\% V_{\text{drop}} = \left(\frac{V_{\text{drop}}}{V_{\text{mp,total}}}\right) \times 100$$

Operating current ($I_{\text{mp}}$) dictates everyday power loss during energy harvest, while short-circuit current ($I_{\text{sc}}$) dictates wire thermal safety.

⚡ Daily Harvest Energy Loss (Watt-Hours Lost)

$$P_{\text{loss}} = I_{\text{mp}}^2 \times R_{\text{loop}}$$ $$E_{\text{daily,loss}} = P_{\text{loss}} \times \text{PSH}$$

For example, 25 Watts of wire drop over 5 peak sun hours results in 125 Wh of daily harvest lost, totaling over 45 kWh of lost solar generation every year.

Frequently Asked Questions

What size wire do I need between solar panels and charge controller?

For residential and mobile solar arrays with short runs under 30 feet, 10 AWG (6 mm²) UL 4703 solar cable is standard and handles up to 30A. For high-current parallel arrays (3 or more parallel strings) or longer rooftop conduit runs exceeding 40 feet, 8 AWG or 6 AWG is required to keep voltage drop below 2%.

Why does NEC require a 1.56 safety factor for solar PV wire sizing?

NEC Article 690.8 requires two independent multipliers: a 1.25 factor for peak cloud-edge irradiance surge (where reflected sunlight pushes solar irradiance above 1,000 W/m²), and a 1.25 continuous duty factor because PV arrays generate peak current for three hours or more (1.25 * 1.25 = 1.56 * Isc).

What is the difference between standard THHN and UL 4703 PV wire?

UL 4703 photovoltaic wire features thick, dual-layer cross-linked polyethylene (XLPE) insulation that provides superior UV resistance, high-temperature rating (90°C wet / 125°C dry), extreme cold flexibility (-40°C), and sunlight resistance for exposed outdoor wiring where conduit is not practical.

How does rooftop temperature adder impact solar conduit ampacity?

Sunlight striking dark vehicle roofs or metal building roofs creates severe localized ambient temperatures. NEC 310.15 requires adding 17°C to 33°C (30°F to 60°F) to outdoor ambient temperature when conduit is mounted directly against or close to the roof surface, drastically reducing conductor ampacity.