Solar Cable Length Calculator 2026: DC Voltage Drop Tables for PV Systems

A 15 kW commercial array in Phoenix lost 4.2% of annual production to cable undersizing. The installer had run 12 AWG string cable across a 35-meter roof run to save $180 on copper. The voltage drop pushed string voltage below the inverter’s minimum MPPT threshold for 3–4 hours every summer morning. The client noticed the production gap on monitoring. The fix — pulling 8 AWG replacement cable — cost $2,400 in labor and downtime. The $180 savings had become a $2,400 problem.

This is not a rare story. Voltage drop from undersized DC cable is the most common electrical design error I see in field inspections. It is also the easiest to prevent. You need a calculator, a resistance table, and five minutes before you pull cable.

This guide gives you all three. It covers the voltage drop formula, NEC 690.8 and IEC cable sizing rules, pre-computed voltage drop tables for common cable sizes and system voltages, temperature derating, string vs. home run sizing, and a complete step-by-step worked example. Use it as a field reference or a design checklist. Professional solar design software automates cable sizing with NEC 690.8 and IEC 62548 compliance built in.

In this guide:

• Why DC cable sizing matters: voltage drop, power loss, and safety
• The DC voltage drop formula: Vdrop = I × R × L explained
• NEC 690.8 and IEC cable sizing standards
• Cable resistance tables by AWG, mm², and material
• Voltage drop limits: 1%, 2%, 3% — when to use which
• Step-by-step solar cable length calculator: worked example
• String cable sizing (PV source circuits)
• Home run cable sizing (PV output circuits)
• Temperature derating for cable ampacity
• Pre-computed DC voltage drop tables for common sizes
• Common mistakes in DC cable sizing
• When oversizing cable is wasteful

Why DC Cable Sizing Matters

DC cables carry power from modules to the inverter. Every meter of cable adds resistance. Every ohm of resistance converts power to heat. That heat is lost production. In extreme cases, it is a fire risk.

Three factors make DC cable sizing critical in solar design:

1. Low voltage means high current. A 400 W module at 40 Vmp produces 10 A. A string of 10 modules at 400 Vmp still produces 10 A. That 10 A flowing through 50 meters of 12 AWG cable drops 6.4 V — 1.6% of string voltage. On a 400 V string, 1.6% drop can push the operating voltage below the inverter’s MPPT window during hot afternoons when module voltage already sags 10–15%.

2. DC arcs do not self-extinguish. Unlike AC, which crosses zero 100–120 times per second, DC current is continuous. A loose connection or damaged cable on a DC circuit can sustain an arc indefinitely. Proper cable sizing — with correct ampacity margins and quality terminations — is the first line of defense against arc faults.

3. Cumulative losses compound. A 2% voltage drop on string cable plus 1.5% on the home run plus 1% on inverter efficiency plus 0.5% on mismatch adds to 5% total system loss before you account for soiling, degradation, or shading. Cable losses are the only ones you can eliminate with a five-minute calculation.

The DC Voltage Drop Formula

The fundamental equation for DC voltage drop is:

Vdrop = I × R × L × 2

Where:

To express voltage drop as a percentage:

%Vdrop = (Vdrop / Vsystem) × 100

Worked Example: Single String

A residential string has the following parameters:

• Isc = 9.8 A
• String Vmp = 405 V
• One-way cable length = 18 m
• Cable: 10 AWG copper PV wire

Step 1: Apply NEC 690.8 current multiplier I = 9.8 A × 1.25 = 12.25 A

Step 2: Find cable resistance 10 AWG copper = 3.28 mΩ/m (0.00328 Ω/m)

Step 3: Calculate voltage drop Vdrop = 12.25 A × 0.00328 Ω/m × 18 m × 2 = 1.45 V

Step 4: Calculate percentage %Vdrop = (1.45 V / 405 V) × 100 = 0.36%

This is well within the 1–2% target. The installer could even use 12 AWG for this short run if cost-sensitive, though 10 AWG is the safer default.

Alternative Formula Using Cable Cross-Section

When you know the conductor area in mm², use:

Vdrop = (2 × I × L × ρ) / A

Where:

• ρ (rho) = resistivity of copper = 0.0172 Ω·mm²/m at 20°C
• A = conductor cross-sectional area in mm²

For the same example with 10 AWG (approximately 5.26 mm²): Vdrop = (2 × 12.25 × 18 × 0.0172) / 5.26 = 1.44 V

The small difference (1.45 V vs. 1.44 V) comes from rounding in the resistance table. Both methods are valid.

NEC 690.8 and IEC Cable Sizing Standards

NEC 690.8 (United States)

NEC Article 690 governs solar PV systems. Section 690.8 specifies how to calculate circuit currents for conductor and overcurrent device sizing.

690.8(A)(1) — PV Source Circuit Current: The maximum current is 125% of the module short-circuit current (Isc) as rated at standard test conditions (STC).

690.8(B)(1) — Conductor Ampacity: Conductors must be sized at 125% of the current calculated in 690.8(A).

Combined multiplier: 1.25 × 1.25 = 1.56× Isc

This means a module with 9.5 A Isc requires conductors rated for: 9.5 A × 1.56 = 14.8 A minimum ampacity

690.31 — Wiring Methods:

• PV source and output circuits must use wiring methods suitable for wet locations
• Exposed single-conductor cables must be listed as PV wire (USE-2) or PV1-F
• Cable must be secured at intervals not exceeding 1.2 m (4 ft)

690.31(C) — Temperature Rating: PV wire must have a temperature rating of 90°C or higher for rooftop installations.

IEC 62548 (International)

IEC 62548 is the international standard for PV array design requirements. Its cable sizing approach differs slightly from NEC. Most solar software platforms default to NEC 690.8 for US projects and IEC 62548 for international work.

Current calculation:

• Minimum cable current rating = 1.25 × Isc (string level)
• No additional 1.25 continuous operation multiplier
• Effective multiplier: 1.25× Isc (vs. 1.56× under NEC)

Voltage drop:

• IEC recommends maximum 1% voltage drop for DC cabling
• Some national implementations allow up to 1.5%

Cable selection:

• PV1-F (flexible) for module interconnections
• PV1-R (rigid) for fixed installations
• Minimum conductor temperature rating: 90°C

Key Difference: NEC vs. IEC

For international projects, the IEC 1.25× multiplier produces slightly smaller cable sizes than NEC. If you design for both markets, size to NEC and you automatically meet IEC.

Cable Resistance Tables

Copper Cable Resistance at 20°C

Resistance values for annealed copper per ASTM B3. Ampacity values for single conductors in free air at 30°C ambient, 90°C insulation per NEC 310.16.

Aluminum Cable Resistance at 20°C

Aluminum resistance is approximately 61% higher than copper for the same gauge. Ampacity is approximately 78% of copper equivalent.

Resistance Temperature Correction

Cable resistance increases with temperature. At operating temperatures of 60–70°C on a rooftop, resistance is 15–20% higher than the 20°C table values.

Correction formula: R(T) = R(20°C) × [1 + α × (T − 20)]

Where α (temperature coefficient) = 0.00393 per °C for copper

For conservative solar design, use the 60°C correction factor (1.16) for exposed rooftop cable runs.

Voltage Drop Limits: 1%, 2%, or 3%?

There is no single correct answer. The right limit depends on system voltage, inverter characteristics, and economic tradeoffs.

1% Drop: High-Performance Systems

Use a 1% limit when:

• String voltage is near the inverter’s minimum MPPT voltage
• The system uses string inverters with narrow MPPT windows
• You are designing for a client who monitors production closely
• The installation is in a hot climate where module voltage sags are severe

A 1% limit adds cable cost but eliminates a class of production losses that are hard to diagnose after installation.

2% Drop: Standard Practice

A 2% limit is the industry default for most residential and commercial installations. It balances material cost against energy loss. At $0.12/kWh and 1,500 equivalent peak sun hours, a 2% drop on a 10 kW system costs approximately $36/year in lost production. Upsizing cable to cut drop from 2% to 1% might cost $200–$400 in additional copper. The payback on that investment is 5–10 years — reasonable but not automatic.

3% Drop: Acceptable for Long Home Runs

A 3% limit is acceptable for:

• Very long home runs where upsizing is prohibitively expensive
• Systems with high string voltage (800 V+) where percentage drop is smaller in absolute terms
• Battery systems where the battery voltage range accommodates wider swings

Never exceed 3% on any single cable segment. If your string cable is at 2% and your home run is at 2%, total DC drop is 4%. That is too much.

Solar Cable Length Calculator: Step-by-Step Worked Example

Let us walk through a complete residential design.

System Parameters

Step 1: Calculate Design Current (NEC 690.8)

I_design = Isc × 1.25 × 1.25 = 10.2 A × 1.56 = 15.9 A

Step 2: Size String Cable (PV Source Circuit)

Each string carries 15.9 A design current over 22 m one-way.

Try 10 AWG copper (5.26 mm²):

• Resistance at 20°C: 3.28 mΩ/m
• Resistance at 60°C: 3.28 × 1.16 = 3.80 mΩ/m
• Vdrop = 15.9 A × 0.00380 Ω/m × 22 m × 2 = 2.66 V
• %Vdrop = 2.66 V / 402 V × 100 = 0.66%

Result: 0.66% is well within the 1–2% target. 10 AWG is acceptable.

Try 12 AWG copper (3.31 mm²) for comparison:

• Resistance at 60°C: 5.31 × 1.16 = 6.16 mΩ/m
• Vdrop = 15.9 A × 0.00616 Ω/m × 22 m × 2 = 4.31 V
• %Vdrop = 4.31 V / 402 V × 100 = 1.07%

Result: 1.07% is still acceptable but near the upper limit. For a 22 m run, 12 AWG works but 10 AWG is the safer choice, especially if the run includes bends or junction boxes that add effective length.

String cable selection: 10 AWG copper PV wire

Step 3: Size Home Run Cable (PV Output Circuit)

The home run carries combined current from all 3 strings.

I_home_run = 3 × 15.9 A = 47.7 A

Try 6 AWG copper (13.3 mm²):

• Resistance at 60°C: 1.30 × 1.16 = 1.51 mΩ/m
• Vdrop = 47.7 A × 0.00151 Ω/m × 18 m × 2 = 2.59 V
• %Vdrop = 2.59 V / 402 V × 100 = 0.64%

Result: 0.64% is excellent. But check ampacity: 6 AWG at 90°C is rated 75 A at 30°C ambient. At 50°C ambient with correction factor 0.75: 75 A × 0.75 = 56.25 A. This exceeds the 47.7 A design current. 6 AWG is acceptable.

Try 8 AWG copper (8.37 mm²) for comparison:

• Resistance at 60°C: 2.06 × 1.16 = 2.39 mΩ/m
• Vdrop = 47.7 A × 0.00239 Ω/m × 18 m × 2 = 4.10 V
• %Vdrop = 4.10 V / 402 V × 100 = 1.02%

Ampacity check: 8 AWG at 90°C = 55 A at 30°C. At 50°C: 55 × 0.75 = 41.25 A. This is below the 47.7 A design current. 8 AWG is undersized for ampacity.

Home run cable selection: 6 AWG copper PV wire

Step 4: Verify Total DC Voltage Drop

Total DC drop = string drop + home run drop = 0.66% + 0.64% = 1.30%

This is within the 2.5% total budget. The design is acceptable.

Step 5: Check Inverter MPPT Compatibility

Minimum string voltage at maximum power point:

• Vmp at STC: 402 V
• Temperature coefficient: −0.30%/°C (typical for monocrystalline)
• Hot cell temperature: 70°C (25°C ambient + 45°C rise)
• Voltage sag: 402 V × [1 − (0.0030 × 45)] = 402 V × 0.865 = 348 V

With 1.30% cable drop, operating voltage at inverter = 348 V × 0.987 = 343 V

Inverter MPPT minimum: 200 V. Margin: 143 V. The design is safe.

String Cable Sizing (PV Source Circuits)

String cables run from the module junction box to the combiner box or inverter. They carry string current at string voltage.

Typical String Configurations and Cable Sizes

String Cable Selection Flowchart

1. Determine string Isc from module datasheet
2. Multiply by 1.56 (NEC) or 1.25 (IEC) for design current
3. Measure one-way cable length from last module to combiner
4. Select initial cable size (10 AWG default for residential)
5. Calculate Vdrop = I × R × L × 2
6. Check %Vdrop against 1–2% target
7. If exceeded, upsize one gauge and recalculate
8. Verify ampacity at operating temperature

Opinion: 12 AWG Should Not Be the Default

Many installers default to 12 AWG for all residential strings because it is cheaper and easier to work with. This is a mistake. 12 AWG saves approximately $0.15 per meter. On a 20-meter string run, that is $3 per string. For a 20-string commercial array, it is $60 total. If even one string runs longer than 15 meters or carries more than 10 A, the voltage drop exceeds 1.5%. The annual production loss from that drop exceeds the cable savings in under two years.

Use 12 AWG only for short runs under 12 meters with low-current modules. Make 10 AWG your default. The cost difference is negligible. The performance difference is measurable.

Home Run Cable Sizing (PV Output Circuits)

Home run cables carry combined current from the combiner box to the inverter DC input. They operate at system voltage (same as string voltage) but carry the sum of all string currents.

Home Run Sizing by Number of Strings

The table below assumes 10 A string Isc, 1.56× NEC multiplier, 10 AWG string cable, and 2% voltage drop target for the home run.

Max run is one-way cable length at 2% voltage drop limit, 60°C operating temperature, copper conductor.

Home Run Design Tips

• Keep the combiner box as close to the inverter as practical
• If the home run exceeds 30 meters, consider relocating the inverter or using a string inverter architecture instead of central
• For very long runs, higher system voltage (1,000 V or 1,500 V) reduces current for the same power, allowing smaller cable or longer runs
• Always include a DC disconnect at the inverter input rated for the home run current

Temperature Derating for Cable Ampacity

Rooftop temperatures routinely exceed 50°C on dark surfaces in summer. Cable ampacity must be derated accordingly.

NEC Temperature Correction Factors (from 310.16)

Practical Derating for Solar Installations

A 10 AWG cable on a dark shingle roof in Phoenix at 50°C ambient has an effective ampacity of only 30 A. If your design current is 15.9 A per string and you have 2 strings in conduit together, you must also apply the conduit fill derating. Two current-carrying conductors in conduit at 50°C: 40 A × 0.80 × 0.75 = 24 A. A 2-string home run in conduit on a hot roof is marginal with 10 AWG. Use 8 AWG or larger.

Conduit Fill Derating (NEC 310.16)

For solar DC circuits, remember that both positive and negative conductors count as current-carrying. A single string in conduit = 2 conductors = no derating. Two strings = 4 conductors = 0.80 derating.

DC Cable Voltage Drop Tables (Pre-Computed)

The tables below show voltage drop percentage for common cable sizes, currents, and system voltages. All values assume copper cable at 60°C operating temperature and include both conductors (×2).

Table 1: 400 V System Voltage (Residential String Inverters)

Values shown as ”—” exceed 2% voltage drop and are not recommended.

Table 2: 600 V System Voltage (Commercial String Inverters)

Table 3: 800 V System Voltage (Commercial/Utility Central Inverters)

Table 4: 1,000 V System Voltage (Utility-Scale)

How to Use These Tables

1. Find your system voltage table
2. Locate your cable run length row
3. Find your design current column
4. Read the voltage drop percentage
5. If the value exceeds your target (1–2%), move one column right to a larger cable size
6. Verify the ampacity at your operating temperature

Common Mistakes in DC Cable Sizing

Mistake 1: Using AC Ampacity Tables for DC

NEC 310.16 ampacity tables are calculated for AC circuits with skin effect and proximity effect. DC current distributes uniformly across the conductor cross-section. For small cables (≤ 1/0 AWG), the difference is negligible. For large cables (≥ 250 kcmil), DC ampacity is slightly higher than AC because there is no skin effect. The practical error is not in the ampacity but in using indoor AC cable (THHN) outdoors where PV wire is required.

Mistake 2: Ignoring Temperature Derating

The most common field failure I see: an installer sizes cable for 30°C ambient using the table ampacity, then installs it on a dark roof that reaches 60°C. The cable operates at 80–85% of its rated ampacity. Over time, insulation degrades. Connections loosen. Resistance rises. The problem compounds.

Always apply both temperature derating and conduit fill derating before finalizing cable size. If you are unsure of peak rooftop temperature, measure it on a sunny summer afternoon with an infrared thermometer. Add 10°C for conduit interior temperatures.

Mistake 3: Forgetting the Return Conductor

Some installers calculate Vdrop = I × R × L, forgetting the ×2 for the return path. This halves the calculated voltage drop. A “1% drop” design becomes a 2% drop reality. The error is most common among electricians transitioning from AC work, where three-phase calculations use √3 instead of 2.

Mistake 4: Mixing Copper and Aluminum Without Conversion

An installer extends a copper string with aluminum cable to save cost on a long run. They size the aluminum at the same AWG as the copper. The aluminum has 61% higher resistance. Voltage drop increases proportionally. Terminations between dissimilar metals corrode without anti-oxidant compound. The connection resistance rises over months.

If you must mix materials, upsize aluminum by two AWG sizes (e.g., replace 6 AWG copper with 2 AWG aluminum) and use listed bimetallic connectors with anti-oxidant paste.

Mistake 5: Sizing for Isc Without Checking Imp

NEC 690.8 requires sizing for 1.56 × Isc. Some installers use this value for voltage drop calculations, producing conservative (larger) cable sizes. This is safe but wasteful. Voltage drop occurs at operating current (Imp), not short-circuit current. Size conductors for 1.56 × Isc for ampacity and protection. Size for actual operating current (or 1.25 × Isc as a margin) for voltage drop.

When Oversizing Cable Is Wasteful

The conventional wisdom in solar design is “size up, never down.” This is defensible for safety. It is not always correct for economics.

The Break-Even Calculation

Upsizing from 10 AWG to 8 AWG on a 20-meter string run costs approximately $25 more in copper. The resistance drops from 3.28 mΩ/m to 2.06 mΩ/m. For a 10 A string at 400 V, voltage drop falls from 0.33% to 0.21% — a 0.12% improvement.

On a 5 kW string at 1,500 equivalent sun hours, annual production is 7,500 kWh. A 0.12% improvement saves 9 kWh/year. At $0.12/kWh, that is $1.08/year. The payback on the $25 investment is 23 years — longer than most inverter warranties.

When to Size Up

• When voltage drop is near the inverter MPPT threshold
• When the cable run is long (>>30 m)
• When future expansion is likely
• When the client values performance over first cost

When Standard Size Is Fine

• Short runs (under 15 m) with low current (under 12 A)
• Systems with high string voltage (800 V+) where percentage drop is small
• Budget-constrained projects where every dollar matters

The correct approach is to calculate the actual voltage drop for every cable segment, compare it against your limit, and size accordingly. Do not default to the largest cable “just to be safe.” That is not engineering. That is guessing with expensive materials.

Conclusion

DC cable sizing is not complex. It is arithmetic that too many installers skip. The formula is Vdrop = I × R × L × 2. The tables are in this guide. The standards are NEC 690.8 and IEC 62548. A five-minute calculation before pulling cable prevents the $2,400 rework that opened this guide.

Three actions to take from here:

1. Print the pre-computed tables and keep them in your field truck. Reference them during every design review.
2. Measure actual rooftop temperatures in summer with an IR thermometer. Do not guess. Apply the real temperature correction factor.
3. Model voltage drop in your design software before finalizing the BOM. Solar design software with integrated cable sizing catches errors that manual calculations miss.

Cable losses are permanent. You cannot fix them with monitoring or maintenance. Size correctly once, and the system performs for 25 years.

Frequently Asked Questions

How do you calculate voltage drop for solar DC cables?

DC voltage drop for solar cables is calculated using Vdrop = I × R × L, where I is the maximum circuit current (1.25 × Isc per NEC 690.8), R is the cable resistance in ohms per meter, and L is the one-way cable length in meters. For a complete circuit, multiply by 2 to account for both positive and negative conductors. The result is compared against the allowable voltage drop percentage — typically 1–2% for DC source circuits and 1–3% for DC output circuits.

What is the maximum allowable voltage drop for solar DC cables?

NEC 690 does not specify a mandatory maximum voltage drop, but industry best practice limits DC voltage drop to 1–2% for PV source circuits (string wiring) and 1–3% for PV output circuits (home runs to inverter). A 1% limit is recommended for systems where inverter efficiency is critical or where string voltage is already near the inverter’s minimum MPPT threshold. A 2% limit is standard for most residential and commercial installations. Exceeding 3% wastes measurable energy and can cause inverter shutdown during low-irradiance conditions.

What size cable do I need for a 10 kW solar system?

A 10 kW solar system typically uses 10 AWG (6 mm²) or 8 AWG (10 mm²) PV wire for string cabling, depending on string current and cable run length. For a typical residential 10 kW system with 2 strings of 5 kW each at approximately 400 Vdc, string current is roughly 12.5 A per string. With a 15-meter string run, 10 AWG copper produces about 0.6% voltage drop. The home run from the combiner box to the inverter — carrying the combined 25 A — typically requires 6 AWG (16 mm²) for runs up to 20 meters to stay under 2% drop. Always verify with the actual Vdrop = I × R × L × 2 calculation for your specific layout.

Does temperature affect solar cable ampacity?

Yes. Cable ampacity decreases as ambient temperature rises. NEC 310.16 provides temperature correction factors: at 40°C ambient, copper THHN/THWN-2 ampacity is multiplied by 0.88; at 45°C, by 0.82; at 50°C, by 0.75. For rooftop PV installations where ambient temperatures can reach 60–70°C on dark surfaces, ampacity can drop to 50–58% of the 30°C baseline rating. This is why PV wire (USE-2 or PV1-F) with 90°C or 105°C insulation is mandatory for exposed rooftop conduit runs — standard THHN at 75°C would be dangerously undersized.

What is the difference between PV source circuits and PV output circuits?

PV source circuits carry current from individual strings of modules to the combiner box or inverter. They operate at string voltage (typically 300–600 Vdc for residential, up to 1,500 Vdc for utility-scale) and carry string current (typically 8–15 A per string). PV output circuits carry the combined current from all strings after the combiner box to the inverter DC input. They operate at the same system voltage but carry the sum of all string currents. NEC 690 defines both circuit types separately because the overcurrent protection, disconnect, and cable sizing requirements differ. Source circuits are protected by string fuses or DC breakers; output circuits require a DC disconnect rated for the combined current.

Should I use copper or aluminum for solar DC cables?

Copper is the standard for solar DC cabling up to 50 mm². It has lower resistance (1.72 µΩ·cm vs. 2.82 µΩ·cm for aluminum), better corrosion resistance in outdoor environments, and more reliable terminations. Aluminum is sometimes used for large commercial or utility-scale DC trunk cables above 95 mm² where cost savings outweigh the handling complexity. If using aluminum, you must use anti-oxidant compound on all terminations, torque connections to manufacturer specifications, and account for aluminum’s 61% higher resistance in voltage drop calculations. For residential and small commercial solar, copper is the only sensible choice.

How long can a solar string cable run be?

Solar string cable length is limited by voltage drop, not by a hard maximum. For a typical 400 Vdc string at 12 A, 10 AWG copper cable can run approximately 45 meters one-way before hitting a 2% voltage drop limit. At 1% drop, the same cable limits you to about 22 meters. If your layout requires longer runs, you have three options: (1) upsize to a larger conductor (e.g., 8 AWG doubles your allowable length), (2) increase string voltage by adding more modules per string (higher voltage means lower current for the same power, reducing I²R losses), or (3) relocate the inverter closer to the array. Always model the actual voltage drop for your specific string voltage, current, and cable size before finalizing the layout.

What is NEC 690.8 and how does it affect cable sizing?

NEC 690.8 is the article governing circuit sizing and current calculations for PV systems. It requires that all conductors and overcurrent devices be sized at 125% of the maximum system current (Isc × 1.25) plus an additional 125% derating for continuous operation, resulting in an overall multiplier of 1.56× (1.25 × 1.25) on short-circuit current. In practice, this means a string with 9.5 A Isc requires cable and protection rated for at least 14.8 A (9.5 × 1.56). The 690.8 multiplier is the reason 10 AWG PV wire (rated 40 A at 90°C) is commonly used even for strings under 15 A — it provides the necessary safety margin.

Can I use standard household wire for solar DC cables?

No. Standard household wire (NM-B, THHN in dry locations) is not rated for the outdoor, UV-exposed, high-temperature environment of rooftop solar installations. Solar DC cables must be listed as PV wire (USE-2) or PV1-F, with insulation rated for 90°C or higher, UV resistance, and moisture resistance per UL 4703 or IEC 62930. Using standard indoor wire on a rooftop installation creates fire risk, voids insurance coverage, and violates NEC 690.31(C). The incremental cost of proper PV wire is negligible compared to the cost of a failed inspection or a roof fire.

What is the voltage drop formula for three-phase AC vs DC solar circuits?

For DC solar circuits, voltage drop is Vdrop = I × R × L × 2 (multiply by 2 for both conductors). For single-phase AC circuits, Vdrop = I × R × L × 2 × power factor (typically 0.95–1.0 for inverter output). For three-phase AC circuits, Vdrop = √3 × I × R × L × power factor. The key difference is that DC circuits always carry current in both conductors simultaneously, so the round-trip resistance always matters. AC circuits have the additional complexity of phase angles and power factor. For solar installers, the practical implication is that DC voltage drop calculations are simpler — but the allowable drop is also smaller because DC circuits operate at lower voltages than the AC grid.