Solar Cable Voltage Drop Calculation: DC and AC

Cable voltage drop is the voltage lost between the sending end and the receiving end of a conductor. This page gives the formulas for DC, single-phase and three-phase circuits, with worked numbers you can check by hand.

Last updated: November 2026.

Voltage drop is one check. Also size the cable for the current it must carry, called ampacity. In the US, see our guide to NEC 690.8 circuit sizing. The resistance values in the examples below are assumed. They are not approved cable specifications.

To run a quick check, use the voltage drop calculator. To size a cable for a whole system, read our solar cable sizing calculation guide.

What You Need Before You Calculate

Measure the real route length. Include risers and the runs into equipment, not only the distance between devices. Note whether your length is one-way or already counts both conductors.

You also need:

• The circuit type: DC, single-phase AC or balanced three-phase AC.
• The operating voltage and the current for your case.
• The cable material and the maker’s resistance data.
• The temperature for that resistance, and the expected cable temperature.
• For AC: frequency, reactance and power factor. Reactance is the extra opposition to AC current that cables add.
• The voltage drop limit you must meet, and where it comes from.

Use the operating current for a loss estimate. Check maximum current, fault current and ampacity in separate calculations. The solar stringing and wiring guide covers module, inverter input and MPPT checks.

What Voltage Drop Is Acceptable?

There is no single number for every site. Here is what the sources we read say.

Sources: PVsyst cable resistance page, CEC Guidelines Issue 13, Schneider Electrical Installation Guide.

Imp is the current at maximum power. Vmp is the voltage at maximum power. STC means standard test conditions.

Three notes on the table:

• The IEC limits are for general installations, not only solar. Schneider says allowed drops vary by country.
• The CEC says its guidelines were partly superseded when AS/NZS 5033:2021 came out on 19 November 2021. The CEC’s newer Issue 14 (May 2022) does not state a voltage drop or voltage rise limit. Check the current standards and your network operator. The CEC also says states and territories may add rules.
• For the US, the National Electrical Code (NEC) text is sold by NFPA, and we did not use it here. Check the edition your authority having jurisdiction (AHJ) enforces.

DC Formula: Count the Loop Once

For steady DC, the cable drop is:

ΔV = I × R_loop
Power loss = I² × R_loop
Voltage drop % = 100 × ΔV / V_reference

If the positive and negative cables have the same length and resistance per unit length:

R_loop = 2 × L × r
ΔV = 2 × L × r × I

L is one-way length. If r is in Ω/km and L is in metres, divide the length by 1,000. For feet, use resistance in Ω per 1,000 ft. If your resistance already covers the whole loop, do not multiply by 2 again.

For unequal cables or several segments in series, add up their real resistances. PVsyst’s cable resistance guide explains the same relationships.

Worked DC Example: A 200 m Route

Assume this DC circuit:

R_loop = 2 × 0.200 × 0.505 = 0.202 Ω
ΔV = 90 × 0.202 = 18.18 V
Drop = 100 × 18.18 / 1,000 = 1.818%
Cable power loss = 90² × 0.202 = 1,636.2 W
Receiving voltage = 1,000 − 18.18 = 981.82 V

With an assumed 0.400 Ω/km, the loop is 0.160 Ω. The drop is 14.4 V, or 1.44%, at the same current.

A Resistance-per-1,000-ft Example

Suppose the cable data says 0.154 Ω per 1,000 ft. One 200 ft conductor has this resistance:

0.154 × 200 ÷ 1,000 = 0.0308 Ω.

A circuit with two 200 ft conductors has a loop of 0.0616 Ω. At an assumed 10 A, the drop is 0.616 V. The resistance of one wire and of the whole circuit are different numbers. Keep them apart.

Voltage Drop by Cable Size and Length

This table shows the DC drop for copper cable at 10 A on a 600 V circuit. It uses the copper resistivity that PVsyst gives for 50 °C: 1.88E-8 Ω·m. Real cables have higher resistance than pure copper, so use the maker’s data for a real design.

Calculated: drop % = 100 × 2 × L × (1.88E-8 ÷ cross-section in m²) × 10 ÷ 600. Example for 6 mm² at 100 m: r = 1.88E-8 ÷ 6E-6 = 0.003133 Ω/m, ΔV = 2 × 100 × 0.003133 × 10 = 6.27 V, and 6.27 ÷ 600 = 1.04%.

The drop grows in step with current. At 20 A, double every number. It falls as voltage rises. At 1,200 V, halve every number.

How far is real cable from the ideal? The Prysmian TECSUN H1Z2Z2-K 1x6 datasheet gives 3.39 Ω/km at 20 °C for its 6 mm² solar cable. Pure copper at 20 °C is 1.68E-8 ÷ 6E-6 = 2.80 Ω/km. The datasheet value is about 21% higher (3.39 ÷ 2.80 = 1.21).

AC Formulas: Resistance, Reactance and Power Factor

For AC, include both resistance and reactance:

Single-phase two-wire:
ΔV ≈ 2 × L × I × (r × cosφ + x × sinφ)

Balanced three-phase, line-to-line drop:
ΔV ≈ √3 × L × I × (r × cosφ + x × sinφ)

L is one-way route length. The values r and x are the resistance and reactance of one conductor in matching units. I is the line current. Divide by the single-phase voltage, or the three-phase line-to-line voltage, to get percent.

Power factor is cosφ. If it is 0.95, use 0.95 for that term. Do not calculate cos(0.95). If you have power instead of current, find current first: P = V × I × cosφ for single-phase, and P = √3 × V_LL × I × cosφ for three-phase. The Schneider steady-load guide shows the same relationships.

These formulas give an approximate size of the voltage change, not an exact phasor result. Use a full network model for unbalanced phases, large harmonics or parallel cable runs.

Worked Three-Phase Example

Assume a 100 m route, 100 A line current, r = 0.20 Ω/km, x = 0.08 Ω/km, lagging power factor 0.95 and 400 V line-to-line.

L = 0.100 km
sinφ = √(1 − 0.95²) ≈ 0.31225
r × cosφ + x × sinφ ≈ 0.21498 Ω/km
ΔV ≈ √3 × 0.100 × 100 × 0.21498 ≈ 3.724 V
Drop ≈ 100 × 3.724 / 400 ≈ 0.931%

Without reactance, the result is about 3.291 V, or 0.823%. The cable heating loss is a separate figure: 3 × I² × r × L = 3 × 100² × 0.20 × 0.100 = 600 W.

Voltage Rise on the AC Side (Australia)

When a PV system exports power, the voltage at the inverter is higher than at the grid connection. This is voltage rise. The CEC Guidelines (Issue 13) say the overall rise from the point of supply to the inverter AC terminals must not exceed 2% of the nominal voltage at the point of supply. They also recommend keeping the drop between the inverter and the main switchboard below 1%.

Table 1 of the same guide lists the longest copper cable runs that give 1% voltage rise:

Source: CEC Guidelines for Accredited Installers, Issue 13, April 2019, Table 1 (maximum route length in metres). As noted above, parts of Issue 13 were superseded by AS/NZS 5033:2021. States and territories may set their own rules.

Temperature: Cable Heat Is Not Air Temperature

Use the resistance at the cable’s real temperature. Air temperature, current, install method and nearby cables all change it. A 50 °C day does not mean a 50 °C conductor.

When the data allows a straight-line correction, write:

r(T) = r(T_ref) × [1 + α × (T − T_ref)].

PVsyst’s metal resistivity page gives copper as 1.68E-8 × (1 + 0.0039 × (Temp − 20)) Ω·m. So α = 0.0039 per °C with a 20 °C reference. Do not correct a value that is already stated at a higher design temperature.

Here is the effect on the DC example. At 75 °C, the multiplier is 1 + 0.0039 × 55 = 1.2145. If the 0.505 Ω/km were a 20 °C value, the drop would be 18.18 × 1.2145 = 22.08 V, or about 2.208%.

Voltage Drop Is One Sizing Check

A low drop does not make a cable right. Also check:

• ampacity for the install method
• protection devices
• short-circuit withstand
• insulation and voltage rating
• terminals and connectors
• the environment, such as sun, heat and water

For parallel cables, check current sharing. Do not divide the current by the number of runs and stop there. Schneider’s general cable sizing method treats install methods and correction factors as part of current capacity. Do not replace them with an extra voltage drop margin.

On the DC side, the drop at operating current lowers the voltage at the inverter. Check that hot operating voltage stays inside the inverter’s MPPT range. Check cold open-circuit voltage (Voc) on its own. At open circuit there is no current, so a drop calculation cannot make a high Voc safe. See maximum power point for the difference.

Does a 2% Drop Mean 2% Energy Loss?

No. A drop at full current is for one moment. Cable loss follows current squared, and current changes through the day. PVsyst’s ohmic losses page says the annual wiring energy loss is usually around 60% of the specified nominal loss fraction, depending on the irradiance distribution.

To compare layouts, add up the energy lost over the year for each option. Then compare it with the installed price of each. Higher voltage lowers current for the same power, but equipment voltage ratings and cold Voc still set the limit.

Copper or Aluminum?

PVsyst’s resistivity page gives 1.68E-8 Ω·m for copper and 2.82E-8 Ω·m for aluminum at 20 °C. Aluminum is about 1.7 times higher (2.82 ÷ 1.68 = 1.68). It needs a larger cross-section for the same resistance. Terminals, rules and price decide the rest.

Record the Result for Your Design Handoff

Tag each circuit on the single line diagram. For each segment, write down the route, the length rule, the current, the voltage, the cable data and temperature, the AC reactance and power factor, the drop and the limit you used. Also note which ampacity and protection checks go with it.

You can also read our guide to the solar DC cable length calculator.

If your team runs these checks across many projects, a solar design platform that keeps layout and electrical data together saves re-entering numbers. Book a demo to see how SurgePV handles it.

Frequently Asked Questions

How do you calculate voltage drop for a DC solar cable?

Multiply the operating current by the resistance of the full loop. For two equal conductors, ΔV = 2 × L × r × I, where L is one-way length. Divide by the circuit voltage and multiply by 100 to get percent.

What is acceptable voltage drop for solar?

There is no single number. PVsyst says design targets are often 1 to 3% over a cable section. Australia’s CEC Guidelines (Issue 13, April 2019) say PV array DC drop should not exceed 3% of Vmp at STC, and AC voltage rise should not exceed 2% of nominal voltage. Check the code, standard and utility rules for your site.

Is one-way or round-trip length used for voltage drop?

Use one-way length when your formula already has the factor of 2 for an equal-conductor DC or single-phase circuit. If your resistance or length already covers the full loop, do not double it again. Balanced three-phase formulas use their own factor.

How does temperature affect voltage drop calculations?

Copper resistance rises with temperature. PVsyst uses Rho = 1.68E-8 × (1 + 0.0039 × (Temp - 20)) Ω·m for copper. Use the conductor temperature, not the air temperature, and do not correct a value that is already corrected.

What is the difference between voltage drop and voltage rise?

Voltage drop is the voltage lost along a cable that supplies a load. Voltage rise is the higher voltage at the inverter when it exports power to the grid. The CEC Guidelines (Issue 13) limit AC voltage rise to 2% of nominal voltage at the point of supply.

Does a 2% voltage drop mean 2% annual solar energy loss?

No. A 2% drop is for one operating point, usually full current. PVsyst says the annual wiring energy loss is usually around 60% of the specified nominal loss fraction, depending on the irradiance distribution.

When should I use aluminum instead of copper for solar cables?

Compare the real resistance, the permitted use, the terminals and the price. By resistivity alone, aluminum (2.82E-8 Ω·m at 20 °C) is about 1.7 times copper (1.68E-8 Ω·m), so it needs a larger cross-section for the same resistance. No single length or current makes one metal right for every job.