Solar Cable Voltage Drop Calculation: DC and AC Worked Examples

Answer: For an equal-conductor DC circuit, voltage drop is ΔV = 2 × L × r × I, where L is one-way length and r is resistance per matching length unit. Single-phase and balanced three-phase AC calculations also account for reactance and power factor. Calculate the percentage against the stated circuit voltage, then separately check conductor ampacity, equipment limits and applicable installation requirements.

A cable calculation should explain the circuit, units and operating point before producing a conductor size. The same percentage can mean different things at a PV string, battery connection or three-phase inverter feeder. This guide provides calculation examples; the assumed resistances are not approved cable specifications.

Gather the circuit inputs

Record the actual routed length, including risers and equipment connections, rather than only the distance between devices. State whether that length is one-way or already includes both conductors.

The calculation also needs:

• DC, single-phase AC or balanced three-phase AC circuit type.
• Operating voltage and the current associated with the calculation case.
• Cable material, construction and manufacturer resistance data.
• Resistance reference temperature and expected conductor temperature.
• AC frequency, reactance, cable arrangement and power factor where relevant.
• The voltage-drop target, its source and the equipment operating limits.

Use operating current for an operating loss estimate. Keep maximum-current, fault and ampacity calculations separate, with the factors required by the applicable design method. A nameplate Imp calculation is one reference case, not necessarily the maximum current encountered in service. Parallel strings and shared feeders require their own segment currents.

The solar stringing and wiring guide explains the separate module, input and MPPT compatibility checks.

DC formula: use loop resistance once

For steady DC, the resistive cable drop is:

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

If the positive and negative conductors have equal length and resistance per unit length:

R_loop = 2 × L × r
ΔV = 2 × L × r × I

Here, L is one-way length. If r is in Ω/km and L is entered in metres, divide the length by 1,000. The same conversion applies to feet with resistance in Ω per 1,000 ft. If a supplied resistance already represents the entire loop, do not multiply it by two again.

For unequal conductors or multiple series segments, add their actual resistances. Connector or termination resistance needs its own justified data; a generic percentage allowance does not identify a defective connection. The underlying relationships are explained in PVsyst’s cable resistance guide.

Worked DC example: a 200 m route

Assume a hypothetical DC circuit with:

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

An alternative assumed resistance of 0.400 Ω/km gives a 0.160 Ω loop, 14.4 V drop and 1.44% at that same operating point. This comparison does not establish an AWG size, ampacity, permitted installation or annual energy saving.

A resistance-per-1,000-ft example

Suppose the provided conductor data states 0.154 Ω per 1,000 ft. A single 200 ft conductor has resistance:

0.154 × 200 ÷ 1,000 = 0.0308 Ω.

A circuit with two such 200 ft conductors has a 0.0616 Ω loop. At an assumed 10 A, its resistive drop is 0.616 V. Keep this distinction explicit: the resistance of one wire and the resistance of the complete circuit are different quantities. The supplied value is an arithmetic input, not a claim that every conductor of a named gauge has that resistance at all temperatures.

AC formulas: resistance, reactance and power factor

Common steady-state approximations 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; r and x are per-conductor resistance and reactance in matching units. I is the line current. Use the appropriate single-phase circuit voltage or three-phase line-to-line voltage as the percentage denominator.

For the displayed positive reactive term, the current convention is lagging. Leading current and exported power need their corresponding signs and reference direction. The Schneider Electric steady-load calculation guide sets out the single-phase and balanced three-phase relationships.

These are voltage-magnitude approximations, not an exact replacement for phasor or network analysis. Unbalanced phases, significant harmonics, parallel cable arrangements and upstream network impedance require an appropriate model. Omitting reactance is a declared simplification, not an accuracy guarantee based only on project size.

Power factor equals cosφ. If it is 0.95, use 0.95 for that term; do not calculate cos(0.95). Where real power is given instead of current, first derive current consistently: P = V × I × cosφ for single-phase and P = √3 × V_LL × I × cosφ for balanced three-phase.

Worked balanced 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 reference voltage. These are hypothetical inputs.

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%

With reactance omitted, the result is about 3.291 V, or 0.823%. The difference illustrates why the assumption matters. Under the stated balanced conditions, conductor heating loss is separately 3 × I² × r × L = 600 W. Reactive impedance is not itself dissipated energy.

Temperature: conductor temperature is not ambient temperature

Use resistance at the relevant conductor temperature. Ambient conditions, current, installation method and neighbouring cables influence that temperature; a 50°C ambient does not establish a 50°C conductor.

When the conductor data permits a linear correction, write:

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

Check the coefficient, reference and valid range. Do not apply a 20°C correction to resistance already stated at the design temperature. PVsyst’s metal resistivity documentation uses temperature-dependent material values and distinguishes DC loop length from AC route length.

For illustration, assume α = 0.0039/°C and a 20°C reference. At an assumed 75°C conductor temperature, the multiplier is 1.2145. If the DC example’s 0.505 Ω/km were a 20°C value, its corrected drop would be 18.18 × 1.2145 = 22.07961 V, or about 2.208%. This temperature scenario does not establish the cable’s allowable operating temperature.

Voltage drop is one conductor-sizing check

A low calculated drop does not demonstrate an acceptable cable. Check installation-dependent ampacity, protection, short-circuit withstand, insulation and voltage rating, terminal ratings, connector compatibility and environmental suitability. For parallel conductors, verify the installation and current-sharing requirements instead of simply dividing current by the number of runs.

Schneider’s general cable-sizing method treats installation methods and correction factors as part of current-carrying capacity. They should not be replaced by an arbitrary extra voltage-drop margin.

There is no universal 2% DC/1% AC approval rule for every solar installation. Identify the adopted jurisdiction and standard edition, contract, utility requirements and equipment instructions. The Schneider voltage-drop limit guide explicitly distinguishes country-dependent limits and circuit applications. Its load-installation examples are not universal PV export limits.

Export voltage rise and inverter operating windows

During grid export, the inverter terminal voltage can be higher than the downstream grid reference. Assess the cable and upstream network together, including reactive-power behaviour and the actual utility voltage range. A transformer’s percentage impedance is not equal to a fixed percentage real-energy loss or a fixed voltage rise.

On the DC side, cable drop at operating current affects voltage available at the inverter. Check hot operating voltage against the exact MPPT range and any power-dependent restrictions. Check cold open-circuit voltage separately: at open circuit, current is zero, so a loaded cable-drop calculation cannot make excessive Voc safe. See maximum power point for the distinction between operating and open-circuit conditions.

Compare layouts and costs using time-dependent losses

Shorter routes and lower resistance can reduce losses. Higher operating voltage can reduce current at fixed power in a simplified DC comparison, but equipment voltage ratings, cold Voc, installation restrictions and system topology still govern the design. Do not default every commercial array to 1,500 V.

A full-load drop percentage is not an annual generation-loss percentage. Evaluate cable heating losses at the relevant current and resistance over time, then sum energy. PVsyst’s ohmic-loss documentation distinguishes reference-power loss fractions from simulated energy losses.

Compare the resulting energy difference with current installed quotations, energy value, discount rate and project duration. Battery charge and discharge paths also require separate operating histories; two 5% losses would multiply to 90.25% retention under that simplified assumption, rather than exactly 90%.

Record a reproducible design handoff

Attach circuit IDs to the single-line diagram. For each segment record route, length convention, current case, voltage reference, conductor data and temperature, AC reactance/power factor, calculated drop and the applicable acceptance criterion. State which ampacity, protection and equipment checks accompany the calculation.

Discuss the required electrical design handoff when evaluating the SurgePV design workflow. Confirm the exact supported calculations and exports with current product documentation rather than assuming automated code approval.

Frequently asked questions

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

Multiply operating current by the complete loop resistance. For equal conductors, ΔV = 2 × L × r × I with one-way length and matching resistance units. Divide by the stated operating voltage and multiply by 100 for percentage drop.

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

Use one-way length when the formula already includes the factor of two for an equal-conductor DC or single-phase circuit. If resistance or length already includes the full loop, do not double it again. Balanced three-phase calculations use their own geometry and voltage reference.

What is the maximum allowable voltage drop in solar PV systems?

The applicable criterion depends on jurisdiction, adopted standard edition, circuit type, equipment instructions and project or utility requirements. A generic 2% or 3% target does not demonstrate code compliance or equipment compatibility.

How does temperature affect voltage drop calculations?

Resistance changes with conductor temperature. Use verified cable data at the relevant temperature, or an applicable correction with an explicit reference and coefficient. Ambient temperature alone does not establish conductor temperature; avoid correcting an already temperature-adjusted value twice.

What is the difference between voltage drop and voltage rise?

Voltage drop describes a reduction along a supplying circuit. Exporting generation can instead produce a higher inverter-terminal voltage relative to the grid reference. Evaluate current direction, reactive power, cable impedance and upstream network conditions with consistent signs.

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

No. A percentage at one operating point does not describe the annual current history. Resistive losses vary with current squared and resistance; integrate them over the operating period and account for the PV/inverter operating behaviour.

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

Compare actual cable resistance, permitted installation, ampacity, compatible terminals, handling requirements and installed quotations. There is no universal run-length or current threshold that makes one material correct for every project.