Solar Cable Voltage Drop Calculator

A solar cable voltage drop calculator determines the voltage lost in a PV conductor run from the entered Array current, One-way cable length, Conductor resistance, System voltage, and Circuit length basis.

The primary result is Voltage drop in volts and as a percentage of the entered system voltage. For a DC PV output circuit using a positive and negative conductor between the source and load/reference point, the calculator can apply a DC two-wire loop basis, doubling the one-way run length to represent the complete circuit path.

Voltage-drop results are used during PV conductor selection and layout review. A long homerun from a combiner, array, or source circuit to inverter equipment can lose enough voltage to reduce the voltage delivered at the reference point. The result helps compare alternate conductor resistances, routing distances, and system-voltage options before final conductor sizing and installation review.

Circuit Length Basis

One-way cable length is the physical distance from the source to the load or reference point. It is not automatically the electrical path length used in every voltage-drop calculation.

For a typical ungrounded DC PV circuit with one outgoing conductor and one return conductor, current travels through both conductors. Selecting DC two-wire loop causes the calculator to use:

\(\displaystyle L_\text{circuit} = 2 \times L_\text{one-way}\)

For a 120 ft one-way PV cable route:

\(\displaystyle L_\text{circuit} = 2 \times 120 = 240\text{ ft}\)

The displayed Circuit length used is the length actually applied to the resistance calculation. Confirm that the selected circuit basis matches the conductor path being evaluated. A one-way physical route and a complete DC circuit loop are different quantities.

Conductor Resistance and Voltage Loss

Conductor resistance is entered in ohms per 1,000 ft. The value should come from the selected conductor reference for the specific conductor being evaluated. Resistance varies by conductor material, AWG or kcmil size, conductor construction, and applicable reference conditions.

The calculator converts the entered resistance into total circuit resistance:

\(\displaystyle R_\text{circuit} = \left( \frac{R_{\text{ohm}/1000\text{ ft}}}{1000} \right) \times L_\text{circuit}\)

Voltage drop is then calculated using Ohm’s law:

\(\displaystyle V_\text{drop} = I \times R_\text{circuit}\)

Where:

Input or result	Electrical meaning
Array current	Current used for the voltage-drop calculation, in amperes
One-way cable length	Physical source-to-load/reference-point distance, in feet
Conductor resistance	Entered conductor resistance in ohm/1000 ft
Circuit length basis	Determines whether the one-way length is doubled for the DC loop
Circuit length used	Actual length applied in the resistance calculation
Circuit resistance	Total resistance of the evaluated circuit path
Voltage drop	Voltage lost across the circuit resistance
System voltage	Voltage used to calculate percentage voltage drop

The percentage result is:

\(\displaystyle \%\text{ Voltage Drop} = \frac{V_\text{drop}}{V_\text{system}} \times 100\)

A conductor with lower resistance, such as a larger AWG or kcmil conductor under the same reference basis, produces less voltage drop for the same current and circuit length. Reducing cable length also reduces resistance and voltage loss directly.

Calculation Example

Using the displayed example values:

Field	Entered value
Array current	18 A
One-way cable length	120 ft
Conductor resistance	0.6282 ohm/1000 ft
System voltage	600 V
Circuit length basis	DC two-wire loop

The DC two-wire loop doubles the one-way length:

\(\displaystyle 120\text{ ft} \times 2 = 240\text{ ft}\)

The resulting circuit resistance is:

\(\displaystyle R_\text{circuit} = \frac{0.6282}{1000} \times 240 = 0.1508 \Omega\)

The voltage drop is:

\(\displaystyle V_\text{drop} = 18\text{ A} \times 0.1508 \Omega = 2.7138\text{ V}\)

Using the entered 600 V system voltage:

\(\displaystyle \%\text{ Voltage Drop} = \frac{2.7138\text{ V}}{600\text{ V}} \times 100 = 0.4523\%\)

The voltage drop is 0.4523% of the entered system voltage.

The calculator therefore reports:

• Circuit length used: 240 ft
• Circuit resistance: 0.1508 ohm
• Voltage drop: 2.7138 V
• Voltage drop: 0.4523%
• Remaining to 3 percent reference: 2.5477%

The remaining value is the difference between the calculated percentage drop and a 3% reference:

\(\displaystyle 3.0000\% - 0.4523\% = 2.5477\%\)

It is a comparison value, not a conductor approval or a complete design determination.

PV Conductor Review

For PV work, voltage drop should be evaluated alongside the actual circuit arrangement. A source circuit, output circuit, feeder, or inverter connection may have different current values, conductor lengths, circuit topologies, and voltage references. The Array current entered for this calculation should match the current being reviewed for that specific conductor segment.

Use the result to compare practical alternatives such as:

• A shorter array-to-combiner or combiner-to-inverter route.
• A lower-resistance conductor selected from the applicable conductor reference.
• A different route that changes the actual conductor length.
• A higher system voltage where appropriate to the equipment and design.
• Separate evaluations for individual PV source circuits and aggregated output conductors.

Voltage-drop arithmetic does not establish conductor ampacity. A conductor that produces an acceptable voltage-drop percentage may still be unsuitable because of ampacity, terminal limitations, insulation temperature rating, environmental conditions, routing, overcurrent protection, or equipment requirements.

Field Verification

Verify the installed route rather than relying only on plan distance. Raceway offsets, vertical risers, equipment approach lengths, roof transitions, pull-box routing, and conductor slack can increase the actual one-way cable length.

Confirm the resistance value used for the conductor under evaluation. Do not substitute an AWG, kcmil, copper, aluminum, or temperature assumption unless the selected conductor reference supports that value. The calculator uses the entered Conductor resistance directly; it does not select a conductor size or correct resistance for field temperature.

This calculation performs voltage-drop arithmetic only. It does not determine conductor size, ampacity, PV current factors, overcurrent protection, terminal rating, insulation temperature rating, raceway fill, adjustment factor, correction factor, current-carrying conductor count, or NEC compliance. Final PV conductor selection must be reviewed against the applicable installation conditions, equipment instructions, and AHJ requirements.