Solar PV Wire Size & Voltage Drop Calculator
Enter the PV module current, parallel strings, run length, conductor type, temperature, and voltage-drop target. The calculator compares ampacity and voltage drop, then shows which one controls the selected conductor size.
Recommended wire size6 AWG
- Maximum PV circuit current
- 33.75
- Required conductor ampacity
- 42.19
- Ampacity-only wire size
- 8 AWG
- Voltage-drop-only wire size
- 6 AWG
- Voltage drop at recommended size
- 1.25
- Percentage voltage drop
- 1.56%
- Full-power wire loss
- 31.72
- Maximum one-way run at this size
- 64.07
- Controlling constraint
- Voltage drop
Current values are in amperes, voltage drop is in volts, wire loss is in watts, and length is one-way feet.
Uses NEC-style 125% PV maximum current, another 125% continuous-load sizing factor, ampacity corrections, terminal ampacity, and DC round-trip voltage drop.
How to use this calculator
- Enter module Isc and Imp from the module nameplate or datasheet.
- Enter the number of parallel strings on the circuit and the string operating voltage.
- Enter the one-way conductor length and select copper or aluminum.
- Choose the insulation rating, terminal rating, ambient temperature, and number of current-carrying conductors together.
- Set the voltage-drop target and compare the ampacity-only, voltage-drop-only, and recommended sizes.
How the solar conductor size is calculated
This calculator separates the code-style ampacity check from the design voltage-drop check. That matters because a conductor can be large enough for current and still be too small for a long, low-voltage solar run.
For the current calculation, module short-circuit current is multiplied by the number of parallel strings. Series modules raise voltage, not current. The maximum PV circuit current is:
Imax = 1.25 x Isc x parallel strings
The conductor ampacity target is then:
Ireq = 1.25 x Imax
Together, those factors equal 1.5625 x Isc x parallel strings when no listed-system exception is being applied.
For each supported AWG or kcmil size, the calculator starts with the selected 75 degree C or 90 degree C ampacity column. It multiplies that insulation ampacity by the ambient-temperature correction factor and by the current-carrying-conductor adjustment factor. The same conductor must also pass the selected 60 degree C or 75 degree C terminal column, and small-conductor limits are applied where this table includes them.
Voltage drop uses operating current rather than short-circuit current:
VD = 2 x K x Imp x parallel strings x one-way length / circular mils
K is 12.9 for copper or 21.2 for aluminum in ohm-cmil per foot. The factor of 2 accounts for the outgoing and return conductors in a two-wire DC circuit. Percentage drop is 100 x VD / string operating voltage, and full-power wire loss is operating current x VD.
What moves the recommended size
Parallel strings, low string voltage, and long conductor runs push the answer up quickly. High ambient temperature and grouped current-carrying conductors reduce usable ampacity. A lower terminal rating can also force a larger conductor even when the insulation itself has a higher temperature rating.
What this calculator leaves out
The result is a design estimate, not an installation approval. It does not check conduit fill, rooftop temperature adders unless you enter them in the ambient field, connector ratings, equipment wire-range listings, grounding conductors, fault current, overcurrent-device sizing, voltage ratings, or local amendments. Conductor resistance also changes with temperature, so the actual voltage drop can differ from this 75 degree C resistance estimate.
Worked example
Suppose each module has 13.5 A Isc and 12.7 A Imp, with two parallel strings at 80 V and a 50 ft one-way copper run. The maximum PV circuit current is 1.25 x 13.5 x 2 = 33.75 A. The required conductor ampacity is 1.25 x 33.75 = 42.19 A.
With 90 degree C insulation, 75 degree C terminals, 40 degree C ambient temperature, and two current-carrying conductors, #8 copper passes the ampacity check: 55 x 0.91 = 50.05 A, and the 75 degree C terminal value is 50 A. But voltage drop at #8 is about 1.98 V, or 2.48% of an 80 V string, which misses a 2% target.
#6 copper drops about 1.25 V, or 1.56%, and loses about 31.7 W at full power. The recommended size is therefore #6 AWG, controlled by voltage drop rather than ampacity.
Common questions
Why is PV wire sometimes sized at 156.25% of module Isc?
NEC PV source and output circuit current is commonly calculated as 125% of module Isc times the number of parallel strings. Conductors for continuous current are then sized at another 125% unless a specific listed-system exception applies. Multiplying those together gives 1.5625 times Isc times parallel strings.
Should voltage drop use Isc or Imp?
Voltage drop is normally evaluated at operating current, so this calculator uses module Imp times the number of parallel strings. Isc is used for maximum-current and ampacity sizing because it represents the short-circuit current basis used by PV code rules. Using Isc for voltage drop is conservative, but it is not the normal full-power operating point.
Does connecting panels in series change the required wire size?
Series-connected modules increase string voltage, but they do not increase current through the string conductors. Higher string voltage can reduce percentage voltage drop for the same wire and length. Parallel strings increase current, so they increase both ampacity requirements and voltage drop.
Why can 90 degree C wire still be limited by a 75 degree C terminal?
The higher insulation rating can be used for adjustment and correction, but the final conductor ampacity must also be acceptable for the equipment terminals. If the terminal is rated 75 degree C, the conductor cannot be used above the applicable 75 degree C ampacity at that termination. That is why terminal rating can control even with 90 degree C PV Wire.
When do bundled conductors require ampacity derating?
When more than three current-carrying conductors share a raceway or cable, the available ampacity is usually adjusted downward. This calculator applies the common NEC adjustment table through 30 current-carrying conductors. Larger groupings need a project-specific review.
Can aluminum cable be used for a solar DC circuit?
Aluminum can be used only when the equipment, terminals, connectors, and installation method are rated for it. It has higher resistance than copper, so voltage drop often requires a larger size. This calculator does not recommend small aluminum sizes below #8 AWG.