Solar and DC systems

PV String and DC Wiring Calculator

Estimate cold-corrected PV string voltage, compare module ratings with one inverter MPPT input, and review DC homerun voltage drop.

Module and string inputs

PV module ratings

Module open-circuit voltage (Voc)

Module maximum-power voltage (Vmp)

Module short-circuit current (Isc)

Module maximum-power current (Imp)

Voc temperature coefficient

%/°C

Use the negative coefficient shown on the module datasheet.

Voc coefficient unit

Vmp temperature coefficient

%/°C

Use the negative coefficient shown on the module datasheet.

Vmp coefficient unit

Modules in series per string

modules

Parallel strings on this MPPT

strings

Lowest expected ambient temperature

°C

Coldest expected cell temperature

°C

Hottest expected cell temperature

°C

Inverter nameplate limits

Selected MPPT input

Maximum DC input voltage

MPPT minimum voltage

MPPT maximum voltage

Maximum input operating current

Maximum input short-circuit current

DC homerun estimate

Conductor voltage drop

Copper conductor size

Resistance reference only; this does not determine an approved PV conductor.

Estimated conductor temperature

°C

Combined homerun, one-way length

The calculator includes the positive and negative conductors and uses the combined current for this MPPT.

Voltage-drop design target

This is your design target, not a code limit.

PV string and wiring estimates

1.40% voltage drop

Cold-corrected string Voc439.2 V

STC array power5,760 W

MPPT operating current18.00 A

Array short-circuit current20.00 A

Cold Voc vs. inverter maximumWithin entered limit

Hot/cold Vmp vs. MPPT windowWithin entered limit

Operating current vs. input limitWithin entered limit

Nameplate Isc vs. short-circuit limitWithin entered limit

Estimated Vmp range: 264.0–359.2 V. The check compares your entered module and inverter values only.

Smallest listed copper size meeting the entered voltage-drop target: 12 AWG

Copper conductor	DC voltage drop	Percent drop	Target
14 AWG	11.29 V	3.53%	Above target
12 AWG	7.13 V	2.23%	Meets target
10 AWG	4.46 V	1.40%	Meets target
8 AWG	2.80 V	0.88%	Meets target
6 AWG	1.76 V	0.55%	Meets target
4 AWG	1.11 V	0.35%	Meets target
3 AWG	0.88 V	0.28%	Meets target
2 AWG	0.70 V	0.22%	Meets target
1 AWG	0.55 V	0.17%	Meets target
1/0 AWG	0.44 V	0.14%	Meets target
2/0 AWG	0.35 V	0.11%	Meets target
3/0 AWG	0.27 V	0.09%	Meets target
4/0 AWG	0.22 V	0.07%	Meets target
250 kcmil	0.18 V	0.06%	Meets target
300 kcmil	0.15 V	0.05%	Meets target
350 kcmil	0.13 V	0.04%	Meets target
400 kcmil	0.12 V	0.04%	Meets target
500 kcmil	0.09 V	0.03%	Meets target
600 kcmil	0.08 V	0.02%	Meets target
750 kcmil	0.06 V	0.02%	Meets target
1000 kcmil	0.05 V	0.01%	Meets target

Design screening onlyThis tool does not select or approve PV modules, strings, conductors, overcurrent devices, or inverters. It does not apply NEC 690 current factors, conductor ampacity correction, rapid-shutdown rules, equipment listings, or all manufacturer limits. Verify the design against the adopted code, module/inverter documentation, and the authority having jurisdiction.

About this calculator

Enter module datasheet values, the number of modules in series, and the number of parallel strings connected to one MPPT input. The calculator estimates cold-corrected string Voc, hot and cold Vmp, nameplate array current, and DC homerun voltage drop.

The inverter comparisons use the limits you enter from its documentation. A result within those limits is not an approval of the complete PV system.

Method and assumptions

Module voltages are treated as STC ratings at 25°C. The temperature-adjusted voltage is estimated with the module’s linear temperature coefficient. Cold-corrected Voc uses the lowest expected ambient temperature; Vmp uses the entered coldest and hottest cell temperatures.

Array Imp and Isc equal the module nameplate current multiplied by parallel strings on the selected MPPT. The DC voltage-drop estimate uses the combined Imp on a two-conductor homerun, Southwire DC resistance values at 25°C, and a linear copper resistance correction for conductor temperature.

The conductor comparison is voltage-drop-only. It does not check NEC 690 current factors, conductor ampacity, OCPD, equipment listing, wire type, disconnects, rapid shutdown, grounding, or all inverter limits. Replace every example input with the project module, inverter, climate, and installation data.

FAQs

Why does the calculator use the lowest ambient temperature for Voc?

A PV module’s open-circuit voltage rises as cell temperature falls. The cold-voltage estimate applies the entered module Voc temperature coefficient to the lowest expected ambient temperature. Verify the temperature method required by the adopted NEC edition and the equipment manufacturer.

Where do I find the module temperature coefficients?

Use the exact module datasheet. Enter a negative Voc and Vmp coefficient, in either percent per degree Celsius or volts per degree Celsius, matching the unit shown by the manufacturer.

What do the inverter current checks mean?

The tool compares nameplate Imp and Isc multiplied by the parallel strings assigned to the selected MPPT input with the inverter limits you enter. It does not apply NEC current factors or size source-circuit conductors or overcurrent devices.

Does the recommended conductor size mean the wire is code-compliant?

No. The comparison only finds a listed copper size that meets the entered voltage-drop target using the selected current, route length, and conductor temperature. Confirm ampacity, temperature correction, wiring method, PV conductor listing, overcurrent protection, terminals, and equipment instructions separately.

Sources

NFPA 70 (NEC), 2026 edition, Article 690 — verify the adopted edition and all project-specific PV requirements.
U.S. Department of Energy, Solar Photovoltaic Performance and Efficiency Basics — module voltage and output vary with operating temperature.
Southwire SPEC 45201 and SPEC 10000 — copper-conductor DC resistance at 25°C.
Southwire Power Cable Manual, 4th edition — copper resistance-temperature correction relationship.