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PV Conductor & OCPD Sizing (690.8 / 690.9) and Voltage Drop

PV Conductor & OCPD Sizing (690.8 / 690.9) and Voltage Drop

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Alright, PV Conductor & OCPD Sizing and Voltage Drop. Don't let the plain title fool you. Here's the part that actually matters on the job: PV circuits run hot, run continuously, and can be fed from more than one direction, so the NEC sizes them differently from a branch circuit in a house — two 125% factors, rooftop heat, conduit fill, and a separate check for voltage drop. Learn the sequence once and you can size any PV circuit with confidence.

Educational content only — not electrical design, engineering, or a substitute for a licensed electrician, electrical contractor, or professional engineer. Designs must follow the NEC (NFPA 70) edition adopted by your Authority Having Jurisdiction (AHJ) and any local amendments, the equipment manufacturers' installation instructions, and your utility's interconnection rules. Code section numbers below refer to the 2023 NEC unless noted; numbering and requirements change between editions.

Step 1: Maximum circuit current (690.8(A))

For a PV source circuit (a string), the maximum current is the sum of the parallel modules' Isc multiplied by 125%. The first 125% accounts for irradiance above STC — bright days, edge-of-cloud effects, and reflection can push current above the rated Isc.

For a single string, there is one module Isc in the parallel path, so: Max current = Isc x 1.25

For the inverter output circuit, the maximum current is the inverter's continuous output current rating from its datasheet. For DC-to-DC converter circuits (optimizers), use the converter's rated continuous output current.

Step 2: Conductor ampacity and OCPD (690.8(B))

PV currents are treated as continuous, so conductors and overcurrent devices are sized for 125% of the maximum current found in step 1. That is where the famous 1.56 comes from: 1.25 x 1.25. Two separate checks apply, and the conductor must satisfy whichever is larger:

  1. 125% of maximum current, with no adjustment or correction factors, using the terminal temperature rating (often 75°C) for the termination check.
  2. Maximum current after applying ambient temperature correction and conduit fill adjustment factors.

A worked example

Module Isc = 13.9 A, one string per circuit.

Now the conductor: 10 AWG copper PV wire or THWN-2, 90°C insulation. From Table 310.16, the 90°C ampacity is 40 A; at a 75°C termination it is 35 A — both above 21.7 A. Now apply conditions. Suppose the conduit runs through an outdoor area with a design ambient in the 46–50°C range (90°C column correction factor about 0.82) and the conduit holds 4 current-carrying conductors (two strings; adjustment 0.80): 40 x 0.82 x 0.80 = 26.2 A, which is greater than 17.4 A. 10 AWG works. Look up the factors in your adopted edition's tables — they are illustrative here.

Note rooftop conduit: the NEC adds a temperature adder for raceways and cables exposed to sunlight on or very near the roof surface. Keep conduit off the roof deck (follow the spacing in your edition) or account for the adder — it can force a larger wire.

Step 3: Overcurrent protection (690.9)

PV DC circuits need overcurrent protection where a fault could be fed by more current than the conductor or module can handle. A single string — or two strings in parallel — usually cannot backfeed enough to exceed the modules' series fuse rating, so 690.9 permits omitting fuses in those cases when the conductors are sized for the maximum current. Three or more parallel strings generally need fuses, typically in a combiner or in the inverter's input, rated at least 156% of Isc and no more than the module's maximum series fuse rating. Fuses and breakers must be listed for DC use at the system voltage.

On the AC side, the inverter output breaker is sized at 125% of the inverter's continuous output current, rounded up to the next standard size. A 7.6 kW inverter at 240 V with a 32 A continuous rating needs 32 x 1.25 = 40 A.

Step 4: Voltage drop

The NEC treats voltage drop mostly through informational notes, not enforceable limits, but designers care because lost volts are lost energy and can push a string below the MPPT window. A common rule of thumb is to keep DC drop to about 1–2% and total system drop to about 2–3%; project specifications sometimes set stricter targets.

For a two-wire circuit: VD = (2 x L x I x R) / 1,000 L = one-way length in feet, I = current (use Imp for operating drop), R = DC resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8.

Example: one string, Imp = 13.1 A, 10 AWG stranded copper (about 1.24 ohms per 1,000 ft), 150 ft one way. VD = 2 x 150 x 13.1 x 1.24 / 1,000 = 4.87 V String Vmp = 10 x 41.6 = 416 V → 4.87 / 416 = 1.2%. Acceptable. At 300 ft, it would be about 2.3%, and you'd consider 8 AWG.

Going Deeper (Intermediate)

Microinverter AC branch circuits behave differently: current accumulates along the trunk cable, so voltage drop is worst at the far end, and the inverter can trip on high voltage if the drop plus utility voltage push it past its limit. Manufacturers publish maximum micros per branch and voltage-drop calculators — use them, and keep the trunk run to the panel short.

Remember equipment grounding conductors: under 690.45 they're sized from Table 250.122 based on the OCPD rating (or the assumed rating where no OCPD is installed).

Advanced / Pro-Level

Pros run conductor sizing in a spreadsheet that lists every circuit segment with its environment — free air under the array, rooftop conduit, attic, exterior wall, interior — because each segment can have a different temperature correction and conduit fill. The governing segment sets the conductor size. They also verify terminal temperature ratings on every device, since a 90°C conductor landed on a 60°C or 75°C lug must be sized for the lug. On bifacial arrays they include the rear-side current gain in the Isc used for 690.8.

Practice Challenge

Three strings of modules with Isc = 11.5 A are paralleled in a combiner before running to the inverter. The module max series fuse is 20 A. What fuse size is required on each string, and what is the maximum current of the combined output circuit? (Answer: each string fuse must be at least 11.5 x 1.56 = 17.9 A, so a 20 A DC fuse, which does not exceed the module's 20 A series fuse rating. The combined output maximum current is 3 x 11.5 x 1.25 = 43.1 A, and the output conductors must be sized for 43.1 x 1.25 = 53.9 A before adjustment.)

In Practice

Put a conductor sizing table on the calculations sheet, one row per circuit, showing max current, 125% value, conductor, ampacity, correction and adjustment factors, and voltage drop. It doubles as the crew's wire pull list.

Common Mistakes to Avoid

Takeaway: Size PV circuits in order — max current at 125% of Isc, conductors and OCPD at another 125%, corrected for heat and fill, fused where backfeed is possible — then check voltage drop so the energy you designed actually reaches the inverter.

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