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16. Inverter, Conductor & OCPD Sizing

Learning objectives

  • Choose inverter capacity using the DC/AC ratio.
  • Size DC conductors by the NEC 690.8 method, then derate for environment.
  • Size overcurrent protection and respect the AC interconnection (busbar) rule.

16.1 Inverter sizing and the DC/AC ratio

DC/AC ratio (inverter loading ratio): the array's DC nameplate capacity divided by the inverter's AC nameplate capacity. A ratio above 1.0 means the array is intentionally larger than the inverter, so the inverter runs near full output more hours per day.
Clipping: what happens when the array's DC output exceeds the inverter's capacity limit and the inverter holds its output at the maximum, discarding the excess power. Mild clipping of rare peak-production moments is an economic optimization, not a defect.
MPPT (Maximum Power Point Tracking): the inverter function that continuously adjusts its input voltage to extract the highest available power from the array. Inverters may have one or multiple independent MPPT inputs, each with its own current limit.

Arrays are deliberately oversized relative to the inverter (Chapter 6.3). The DC/AC ratio typically runs 1.1–1.3: an 8.4 kW DC array commonly pairs with a ~7.0–7.6 kW AC inverter. Match the array’s string configuration (Chapter 15) to the inverter’s MPPT count and per-MPPT current limits.

16.2 DC conductor sizing: the NEC 690.8 sequence

OCPD (Overcurrent Protective Device): a fuse or circuit breaker that interrupts current when it exceeds a safe level. In PV systems, OCPDs protect DC source circuits and AC output circuits from overcurrent conditions.

This is the calculation that causes an estimated 30–40% of permit rejections, so follow every step:

Step 1: Maximum circuit current (690.8(A)). Multiply the sum of parallel module short-circuit currents by 125% (the “irradiance enhancement” factor, covering conditions where field Isc exceeds STC):

I(max) = Σ Isc × 1.25

Step 2: Continuous-duty ampacity (690.8(B)). PV circuits are continuous, so the conductor (and OCPD) must carry another 125%:

Required ampacity = I(max) × 1.25 = Isc × 1.56

This is the well-known 156% rule (1.25 × 1.25 = 1.5625).

Step 3: Apply correction factors. The chosen wire’s table ampacity must, after temperature correction (rooftop conduit can hit 60–70 °C) and conduit-fill adjustment, still meet or exceed the Step-2 value. Hot rooftops gut conductor ampacity. A 10 AWG conductor rated for ~30 A at 30 °C can fall well below that in a hot raceway, so this step often forces a larger wire than Step 2 alone suggests.

Step 4: Voltage drop. Independently, keep DC voltage drop low (a common design target is ≤2%) for efficiency over the run length.

Example 16.A: String Isc = 13.8 A (single series string, so Σ = 13.8 A). Step 1: 13.8 × 1.25 = 17.25 A. Step 2: 17.25 × 1.25 = 21.6 A minimum ampacity. Select a conductor whose ampacity after temperature/conduit derating still exceeds 21.6 A. Often 10 AWG copper will work, but verify against the corrected table value for your conditions.

16.3 Overcurrent protection (690.9)

Source circuit: the wiring that connects one or more PV modules in series (a string) back to the combiner or inverter. Each source circuit is a discrete DC branch whose fault current must be managed.

Series fuses/OCPD protect source circuits and must not exceed the module’s maximum series fuse rating (datasheet, Chapter 5.4). They are sized to the 156% current and rounded to the next standard rating. Many small residential string systems (one or two strings per MPPT) may not require series fusing, but this is code- and configuration-specific.

16.4 AC side and the interconnection (705) busbar rule

Busbar: the internal copper or aluminum bus inside a panelboard that all breakers connect to. The busbar has a current rating that limits how much total current can flow through it, regardless of how many individual breakers are installed.
AHJ (Authority Having Jurisdiction): the organization, office, or individual responsible for enforcing the applicable codes in a given location, typically a local building or electrical inspector. AHJ interpretations of code provisions can vary.

AC output conductors are governed by Article 705, sized to the inverter’s maximum continuous AC output current × 125%. The classic interconnection limit is the 120% busbar rule: where PV back-feeds a panelboard, main breaker + PV breaker ≤ 120% of the busbar rating, with the PV breaker landed at the opposite end from the main.

Example 16.B: A 200 A busbar with a 200 A main allows up to 200 × 1.20 − 200 = 40 A of PV back-feed breaker. A larger inverter would require a different interconnection method (e.g., line-side/supply-side connection or a busbar derate).

⚠️ Every number here depends on the adopted NEC edition and AHJ interpretation. The 2023 and 2026 editions differ in details (e.g., bifacial current treatment), so confirm before stamping a plan set.

16.5 The 120% busbar rule, drawn

        ┌──────────── 200 A busbar ────────────┐
        │                                       │
   [200 A MAIN]                          [40 A PV breaker]
   (utility/grid end)                    (opposite end, required)

   Allowed PV breaker = (1.20 × 200) − 200 = 240 − 200 = 40 A

The main and PV breakers feed the busbar from opposite ends so their combined current can’t overload the middle of the bar. Main + PV ≤ 1.20 × busbar rating, and the PV breaker sits at the far end from the main.

16.6 Worked example: conductor + OCPD + interconnection

A residential string: module Isc = 13.85 A (single series string), inverter rated 7.6 kW AC, 32 A max output, interconnecting to a 200 A busbar with a 200 A main.

DC conductor (690.8): 13.85 × 1.25 × 1.25 = 13.85 × 1.5625 = 21.6 A minimum ampacity. Select a conductor whose derated ampacity (after rooftop temperature + conduit fill, Ch 16.2) still exceeds 21.6 A. Frequently 10 AWG copper will work, but confirm against the corrected table value.

Series OCPD (690.9): size to ~21.6 A, rounded to the next standard rating (e.g., 25 A), and confirm it does not exceed the module’s max series fuse rating (e.g., 25 A, which is OK here).

AC interconnection (705): allowed PV back-feed breaker = (1.20 × 200) − 200 = 40 A. The inverter’s 32 A output needs ≥ 32 × 1.25 = 40 A breaker, which is exactly at the 40 A limit, so it just fits on the load side. A larger inverter would force a busbar derate or a line-side (supply-side) connection.

Chapter 16 summary

Pick the inverter for a ~1.1–1.3 DC/AC ratio. Size DC conductors at 156% of Isc (125% irradiance × 125% continuous), then verify the wire survives temperature and conduit-fill derating and meets voltage-drop targets. Size OCPD to the module’s max series-fuse rating. On the AC side, apply Article 705 and the 120% busbar rule. Verify every figure against your code edition.

  • DC/AC ratio: the array’s DC nameplate capacity divided by the inverter’s AC rating; typically 1.1–1.3 for grid-tied systems.
  • Clipping: inverter output held at its capacity ceiling while excess DC power is discarded; acceptable in small amounts.
  • MPPT (Maximum Power Point Tracking): inverter function that finds and holds the array’s highest-power operating point.
  • OCPD (Overcurrent Protective Device): fuse or breaker that interrupts excess current to protect conductors and equipment.
  • 156% rule: the NEC 690.8 combined factor (1.25 × 1.25) applied to Isc to establish minimum conductor ampacity.
  • Source circuit: series string of modules forming one DC branch back to the combiner or inverter.
  • 120% busbar rule: NEC limit requiring main breaker + PV breaker ≤ 120% of the panelboard busbar rating.
  • Busbar: internal current-carrying bar in a panelboard to which all breakers connect.
  • AHJ (Authority Having Jurisdiction): the local official or agency that enforces and interprets applicable codes.

Full definitions: Appendix A (glossary).

Practice Problems: Chapter 16

  1. A string has Isc = 11.2 A. What is the minimum conductor ampacity per NEC 690.8 (before environmental derating)?
  2. Three strings (each Isc = 10 A) are combined in parallel before the OCPD. What is the 690.8 minimum ampacity for the combined conductor?
  3. A 150 A busbar has a 150 A main. What is the largest PV back-feed breaker allowed under the 120% rule?
  4. An inverter outputs 48 A continuous AC. What AC breaker size does it require (125% continuous), and will it fit on the 150 A busbar from Q3?
  5. An 8.0 kW DC array pairs with a 6.6 kW AC inverter. What is the DC/AC ratio, and is it in the normal range?
  6. Why does rooftop conduit temperature often force a larger conductor than the bare 156% calculation suggests?

Solutions: Chapter 16

  1. 11.2 × 1.5625 = 17.5 A minimum.
  2. Combined Isc = 30 A; 30 × 1.5625 = 46.9 A minimum.
  3. (1.20 × 150) − 150 = 180 − 150 = 30 A.
  4. 48 × 1.25 = 60 A breaker. That exceeds the 30 A allowed on that busbar, so it will not fit load-side; you’d need a line-side connection or service upgrade.
  5. 8.0 ÷ 6.6 = 1.21, which is within the typical ~1.1–1.3 range.
  6. Hot conduit (often 60–70 °C) sharply derates a conductor’s table ampacity, so a wire that meets 156% at 30 °C may fall short once corrected, forcing the next size up.