Electrical Design, Single-Line Diagrams, and Component Selection

What Is a Single-Line Diagram?

A single-line diagram (SLD), sometimes called a one-line diagram, is a simplified electrical drawing that uses a single line to represent each current-carrying conductor or set of conductors between components. It is not a wiring diagram showing every individual wire; instead, it communicates the system's architecture, component ratings, and safety devices in a format that inspectors, engineers, and installers can read quickly. For residential grid-tied PV systems, the SLD is typically the centerpiece of a permit package submitted to the authority having jurisdiction (AHJ).

Why the SLD Matters for PV Work

AHJs use the SLD to verify that the system meets the National Electrical Code (NEC), specifically Article 690 for PV systems, before issuing a permit. Utilities review it before approving interconnection. Field crews use it to confirm wiring sequences and equipment locations. Because safety and code compliance are at stake, every element must be accurate. A missing disconnect or an incorrectly sized overcurrent protection device (OCPD) on a submitted drawing can delay a project and, if built that way, can create a shock or fire hazard.

Required Elements on a Residential Grid-Tied SLD

A complete SLD for a residential grid-tied system must show, at minimum:

• PV modules or array: Indicate the number of modules, series/parallel arrangement (strings), and module model.
• DC conductors: Label each conductor segment with wire gauge (e.g., #10 AWG) and insulation or conduit type (e.g., USE-2 in conduit, or PV Wire).
• DC disconnect: Required to isolate the DC side. Label voltage and current rating.
• Inverter: Show the make and model. For string inverters, indicate the number of MPPT inputs. For microinverters, indicate the AC output arrangement.
• AC conductors: Label gauge and conduit type (e.g., THWN-2 in EMT).
• AC disconnect (PV system AC disconnect): Required between the inverter and the point of interconnection. Label voltage and current rating.
• Overcurrent protection devices (OCPDs): Show each fuse or breaker with its ampere rating. This includes any string fuse in a combiner box, the inverter's output breaker, and the backfed breaker or line-side tap at the service panel.
• Service panel (load center): Show the main breaker rating and the interconnection point. NEC Article 690 contains provisions regarding the sum of breaker ratings relative to the bus bar rating (the "120% rule" is a commonly referenced guideline — always verify the specific NEC edition in use).
• Grounding and bonding path: Show the equipment grounding conductor (EGC) path from the array frames through the inverter to the grounding electrode system. Indicate the grounding electrode conductor (GEC) and grounding electrode (e.g., existing ground rod or water pipe).
• Meter and point of common coupling (PCC): Show the utility meter and the utility side connection point.

Conductor Labeling Conventions

Each conductor segment on the SLD should be labeled with:

1. Wire gauge — AWG or kcmil (e.g., #10 AWG, #6 AWG).

2. Insulation type — relevant to location and temperature rating (e.g., THWN-2, USE-2, PV Wire).

3. Conduit type — EMT (electrical metallic tubing), PVC Schedule 40 or 80, or MC cable where applicable.

4. Number of conductors — often noted as a small hash mark across the line or a numeric notation.

Correct labeling allows the AHJ to verify ampacity and confirm the conductor is appropriate for its environment (wet, sunlight-exposed, or rooftop conditions).

Sizing OCPDs: The Basic Rule

For DC strings, the OCPD must be sized to protect the conductor, not to match the module's short-circuit current (Isc) exactly. The general starting point for PV source circuits is that the OCPD rating must not exceed the conductor's ampacity after all applicable correction and adjustment factors are applied. PV source circuit conductors must be sized at no less than 125% of the module's rated Isc, and the OCPD protecting them must not exceed the conductor's corrected ampacity. Always verify sizing against the applicable NEC edition and AHJ requirements.

For the AC output breaker at the inverter, manufacturers specify a maximum OCPD rating in the inverter's listing; do not exceed it.

Grounding and Bonding: Safety-Critical Paths

Grounding and bonding are among the most safety-critical elements on the SLD. Equipment grounding connects non-current-carrying metal parts (module frames, racking, inverter enclosure) to the grounding system so that a fault current has a low-impedance return path — this trips the OCPD instead of energizing a metal surface that someone might touch. Bonding connects conductive parts to each other to eliminate voltage differences. Always show a continuous grounding path on the SLD from the array to the grounding electrode. A missing ground on a submitted drawing is an automatic AHJ rejection and, in the field, a serious electrocution hazard.

Lockout/Tagout and PPE Reminder

When field work follows an SLD review, remember: both DC and AC sources must be de-energized using proper lockout/tagout (LOTO) procedures before working on any conductors or equipment. PV arrays cannot be fully de-energized by simply opening a disconnect — they produce voltage whenever illuminated. Always wear appropriate PPE (insulated gloves rated for the voltage, safety glasses, arc-flash protection where required) and use a calibrated meter to verify zero energy before touching conductors.

Putting It Together: The Permit Package

The SLD travels with an equipment schedule — a table listing every major component by make, model, quantity, and key rating. Together, the SLD and equipment schedule give the AHJ everything needed to check code compliance on paper before any work begins. Accuracy in this documentation is not a bureaucratic detail; it is the foundation of a safe, inspectable, and interconnectable PV system.