How to read a solar one-line diagram

The one-line diagram is the heart of a solar permit set — a single schematic line that traces power from the array to the grid, labeling every conductor, disconnect, and overcurrent device along the way. Learn to read it left to right and you can check a design the way a plan reviewer does.

Read it as a flow of power

A one-line follows the current from source to grid. The typical residential order is:

• PV array — modules in series strings (or AC modules for microinverters).
• DC or AC conductors — leaving the array, sized and labeled.
• Inverter — string, hybrid, or the combined AC output of microinverters.
• Rapid-shutdown device — micros, optimizers, or a listed PVRSS, with the initiator.
• AC disconnect — a lockable means of disconnect, often utility-required.
• Overcurrent protection (OCPD) — the solar breaker or fuses.
• Point of interconnection — back-fed breaker, load-side tap, or supply-side tap.
• Grounding — the grounding electrode, GEC, and equipment grounding conductors.

The array block

At the source you'll see the module make and model, count, and string configuration. Look for Voc, Vmp, Isc, and Imp and the string length — these feed the voltage and current math a reviewer checks against the inverter window. See our PV string sizing 690.7 reference.

Conductors and their labels

Each conductor segment is tagged with size, type, and count, for example "10 AWG PV wire" or "(2) 8 AWG THWN-2 in EMT." The reviewer confirms the conductor ampacity covers the corrected current for that segment — the ampacity comes from NEC 310.16 with temperature and conduit-fill adjustments. Our wire ampacity 310.16 reference covers the tables.

Overcurrent protection

Next to breakers and fuses you'll see the OCPD rating in amps. On the PV side, the OCPD relates to the module Isc through the 690.8 sizing factors; on the AC side it protects the inverter output conductors. The device rating must not exceed the conductor ampacity it protects.

Disconnects and rapid shutdown

Between the inverter and the grid you'll find one or more disconnecting means — a visible, lockable AC disconnect is commonly required by the utility so a lineman can isolate the system. On the array side, the one-line names the rapid-shutdown device (microinverters, optimizers, or a listed PVRSS) and shows where the initiator is and where the required label goes. A drawing that references rapid shutdown in a note but shows no device is a classic correction.

The point of interconnection

Where the PV ties in is often the most scrutinized part:

• Load-side back-fed breaker — must satisfy the 120% busbar rule of NEC 705.12; see the 120% busbar rule reference.
• Supply-side tap — connects ahead of the main, bypassing the busbar limit but needing its own disconnect and fusing.

A clean one-line answers the reviewer's questions before they ask. Confirm your AHJ's specific sheet requirements, and remember a licensed electrician or PE must review and stamp the set. To see what else belongs in the package, read what is in a solar permit set or try the DIY workflow.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.