How to draw a solar one-line diagram

The one-line is the sheet a plan checker opens first, and the one that decides whether the rest of the set gets a careful read or a suspicious one. It is not an artistic drawing — it is an argument that the system complies, made in a form a reviewer can check in about ninety seconds.

Draw it in the order power flows

Working left to right in the direction of energy keeps the sheet readable and stops you forgetting a device.

• Source. The array, as strings. Show module count per string, strings per MPPT, and the module's Voc, Isc, Vmp and temperature coefficient — the reviewer needs them to check 690.7 cold-voltage.
• DC collection. Combiner or junction box if you have one, with its fuse and bus rating.
• Rapid shutdown. The listed device and its initiator, if the array is on a building — see NEC 690.12.
• Inverter. Make, model, maximum DC input voltage, continuous AC output, and nominal AC voltage.
• AC disconnects. Every one, with rating and whether it is fused and service-entrance rated.
• Point of interconnection. Back-fed breaker on the busbar, or a supply-side tap. This is where 705.12 or 705.11 is proven.
• Grounding. The electrode system, the GEC, and the equipment grounding conductors.

Every run carries four facts

A conductor with no annotation is an unanswered question. On each run, state:

Do not put the wire size in a legend and nowhere else. A reviewer checking ampacity should not have to hunt.

Show the arithmetic, not just the answer

The single fastest way to prevent a correction is to print the calculation next to the result: `Isc 13.94 A x 1.25 = 17.43 A -> 20 A OCPD`, and the AC side as `47.5 A x 1.25 = 59.4 A -> 60 A`. A reviewer who can see the multiplication does not have to redo it, and a number without its derivation invites the question "where did this come from".

The omissions that actually cause corrections

• No grounding electrode system. Extremely common, and an automatic comment.
• Disconnects implied rather than drawn. If it is not on the sheet it does not exist.
• Busbar arithmetic missing. State bus rating, main breaker, and back-fed breaker, and show the 120% sum.
• Voltage drop unstated on long runs. Not a hard NEC requirement, but reviewers ask.
• Equipment that is not listed, or listed for the wrong thing. An off-grid inverter drawn next to a net meter is an interconnection the utility will refuse regardless of your wiring.

One-line, three-line, or both

A one-line collapses each circuit to a single stroke with a tick and a count. A three-line shows every conductor individually. Residential PV is usually accepted as a one-line; some jurisdictions and most commercial work want the three-line. See one-line vs three-line for which to submit.

Draw it once, then let the engine keep it honest

The hard part of a one-line is not the drawing, it is keeping the numbers consistent as the design changes. Swap a module and the string voltage, conductor size, OCPD and label set all move with it. OneLine Studio builds the one-line from a typed model and re-runs the NEC arithmetic on every edit, so the sheet cannot drift from the design — and prints the code section next to every number.

> Design aid — a licensed electrician or PE must review and stamp before filing.

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.