Single-Line Diagram Guide for Electricians

What is a single-line diagram?

A single-line diagram (SLD) — also called a one-line diagram — is a simplified schematic of an electrical power system. Instead of showing every individual conductor as a separate line (hot A, hot B, neutral, ground), it uses a single line to represent the power path and standardised symbols to represent equipment.

For residential and light commercial work, an SLD typically shows:

• The utility service point — meter socket or CT cabinet
• Service entrance conductors — size, insulation type, conduit
• Main disconnect or main breaker with ampacity rating
• Panel board with ampacity and number of spaces
• Main bonding jumper and grounding electrode system
• Branch circuits with breaker sizes (sometimes)
• Subpanels and their feeders, if present

When you need one

Whether your AHJ requires an SLD depends on the jurisdiction and the scope of work. These are the most common triggers:

• Service upgrade — upgrading from 100A to 200A, or 200A to 400A. Almost every AHJ requires an SLD showing the new service size, conductor sizes, and grounding electrode system.
• EV charger permit — Level 2 EVSE (240V circuit) typically requires a permit. Many AHJs want an SLD showing where the new circuit originates in the panel and the EVSE location.
• Solar PV and battery storage — NEC 705 and 706 installs almost always require a single-line showing interconnection, backfeed breaker, and any production meter.
• Subpanel addition — adding a detached garage sub, workshop, or accessory dwelling unit panel. Show the feeder size and both panel ampacities.
• Generator and transfer switch — ATS or manual transfer switch requires an SLD showing the utility feed, generator feed, transfer switch, and load panel.
• Panel replacement — replacing a Federal Pacific or Zinsco panel often requires documentation of the new panel and service size.

What to include

A residential permit SLD typically needs these elements — confirm with your AHJ which are required:

Required on most permits

• Utility service type — overhead or underground, 120/240V single-phase 3-wire
• Meter socket — with utility label or meter number if available
• Service entrance conductors — AWG size, insulation type (THHN, XHHW), conduit type and size
• Main disconnect — ampacity rating (100A, 200A, 400A), 2-pole designation
• Main panel — ampacity, number of spaces, voltage (120/240V, 1φ, 3W)
• Main bonding jumper (MBJ) — notation confirming neutral bonded to panel enclosure at service
• Grounding electrode system (GES) — conductor size, electrode type (ground rod, Ufer, water pipe)

Required for EV charger permits

• Circuit breaker size and pole count (typically 2P 50A or 2P 60A for 48A EVSE)
• Conductor size (6 AWG THHN for 50A circuit)
• Conduit type and size to EVSE location
• EVSE ampacity (continuous amps, not breaker size)
• Load calculation showing panel has capacity — reference your load calculation

Required for solar/battery

• Inverter output breaker size and location in panel
• Backfeed breaker position and "Solar" label
• Production meter or bi-directional meter notation
• Battery storage system breaker and capacity if present
• NEC 705.12 point of connection labeling

Drawing an SLD in DesignedWire

DesignedWire's editor is designed for circuit diagrams — the same symbols and wiring approach work for single-line diagrams. Here's the fastest workflow:

1. 1

   Open the Panel Schedule template

   In the editor, open the template picker (T) and select Panel Schedule. This gives you a pre-built panel with labeled circuits — delete what you don't need and add the service entrance above it.

2. 2

   Add service entrance elements

   Use the rectangle and line tools to place the meter socket and main breaker. Label each with the text tool — conductor size, breaker rating, conduit type. The convention is to read power flow left-to-right: utility → meter → main breaker → panel.

3. 3

   Add the grounding electrode system

   Draw a vertical line down from the panel with the ground symbol at the bottom. Label the GES conductor size (#4 AWG or #6 AWG copper for most residential services) and electrode type.

4. 4

   Add branch circuits or new circuit

   For an EV charger permit, add the new breaker with size and label, draw the conductor run to the EVSE symbol, and note the conductor size and conduit on the wire label.

5. 5

   Add a title block and export PDF

   Use the text tool to add a simple title block: project address, permit number, "Not to scale", and your license number if required. Export → PDF. Most AHJs accept letter-size PDF submissions.

EV charger permit SLD — worked example

The most common single-line diagram an electrician draws today is for a Level 2 EV charger permit. Here's exactly what it needs:

FAQ

• Does a single-line diagram need to be to scale?

  No. Almost all residential and light commercial SLDs are explicitly marked "Not to scale." The purpose is to show component relationships and ratings, not physical dimensions. Add "Not to scale — for permit documentation only" in your title block.

• Does my SLD need an engineer's stamp?

  For residential work in most jurisdictions, no. Some jurisdictions require engineer or architect-stamped drawings for commercial service upgrades above a certain ampacity (commonly 800A or 1,200A). Check your AHJ's requirements — most residential permits are submitted without a stamp.

• What is the difference between a single-line diagram and a wiring diagram?

  A wiring diagram shows every individual conductor — hot A, hot B, neutral, ground — as separate lines with color coding and terminal connections. A single-line diagram abstracts this to one line representing the power path, making service-level relationships readable at a glance. Wiring diagrams are used for circuit-level detail; SLDs are used for service-level permit documentation.

• My panel is already 200A. Do I still need an SLD for an EV charger?

  Many AHJs require a load calculation showing the panel has spare capacity, and an SLD showing the new circuit — even if the service size isn't changing. Some jurisdictions have a simplified EV charger permit process that only needs a one-page application. Always verify with your local building department before assuming.

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