How to draw a single line diagram

To draw a single line diagram, start with the utility source at the top of the sheet, then draw the meter, the service disconnect and the main panel in the order power flows through them. Add each feeder and large load as a branch from the panel bus, label every device with its rating and every line with its conductors, then add the grounding, the notes and a title block.

The steps below work on paper or in any drawing program. Each one also says how it is done in Oneline, the free single line diagram maker on this site. If you have not read one before, start with what a single line diagram is and the symbol chart.

The eight steps

1. Collect the ratings

   Write down the service voltage, phase and amp rating, the main breaker size, the bus rating of each panel, the nameplate ratings of the new equipment, and the conductor sizes and raceways you plan to use.

   In Oneline: If you are describing the job to the drafting assistant, these are the details to put in the description.

2. Draw the source and the meter

   Put the utility at the top of the sheet and label it with the service voltage, such as 120/240V 1φ 3W. Draw the meter below it with its socket rating.

   In Oneline: Every template already starts with the utility and the meter. Click either one to change its label.

3. Draw the service disconnect and the main panel

   Below the meter, draw the main breaker with its amp rating and poles, then the panel bus with its rating. Put a dashed enclosure around the breaker and bus if they share one cabinet.

   In Oneline: Click the main breaker or the panel and type the new rating. The enclosure moves with its contents.

4. Add a breaker for each feeder and large load

   Tap a breaker from the bus for each new circuit, each feeder and each large load. Group the remaining existing circuits as one “existing loads” branch so the drawing stays readable.

   In Oneline: Select the bus and add a circuit. The bus grows to fit.

5. Draw what each breaker feeds

   At the end of each branch draw the equipment: an EV charger, an inverter, a subpanel, a transfer switch, a motor. Add the disconnects that sit between the breaker and the equipment.

   In Oneline: Pick the symbol from the library, or insert a device on an existing wire.

6. Label every device and every line

   Give each breaker its rating, such as 60A 2P, and each piece of equipment its size. Beside each line, write the conductors and the raceway, such as 3 #6 Cu, #10 Cu ground, 3/4 in EMT.

   In Oneline: Click a symbol to edit its label and rating. Click a wire to add its conductor callout.

7. Add the grounding and the notes

   Show the grounding electrode system and where the neutral is bonded. In the notes, write the calculations the reviewer will look for, such as 48A × 125% = 60A for an EV charger or the busbar check for solar.

   In Oneline: The notes column sits beside the drawing. Templates come with notes you can edit.

8. Fill in the title block, check and export

   Add the project address, the client, your company and licence number, the date, the sheet number and the revision. Check the drawing against the equipment, then export it as a PDF.

   In Oneline: Contractor details are entered once and printed on every sheet. The check before export lists anything missing, and the PDF comes out at 11 × 17 or letter size.

The shortcut: start from a drawing that is close

Most jobs are a variation on one you have drawn before. Oneline has 40 finished drawings to start from: EV chargers, solar and storage, generators, service and panels, multi-unit and commercial and industrial. Open the closest one and change only what is different. Or type the job the way you would say it to another electrician, and a first draft is drawn for you to check.

Common mistakes

• Leaving a rating off a breaker, a panel bus or a piece of equipment.
• Lines with no conductor callout, so the reviewer cannot check the wire size.
• Not showing where the neutral is bonded, or showing it bonded in a subpanel.
• Sizing an EV charger or other continuous load at 100% instead of 125%.
• Leaving out the busbar check on a load-side solar connection.
• Not marking what is existing and what is new.
• An empty title block: no address, no contractor, no date, no revision.