Short answer: A one-line diagram shows how power flows from the source to each panel, with breaker sizes and feeder callouts. A panel schedule lists each circuit in one panel and its load by phase. The specification sets required products and methods. To trace a feeder, follow the one-line, confirm the panel schedule, then check the spec.
Reading the drawings is reading the exam questions. The PSI outline lists Plan Reading in the Texas Journeyman NEC Knowledge part, and feeder and service questions on both parts start from the kind of numbers a one-line gives you. The Texas Journeyman Practice Pack has step-by-step solutions for 299 calculation questions, including feeders and services. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. Drawing conventions vary by design firm. Follow the legend and notes on your own drawing set, and ask before you assume.
Three documents, three questions
| Document | Answers | Typical sheet |
|---|---|---|
| One-line (single-line) diagram | Where does this panel get its power, and through what? | Electrical one-line or riser sheet |
| Panel schedule | What is on each circuit in this panel, and how much load is on each phase? | Schedules sheet or on the plan |
| Specification | Which products, materials and methods are required? | Project manual, Division 26 for electrical |
The US Bureau of Labor Statistics lists reading blueprints or technical diagrams among electricians’ typical duties (BLS Occupational Outlook Handbook). For symbols and scales on plan sheets, see blueprint symbols and plan reading.
Reading the one-line
A one-line diagram draws a three-phase system as a single line to keep it readable. Read it from the top (the utility or source) down. A typical feeder callout next to a line looks like this:
225A/3P → 4 #4/0 CU + #4 CU G, 2-1/2″ C → LP-1
| Part | Meaning |
|---|---|
| 225A/3P | A 225-amp, three-pole breaker protects the feeder |
| 4 #4/0 CU | Four 4/0 AWG copper conductors: three phases and a neutral |
| #4 CU G | A 4 AWG copper equipment grounding conductor |
| 2-1/2″ C | In 2-1/2-inch conduit |
| LP-1 | The panel it feeds |
Other things on a one-line: transformer ratings and voltages (for example 75 kVA, 480 V to 208Y/120 V), the available fault current at key points, and which equipment has surge protection.
Reading the panel schedule
A panel schedule lists, for one panel: the panel name, voltage, phases and wires, bus rating, main breaker or main lugs, short-circuit rating, then each circuit with its number, description, breaker size and poles, and its load in volt-amperes under the phase it connects to. The bottom totals the load by phase. The VA’s master specification for panelboards, for example, calls for a printed schedule of circuits, with final load descriptions and room names, on the directory card in each panelboard (VA specification section 26 24 16, Panelboards).
Worked example 1: phase totals and balance
Panel LP-1 is 208Y/120 V, three-phase. The schedule totals are:
| Phase | Total load |
|---|---|
| A | 12,480 VA |
| B | 10,800 VA |
| C | 11,520 VA |
- Current on each phase at 120 V to neutral: A 12,480 ÷ 120 = 104 A, B 10,800 ÷ 120 = 90 A, C 11,520 ÷ 120 = 96 A.
- Total load: 12,480 + 10,800 + 11,520 = 34,800 VA.
- Balanced-load current for that total: 34,800 ÷ (208 × 1.732) = 34,800 ÷ 360.3 = 96.6 A.
- Average phase current: (104 + 90 + 96) ÷ 3 = 96.7 A. The heaviest phase, A, is 104 − 96.7 = 7.3 A above average, about 7.6%.
- Simple arithmetic margin on phase A against a 225 A rating: 225 − 104 = 121 A. This is not a determination of available feeder or panel capacity; actual capacity must be evaluated with the applicable load calculation, the equipment ratings and the installation conditions.
Check: in this simplified example, assume all listed phase totals are 120 V line-to-neutral loads. The average current is (104 + 90 + 96) ÷ 3 = 96.7 A. The balanced three-phase formula gives a similar value here (96.6 A) because the line-to-line voltage is approximately √3 times the line-to-neutral voltage. For real panel balancing, review the individual circuit loads, including multipole loads, and confirm the available spaces and circuit arrangement before assigning new loads.
Worked example 2: check the feeder against the schedule
The one-line says LP-1 is fed by 4 #4/0 CU on a 225 A breaker.
- From the ampacity table, 4/0 AWG copper at 75°C is 230 A (Table 310.16).
- The 75°C ampacity of 4/0 AWG copper, 230 A, exceeds the 225 A breaker rating in this simplified example. That is only an initial check. Verify the terminal temperature ratings, the conductor insulation, the ambient temperature, any conductor-count adjustments and all other applicable installation conditions before you accept the feeder design.
- The heaviest phase load (104 A) is well under both.
- The schedule’s main rating (225 A) matches the breaker on the one-line. If it did not, that is a question for your foreman before you order material.
Check: the terminal rating must also be 75°C or higher for the 75°C column to apply. The panel’s listing and label tell you.
Reading the specification
Most commercial project manuals follow the numbering where Division 26 is electrical. Each section is usually split into Part 1 General, Part 2 Products and Part 3 Execution. The VA panelboard section, for example, lists related sections in Part 1, including 26 05 19 for low-voltage conductors and cables, 26 05 26 for grounding and bonding, and 26 05 33 for raceway and boxes (VA 26 24 16).
| Spec part | What to look for as an apprentice |
|---|---|
| Part 1 General | Related sections, submittals, quality requirements |
| Part 2 Products | Required materials, ratings and features (for example, copper bus) |
| Part 3 Execution | How to install, label, test and clean up |
When a drawing and a spec seem to disagree, the contract documents say which one governs. Do not choose; ask your foreman.
Check yourself
Our own practice questions. Answers below each.
- Phase loads of 9,600, 9,600 and 9,600 VA on a 208Y/120 V panel. Current per phase? Answer: 9,600 ÷ 120 = 80 A on each phase.
- Same panel, total load by the three-phase formula? Answer: 28,800 ÷ 360.3 = 79.9 A, which matches 80 A to rounding.
- A callout reads 100A/3P, 4 #3 CU + #8 CU G. What is the #8 CU G? Answer: An 8 AWG copper equipment grounding conductor.
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A printed workbook for the two-part exam
Texas Journeyman Electrician Exam Prep 2026 NEC (Tsai Electro, paperback): 486 practice questions and 2 full two-part mock exams for the Texas Journeyman exam on the 2026 NEC. Use it alongside your own code book. Not affiliated with TDLR or PSI.
FAQ
What is a one-line diagram in electrical drawings?
A simplified diagram that shows how power flows from the source through transformers, switchboards and feeders to each panel, drawing a three-phase system as a single line. It shows breaker sizes, feeder conductors and equipment ratings.
How do you read a panel schedule?
Read the header for voltage, phases, bus and main ratings, then each circuit’s number, description, breaker and load by phase. The totals at the bottom let you compare the load on each phase. They do not, by themselves, tell you how much more load the panel or feeder can take.
What does Division 26 mean in specifications?
Division 26 is the electrical division in the common numbering system for construction specifications. Each section, such as 26 24 16 for panelboards, usually has Part 1 General, Part 2 Products and Part 3 Execution.
How do you balance a three-phase panel?
Add the load on each phase from the schedule and divide by the line-to-neutral voltage to get each phase current. In this simplified example, compare the phase loads and note which phase carries the most. In real panel work, the existing circuit arrangement, multipole loads, the load calculation and equipment limits decide where new circuits go, not the lightest phase alone.
Sources checked October 2026
- US Bureau of Labor Statistics, Occupational Outlook Handbook, Electricians: https://www.bls.gov/ooh/construction-and-extraction/electricians.htm
- US Department of Veterans Affairs master specification section 26 24 16, Panelboards (via WBDG): https://www.wbdg.org/FFC/VA/VAASC/VA%2026%2024%2016.pdf
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70 (NEC), 2026 edition, Article 310 ampacity table, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
