Short answer: Commercial layout starts from building gridlines and a finished-floor benchmark, not from walls. Measure from the nearest gridline, set heights from finished floor, and check other trades’ drawings before you hang anything. When two trades want the same space, record the conflict with location, photo and date, and give it to your foreman. You do not redesign.
Commercial work puts raceways, workspace and feeders in front of you often, and the exam outline covers all three. The PSI outline gives Wiring Methods and Electrical Equipment 10 items each in the Texas Journeyman NEC Knowledge part. The Texas Journeyman Practice Pack has 1,000 practice questions on the 2026 NEC and 10 timed two-part mocks. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR or PSI. Layout methods differ by contractor and general contractor. Follow your foreman, the contract documents and the approved coordination drawings.
How commercial layout differs from houses
On a house, you can set most boxes by measuring from a stud or a corner. On a commercial job the walls may not exist yet, the ceiling is crowded with ductwork, pipe and sprinkler lines, and dozens of people are working from the same reference points.
| Reference | What it is | How you use it |
|---|---|---|
| Gridlines | Lettered and numbered lines through the column centers | Horizontal locations are dimensioned from the nearest gridline |
| Control lines | Offset lines snapped by the layout crew, often 2 or 4 feet off a gridline | Measure from these when the gridline itself is inside a column |
| Finished floor (FF) benchmark | A marked elevation for the finished floor of each level | Device heights and conduit elevations are measured from it |
| Ceiling heights | From the reflected ceiling plan | Tells you how much space is above the ceiling |
| Coordination drawings | Combined drawings of all trades above the ceiling | Shows the agreed route and elevation for each system |
Worked example 1: transfer a plan dimension
The electrical plan is drawn at 1/8 inch = 1 foot. A floor box is 1-3/4 inches on paper from gridline B. The plan has no written dimension for it.
- 1-3/4 = 1.75 inches.
- At 1/8 inch scale, 1 inch = 8 feet.
- 1.75 × 8 = 14 feet from gridline B.
Check: 14 × 1/8 = 1.75 inches. Before you mark it, ask whether a dimensioned architectural plan shows the same box or wall. Written dimensions beat scaled ones.
Worked example 2: spacing between two marks
The plan dimensions two junction boxes from gridline 3: one at 23′-7″, the next at 31′-3″. How far apart are they?
- Convert to inches: 23 × 12 + 7 = 283 inches. 31 × 12 + 3 = 375 inches.
- Difference: 375 − 283 = 92 inches.
- Back to feet and inches: 92 = 7 × 12 + 8, so 7′-8″.
Check: 23′-7″ + 7′-8″ = 30′-15″ = 31′-3″. Same as the plan. Pull both marks from gridline 3 rather than measuring box to box, so a small error does not add up along the run.
Worked example 3: clearance above the ceiling
The reflected ceiling plan shows a 9′-0″ ceiling. Recessed fixtures need 6 inches above the ceiling. The mechanical drawing shows the bottom of a duct at 9′-8″. You want to run 3/4-inch EMT on a strap across the duct path, below the duct.
- Top of fixtures: 9′-0″ + 6″ = 9′-6″.
- Space between fixture tops and duct bottom: 9′-8″ − 9′-6″ = 2 inches.
- 3/4-inch EMT is close to 1 inch in outside diameter, and the strap, couplings and the room to work take more. Two inches is a coordination problem: the conduit, fittings and supports take more room than that, and the fixtures need access.
- This is a conflict. Do not hang the conduit until it is resolved.
Check: convert to inches. 9′-8″ = 116 inches. 9′-6″ = 114 inches. 116 − 114 = 2.
Record the conflict
Your job is to find and record conflicts early, not to decide how they get fixed. A short, clear log entry lets your foreman raise a request for information (RFI) to the general contractor and the design team.
| Field | Example entry |
|---|---|
| Date and your name | 2026-10-08, J. Apprentice |
| Location | Level 2, gridlines C to D at line 4 |
| What conflicts | Duct bottom at 9′-8″ leaves 2 inches above recessed fixtures; no room for the 3/4-inch EMT route shown |
| Drawings | E2.1 lighting plan, M2.1 duct plan, coordination sheet CD-2 |
| Photo | Photo of the duct and the fixture layout marks |
| Told to | Foreman, same day |
| Status | Open, waiting on RFI response |
Electrical rules worth knowing during coordination:
- The space above and below panelboards and switchboards is reserved for electrical equipment, so ducts and pipes that do not serve it should not run there (110.26(E)).
- The 2026 NEC requires a minimum clear space above cable trays for access, according to the City of Denison’s 2026 change summary (Denison 2026 NEC summary) (392.18).
- Raceways must be supported by their own supports, not by ceiling grid wires or other trades’ hangers (Article 300).
Habits that make you useful on a commercial crew
- Check the latest revision of every sheet before you lay out. Superseded drawings cause most layout errors.
- Mark heights as “to center” or “to bottom” exactly as the plans or specs say.
- Photograph your marks with a tape in the frame.
- Keep a small notebook or app log of every conflict and every verbal instruction, with the date.
Check yourself
Our own practice questions. Answers below each.
- At 1/4 inch = 1 foot, a box is 3-1/8 inches from a gridline on paper. Distance? Answer: 3.125 × 4 = 12.5 feet, or 12′-6″.
- Two marks at 14′-9″ and 22′-2″ from the same gridline. Spacing? Answer: 177 and 266 inches; 266 − 177 = 89 inches = 7′-5″.
- A sprinkler main blocks your conduit route. Who reroutes it? Answer: Not you. Log it and tell your foreman so it can go through an RFI.
Working toward the Texas Journeyman exam? TDLR accepts the exam application at 7,000 supervised hours. When you start studying for it, the Texas Journeyman Practice Pack (US$39, one payment) has 1,000 questions and 10 timed two-part mock exams on the 2026 NEC.
FAQ
What are gridlines on construction drawings?
Lettered and numbered reference lines that run through the building’s column centers. Commercial layout is dimensioned from the nearest gridline, so every trade works from the same reference.
What is an RFI in electrical construction?
A request for information: a formal written question from the contractor to the design team when drawings conflict, are unclear or cannot be built as shown. Apprentices usually report the problem to the foreman, who writes the RFI.
What does AFF mean on electrical drawings?
Above finished floor. Device and equipment heights are measured from the finished floor elevation, so check whether the height is to the center, top or bottom of the device.
What should an apprentice do when trades conflict above the ceiling?
Stop, record the location, the systems in conflict, the drawing numbers and a photo, and tell the foreman the same day. Do not reroute another trade’s work or change the design yourself.
Sources checked October 2026
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- City of Denison, 2026 NEC significant changes: https://denisontx.gov/DocumentCenter/View/4131/2026-National-Electrical-Code-Significant-Changes
- NFPA 70 (NEC), 2026 edition, Articles 110, 300 and 392, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
