Reading Drawings & Specs
An electrical drawing set has 30+ sheets, organized by a precise convention. Knowing which sheet number to look at when you have a question is half the skill. Specifications (Division 26) tell you HOW to install — drawings tell you WHAT.
The Electrical Drawing Set
An electrical project's drawing set is organized by sheet number. The number tells you what type of information to expect.
| Sheet number | Content | What you find here |
|---|---|---|
| E001 | Cover sheet, drawing index | Project info, sheet list, applicable codes, abbreviations |
| E002 | Symbols + general notes | Legend of all electrical symbols used; project-wide notes |
| E003-E099 | Site / utility | Service entrance, site lighting, utility coordination |
| E101-E199 | Floor plans — power | Receptacles, equipment locations, panel locations |
| E201-E299 | Floor plans — lighting | Light fixtures, controls, emergency egress lighting |
| E301-E399 | Floor plans — systems | Fire alarm, security, telecom, audio/visual |
| E401-E499 | Single-line diagrams | Power distribution SLD, riser diagram |
| E501-E599 | Schedules | Panel schedules, transformer schedule, MCC schedule, fixture schedule |
| E601-E699 | Details | Mounting details, grounding details, service entrance detail |
| E701-E799 | Special systems | Telecom rooms, AV rooms, equipment rooms |
| E801-E899 | Demolition | Existing-to-remove (renovation projects only) |
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When you have a question, look here
"Where does this circuit feed from?" → Panel schedule (E501). "What's the fault current at this bus?" → SLD (E401). "How is this panel mounted?" → Detail (E601). Drawing-set fluency = knowing where each question lives.
The Specification Document
Drawings tell you WHAT to install. Specifications tell you HOW. The CSI MasterFormat 50-Division system is the industry standard for organizing specifications.
| CSI Division | Subject | Electrical relevance |
|---|---|---|
| Division 1 — General Requirements | Project administration, submittals, etc. | Read first — applies to all trades |
| Division 26 — Electrical | All electrical work | Your home base |
| Division 27 — Communications | Voice + data + AV cabling | Often coordinated with electrical |
| Division 28 — Electronic Safety + Security | Fire alarm, security, access control | Integrated with electrical service |
| Division 23 — HVAC | Mechanical equipment | You provide power for their equipment per MEL |
| Division 33 — Utilities | Site utilities | Coordination with utility company |
Division 26 Sub-Sections (Most Common)
| Section | Content |
|---|---|
| 26 05 00 | Common Work Results for Electrical (general requirements) |
| 26 05 19 | Low-Voltage Electrical Power Conductors and Cables |
| 26 05 26 | Grounding and Bonding |
| 26 05 33 | Raceways and Boxes |
| 26 09 23 | Lighting Control Devices |
| 26 09 43 | Network Lighting Controls |
| 26 12 00 | Medium-Voltage Transformers |
| 26 13 00 | Medium-Voltage Switchgear |
| 26 18 00 | Medium-Voltage Distribution |
| 26 22 00 | Low-Voltage Transformers |
| 26 24 13 | Switchboards |
| 26 24 16 | Panelboards |
| 26 24 19 | Motor-Control Centers |
| 26 27 26 | Wiring Devices (receptacles, switches) |
| 26 28 13 | Fuses |
| 26 28 16 | Enclosed Switches and Circuit Breakers |
| 26 29 13 | Enclosed Controllers (motor starters) |
| 26 32 13 | Engine Generators |
| 26 33 53 | Static Uninterruptible Power Supply |
| 26 36 00 | Transfer Switches |
| 26 41 13 | Lightning Protection for Structures |
| 26 43 13 | Surge Protective Devices |
| 26 51 00 | Interior Lighting |
| 26 56 00 | Exterior Lighting |
Three-Part Specification Format
Every Division 26 spec section follows the CSI 3-part format:
| Part | Content | What you do with it |
|---|---|---|
| Part 1 — General | References, submittal requirements, quality assurance, warranty | Read first — applies to entire section |
| Part 2 — Products | Approved manufacturers, technical specifications, options | Tells you exactly what equipment to buy + what's substitutable |
| Part 3 — Execution | Installation, testing, commissioning, training | Field installation rules |
Drawing-Spec Discrepancies
When drawings and specs disagree (and they often do), which governs?
!
Generally: specifications govern (per Division 1)
Most contracts state specifications take precedence over drawings, and large-scale drawings take precedence over small-scale. ALWAYS submit an RFI when discrepancy is found — never assume which is correct.
Schedules — Where Equipment Lives
| Schedule type | Content | Sheet location |
|---|---|---|
| Panel Schedule | Every breaker, wire, load, phase. Per panel. | E501-E599 typically |
| Transformer Schedule | kVA, voltage, %Z, configuration, location, OCPD | E501 |
| MCC Schedule | Each bucket: starter type, motor served, FLA, CB size | E501 |
| Switchgear Schedule | Each compartment: breaker rating, function, connection | E501 |
| Lighting Fixture Schedule | Each fixture type: model, watts, lumens, mounting, voltage | E501-E599 |
| Cable Schedule | Each cable run: from, to, type, size, length | E501-E599 (industrial only) |
| Conduit Schedule | Each conduit run: type, size, fittings | Industrial only |
| Equipment Schedule | Each piece of major equipment: tag, V, HP/kW, FLA, location | The MEL — usually mech-provided, electrical-augmented |
Worked Example 1 — Atlas DC1 Drawing Set
Example 01 · Atlas DC1 spineAtlas DC1's 87-sheet drawing set + 250-page Division 26 specification
Drawing set (excerpts)
| Sheet | Content |
|---|---|
| E001-E002 | Cover, index, codes, symbols |
| E003-E010 | Site plan, utility coordination, ground ring |
| E101-E110 | Floor plans — IT halls power layout (PDU + RPP locations) |
| E111-E115 | Mech room power (chillers, pumps, MCCs) |
| E201-E210 | Lighting plans (IT halls, mech, office, exterior) |
| E301-E305 | Fire alarm + emergency systems |
| E401 | Main SLD (the canonical Atlas DC1 one-line) |
| E402-E405 | Detailed SLDs for each side, UPS, generator paralleling |
| E501-E520 | Panel schedules (every panel + RPP) |
| E521-E523 | Transformer + MCC + UPS schedules |
| E601-E620 | Mounting details, grounding details, service entrance |
| E701-E705 | UPS room layouts, battery room ventilation, cable tray routes |
Specification (excerpts from 250-page Division 26)
| Section | Excerpt |
|---|---|
| 26 13 00 (MV switchgear) | 3-piece arc-resistant gear, vacuum CBs, withstand 50 kA. Manufacturers: Eaton, Siemens, ABB. Coordination study by mfr. |
| 26 24 13 (Switchboards) | Square D/Eaton/GE acceptable. Bus 4000A Cu. 65 kA AIC. ANSI/NEMA PB2 compliant. |
| 26 32 13 (Generators) | 2 × 2500 kW Tier 4 final, sound-attenuated enclosure, sub-base fuel tank 24 hr. 0.85 PF. Manufacturers: Caterpillar, MTU, Cummins. |
| 26 33 53 (UPS) | Static double-conversion UPS, 1250 kVA, VRLA battery, 5 min ride-through. SCCR 65 kA. Eaton, Schneider, Vertiv accepted. |
| 26 36 00 (ATS) | Bypass-isolation construction. Open transition. 4000 A. NEMA 1. |
Result: Contractor uses these to bid + procure equipment. Engineer reviews submittals against the spec.
Worked Example 2 — Reading a Spec for First Time
Example 02 · Skill walkthroughWhere to start when handed a 200-page Division 26 spec
- Read 26 05 00 first. "Common Work Results" — applies to entire division. Submittal requirements, codes, warranties.
- Then read 26 05 19 (cables) + 26 05 33 (raceways) + 26 05 26 (grounding). The basic infrastructure spec.
- Match each piece of equipment on drawings to its spec section. Switchboard on E401? Read 26 24 13. Panel on E501? Read 26 24 16. Etc.
- Look for "approved manufacturers" lists. Tells you who's bidding. If only one mfr listed = single-source spec.
- Look for "Owner-furnished, contractor-installed" (OFCI). Common for IT switchgear in DCs — owner buys, contractor installs.
Drill — Quick Self-Check
Work each problem mentally; reveal to check. Goal: reflex, not deliberation.
Drill 1 · Sheet number
Where do you find the SLD?
E401
E001 = cover; E101 = power; E201 = lighting; E501 = schedules.
Drill 2 · Division 26
What's Division 26 in CSI MasterFormat?
Electrical
23 = HVAC; 27 = Comm; 28 = Safety/Security.
Drill 3 · 3-part spec
What's in Part 2 of a CSI spec?
Products (manufacturers + technical specs)
Part 1 = General; Part 3 = Execution.
Drill 4 · Discrepancy
Drawings + specs disagree. Generally which governs?
Specifications (per Division 1)
Always submit RFI when unsure.
Drill 5 · OFCI
OFCI means?
Owner-Furnished, Contractor-Installed
Common for sensitive DC equipment.
If You See THIS, Think THAT
| If you see… | Think / use… |
|---|---|
| "Division 26" | Electrical specifications. Companion to drawings. |
| "E001" or "E101" sheet | Cover/index or first floor power plan respectively. |
| "E401" | Single-line diagram. The system map. |
| "E501" | Panel schedules + transformer + MCC schedules. |
| "E601" | Details — mounting, grounding, service entrance. |
| "OFCI" | Owner-Furnished, Contractor-Installed. Common for sensitive equipment. |
| "OFOI" | Owner-Furnished, Owner-Installed. Rare in commercial. |
| "RFI" (Request for Information) | Contractor question — engineer must respond formally. |
| "Submittal" | Contractor's documentation showing equipment selected. Engineer reviews + stamps. |
| "As-built" or "As-recorded" | Final drawings showing what was actually installed (vs. designed). |
| "IFC" (Issued for Construction) stamp | Drawing version released for construction. Subsequent revisions tracked. |
| "Revision cloud" | Marks a change region on a drawing. Triangle marker shows revision number. |
| "NTS" or "Not To Scale" | Drawing not to scale — measure dimensions, don't measure off the drawing. |
| "Coordinate with [other discipline]" | Issue resolved between trades during construction. Common note on drawings. |