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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Locksmith & Security Systems › Access Control Systems: Installation, Programming, and Service › Lesson 2
BUS-LOCK72-05 · Lesson 2 · 14 clock hours · demo

Site Survey, System Design, and Low-Voltage Wiring

Objective. Students will conduct a structured site survey of a practice space, produce a scaled door schedule and basic riser diagram, select appropriate wire gauge and type for each device run, and demonstrate safe low-voltage wiring practices including proper termination and labeling.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Site Survey: Reading the Job Before You Touch a Wire

A successful access control installation begins before any cable is pulled. The site survey is a systematic walk-through that captures every condition affecting design, material selection, and code compliance. Skipping or rushing this step causes mismatched hardware, undersized power supplies, and costly rework.

Arrive with a standardized survey form, a tape measure, a flashlight, and a camera or phone. Walk each door opening and record: door material (hollow metal, wood, glass, aluminum storefront), frame type (hollow metal, wood, aluminum), hinge placement (left-hand or right-hand, count the hinges), door thickness, and the gap between door and frame. Note the existing lockset and closer, whether the door is normally held open or closed, and any fire-rating label. A fire-rated opening requires hardware and wiring methods that do not void the listing — flag it on your form and confirm with the authority having jurisdiction before proceeding.

Next, trace conduit or raceway paths from each door back to the proposed controller location. Measure run lengths, count bends, and identify obstacles such as fire barriers that require fire-stop material around penetrations. Locate the nearest electrical panel and note the available circuit capacity for the power supply. Photograph everything.

System Design: Door Schedule and Riser Diagram

The door schedule is a table — one row per opening — listing door number, location, hardware, reader type, lock type, REX (request-to-exit) device type, door position switch type, and any special conditions. A scaled floor plan with door numbers keyed to the schedule gives installers and inspectors a shared reference.

The riser diagram is a single-line schematic showing the vertical and horizontal relationship of system components: the access controller, each door station (reader, electric lock, REX, door position switch), the power supply, and the communication backbone (Wiegand, OSDP, or network). It does not need to be to scale, but every device, wire run, and cable type must be labeled. Standard electrical symbols keep the diagram readable by any qualified technician.

Draw the riser diagram before ordering materials. It forces you to account for every conductor and exposes design gaps — a door 280 feet from the controller may exceed the signal distance for standard Wiegand wiring, requiring a different cabling strategy or protocol.

Selecting Wire Gauge and Type

Access control systems use low-voltage wiring governed by NFPA 70 (National Electrical Code) Article 725, which covers Class 2 and Class 3 remote-control, signaling, and power-limited circuits. Comply with Article 725 for wiring methods, separation from power wiring, and allowed cable types.

Common selections:

Voltage drop matters. Excessive resistance in an undersized or overly long cable causes a magnetic lock to release or a strike to chatter. Use the manufacturer's maximum run length table or calculate drop yourself: V_drop = I × (2 × L × R_per_foot), where I is current in amps, L is one-way run length in feet, and R_per_foot is the resistance of the conductor from a wire table.

Power Supply Sizing

Add the current draw of every device on the power supply: reader standby current, lock holding current, REX device current, and door position switch current. The power supply's rated output current must exceed the total load with a safety margin — manufacturers typically recommend no more than 80% of rated capacity for continuous loads, matching the NEC principle for continuous loads on overcurrent devices.

If the controller also powers auxiliary outputs, include those in the calculation. Record the math on your wire calculator worksheet so the inspection file documents your design decision.

Safe Low-Voltage Wiring Practices

Low voltage does not mean no hazard. Follow these practices on every job:

Accurate survey data, a clear riser diagram, correctly sized wire, and disciplined termination practices are the foundation of an access control installation that passes inspection and keeps working for years.

Key terms9

Site surveyA structured on-site inspection that documents door hardware, frame types, conduit paths, power sources, and all conditions needed to design and estimate an access control system.
Door scheduleA table listing each door opening in a project with its hardware, reader type, lock type, REX device, door position switch, and any special conditions such as fire rating.
Riser diagramA single-line schematic showing the relationship between the access controller, door-station devices, power supply, and wiring runs, with all components and cable types labeled.
Article 725The section of NFPA 70 (National Electrical Code) covering Class 2 and Class 3 remote-control, signaling, and power-limited circuits, which governs most access control low-voltage wiring.
Voltage dropThe reduction in voltage along a conductor caused by the resistance of the wire; excessive drop can cause electric locks to malfunction or card readers to lose signal integrity.
Drain wireAn uninsulated conductor inside a shielded cable that connects to the foil or braid shield; it is grounded at the controller end only to bleed off induced noise without creating a ground loop.
Request-to-exit (REX) deviceA sensor — typically a passive infrared motion detector or push button — that signals the access controller to unlock the door for egress and suppress an alarm when someone exits.
Door position switch (DPS)A magnetic contact mounted on the door and frame that reports to the controller whether the door is open or closed, enabling alarm and reporting functions.
Current draw calculationThe process of summing the amperage requirements of every device on a power supply to confirm the supply's rated output is sufficient, typically keeping the load at or below 80% of rated capacity.

Worked examples4

1. A single door station has a magnetic lock drawing 0.5 A, a card reader drawing 0.1 A, a REX sensor drawing 0.05 A, and a door position switch drawing 0.01 A. What is the minimum power supply output current needed if the load must not exceed 80% of rated capacity?
  1. Step 1 — Add all device currents: 0.5 + 0.1 + 0.05 + 0.01 = 0.66 A total load.
  2. Step 2 — Set up the equation: total load ÷ 0.80 = minimum rated capacity.
  3. Step 3 — Divide: 0.66 ÷ 0.80 = 0.825 A minimum rated capacity.
Answer: The power supply must be rated for at least 0.825 A; the next standard size up (typically 1 A) should be selected.
2. A magnetic lock draws 0.5 A and is powered by 12 VDC through 18 AWG two-conductor cable with a one-way run length of 100 feet. The resistance of 18 AWG copper wire is approximately 0.00641 ohms per foot. What is the voltage drop in the cable, and what voltage reaches the lock?
  1. Step 1 — Calculate total conductor length (both ways): 2 × 100 ft = 200 ft.
  2. Step 2 — Calculate total resistance: 200 ft × 0.00641 Ω/ft = 1.282 Ω.
  3. Step 3 — Calculate voltage drop: V_drop = I × R = 0.5 A × 1.282 Ω = 0.641 V.
  4. Step 4 — Subtract from supply voltage: 12 V − 0.641 V = 11.359 V at the lock.
Answer: Voltage drop is approximately 0.64 V; the lock receives approximately 11.36 V, which is within the typical ±10% tolerance (10.8 V–13.2 V) for a 12 VDC lock.
3. A card reader cable run is 150 feet one way. The wire table shows 22 AWG copper has a resistance of approximately 0.01619 ohms per foot. If the reader draws 100 mA (0.1 A), what is the total voltage drop on the signal supply conductors?
  1. Step 1 — Calculate total conductor length (both ways): 2 × 150 ft = 300 ft.
  2. Step 2 — Calculate total resistance: 300 ft × 0.01619 Ω/ft = 4.857 Ω.
  3. Step 3 — Calculate voltage drop: V_drop = 0.1 A × 4.857 Ω = 0.486 V.
  4. Step 4 — Evaluate: if the reader is powered at 12 VDC, the drop is about 0.49 V, leaving approximately 11.51 V — generally acceptable, but verify against the manufacturer's minimum input voltage specification.
Answer: Voltage drop is approximately 0.49 V over the 300-foot round-trip conductor length at 100 mA draw.
4. You have a power supply rated at 2 A. The connected loads total 1.44 A. What percentage of the power supply's rated capacity is being used, and is this within the recommended 80% limit?
  1. Step 1 — Calculate percentage: (1.44 A ÷ 2 A) × 100 = 72%.
  2. Step 2 — Compare to limit: 72% is less than 80%.
Answer: The load is at 72% of rated capacity, which is within the recommended 80% limit. The installation is acceptable.

The math in this lesson1

Learners sum device currents to 0.66 A and divide by 0.80 to size a 0.825 A minimum power supply.
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Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates a live site survey of the classroom/lab space: students follow along completing a standardized survey form covering door material, frame type, hinge placement, conduit paths, existing hardware, and power source locations; students then repeat the survey independently at a second mock door station. · Using the survey data, students draw a riser diagram showing controller location, reader drops, REX device, door position switch, and power supply, then calculate total current draw for the door station and verify the power supply rating is adequate using the manufacturer's current draw table. · Wiring lab: students pull and terminate a complete single-door cable bundle (reader cable, lock power cable, REX and door position switch leads) between a mock controller panel and a practice door frame, applying correct wire colors, ring terminals or strip lengths per manufacturer spec, and labeling each conductor with adhesive tags; instructor checks each termination before energizing.
Materials: Site survey forms, graph paper and drafting tools (or CAD/drawing tablet if available), wire calculator worksheet, 22 AWG shielded multi-conductor cable (Belden or equivalent), 18 AWG two-conductor power cable, wire stripper, crimping tool, punch-down tool, ring terminals, label maker or adhesive wire markers, multimeter, mock door frame with conduit, manufacturer wiring diagrams, NFPA 70 low-voltage article reference (Article 725), PPE (safety glasses, gloves)
Check: Instructor inspects each student's wiring termination using a continuity tester: all conductors must show continuity with no shorts, all conductors must be labeled, and no insulation damage is acceptable; students who fail any point re-terminate before proceeding. Survey form and riser diagram graded on a rubric requiring correct component symbols, labeled wire runs, and accurate current draw calculation — minimum 80% rubric score.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

System design and code compliance
1. During a site survey, you notice a door has a fire-rating label on its edge. What is the most important next action regarding the planned access control hardware?
Electrical code knowledge
2. Which NFPA 70 article governs the wiring methods and conductor types used for most access control low-voltage circuits?
Low-voltage wiring practices
3. A card reader cable shield's drain wire should be grounded at:
Power supply sizing and system design
4. When sizing a power supply for a single-door access control station, industry practice recommends keeping the continuous load at or below what percentage of the supply's rated output?
Access control devices and functions
5. A technician installs a door position switch. What is the primary function of this device in an access control system?
Safety and installation practices
6. Before energizing a newly terminated access control door station, a technician should:
System design and documentation
7. A riser diagram for a single-door access control system should include which of the following?
Wire sizing and voltage drop
8. A magnetic lock run uses 18 AWG two-conductor cable over a long distance. The calculated voltage at the lock is below the manufacturer's minimum input voltage. What is the correct corrective action?
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