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newSchoolWorks.com › VoTechWorks › Building & Construction › Solar / PV Installation › Externship / Worksite Placement › Lesson 2
BLD-SOLA09-09 · Lesson 2 · 100 clock hours · field

Hands-On Installation Tasks: Mechanical and Electrical Work

Objective. Student will perform the full mechanical and electrical scope of at least three PV installations—including racking layout, module mounting, DC string wiring, inverter mounting, AC interconnection, and labeling—meeting the supervising installer's quality standard on each task.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson 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.

What Happens on a Real Installation Day

Externship days combine everything you have studied—structural principles, electrical theory, code rules, and safety habits—into a single, fast-moving workflow. Each job site is slightly different, but the scope of work on a residential or light-commercial PV installation follows a repeatable sequence: layout and racking, module mounting, DC wiring, inverter and AC disconnect installation, labeling, and commissioning. Your supervising installer sets the quality bar; your job is to meet it on every task, every time.

Step 1 – Lay Out and Install the Racking System

Before any hardware goes on the roof, study the approved permit set and the engineered racking drawings. Confirm the roof section, orientation, setbacks, and attachment-point spacing shown on the drawing. Racking manufacturers specify the maximum rail span and the required fastener torque for each attachment type—follow those values exactly and record them in your daily field log.

When attaching to a shingle or tile roof, each lag screw or through-bolt is a potential water entry point. Follow the manufacturer's flashing and sealant instructions for every penetration: flash first, set the lag, apply sealant approved for roofing use, and photograph the completed attachment before it is covered by the rail. Document each penetration in the field log with its location (row and column), fastener size, torque value achieved, and waterproofing product used.

Torque fasteners with a calibrated torque wrench, not an impact driver set by feel. Over-torquing crushes flashing and voids the waterproofing warranty; under-torquing allows the rail to move under wind and snow load. After rails are secured, install mid-clamps and end-clamps finger-tight until modules are positioned, then torque to spec in a consistent pattern.

Safety on the roof: Wear slip-resistant footwear and a properly fitted harness connected to an anchor rated for fall arrest whenever you are on a roof with a pitch or height that triggers your employer's fall-protection program. Never set tools on the roof surface where they can slide. Keep your three-point contact on ladders.

Step 2 – Pull and Terminate DC Conductors

Once modules are mechanically secured, route the DC home-run conductors from the array to the inverter or combiner location. Use conduit where required by the design, and maintain the minimum bend radius for the conductor size selected. Use wire management clips or cable trays designed for PV use—never zip-tie conductors to mounting rails without a UV-rated tie.

Before making any connections, verify conductor colors match the design: typically red or positive-marked for the ungrounded conductor and black or negative-marked for the return in a DC system, following your specific design documents. Inspect each MC4 (or listed) connector for pin seating, then pull-test each connection by hand. A properly crimped and locked connector will not pull free.

Before energizing, measure open-circuit voltage (Voc) across each string with a multimeter set to DC voltage. Compare the reading to the calculated Voc at the ambient temperature of the day—a significant deviation indicates a wiring error, a shaded module, or a failed module. Record the measured Voc, string polarity, conductor colors, and connector types in your field log before you report the string ready for connection to the inverter.

Electrical safety: DC circuits cannot be switched off by simply opening a breaker while current is flowing under load. Under sunlight, an energized arc can sustain itself. Work on DC circuits before sunrise, in low-light conditions, or cover modules with opaque material to de-energize the string. Always use properly rated personal protective equipment (PPE) including insulated gloves when working on energized DC conductors. Follow your employer's lockout/tagout (LOTO) procedure for the inverter and AC disconnect before working on the AC side.

Step 3 – Mount the Inverter and AC Disconnect

Mount the inverter at the height and location shown on the permit set—typically on a shaded exterior wall or in a conditioned space per the manufacturer's clearance requirements. Use hardware rated for the wall material and the inverter weight. Route the DC conduit from the array into the inverter's DC input compartment and the AC conduit from the inverter output to the AC disconnect.

Wire the AC disconnect following the single-line diagram. Verify conductor sizing, breaker rating, and conduit fill against the permit set. Torque all terminal lugs to the manufacturer's specification printed on the device label—under-torqued lugs cause resistance heating and are a fire hazard.

Step 4 – Apply Required Labels

NEC Article 690 and Article 705 require specific labels at the inverter, AC disconnect, service panel, conduit runs, and other locations. These include arc-flash warning labels, rapid-shutdown instructions visible to first responders, and conductor identification labels. Use only labels listed for the application—field-made paper labels are not code-compliant. Apply each label flat, without wrinkles that obscure text, before the system is energized.

Step 5 – Commissioning and System Startup

Commissioning is performed by or directly supervised by the licensed installer. Your role is to assist and observe. The sequence typically includes: verify all mechanical and electrical connections are complete; confirm rapid-shutdown devices are functional; close the AC disconnect and enable the inverter per the manufacturer's startup sequence; verify AC output voltage, frequency, and power factor on the inverter display or monitoring portal; record all commissioning readings in the field log.

Documentation Is Part of the Job

Every field-log entry—torque values, Voc readings, label locations, commissioning data—protects the homeowner, the installing contractor, and you. Inspectors and utility representatives review this documentation. Incomplete logs reflect poorly on the crew. Fill out every field log completely before leaving the site.

Key terms10

Open-Circuit Voltage (Voc)The DC voltage measured across a PV string or module with no load connected; used during commissioning to verify correct wiring and module health.
Torque SpecificationThe manufacturer-required rotational force, measured in inch-pounds or foot-pounds, applied to a fastener or terminal lug to ensure proper clamping without damage.
Racking LayoutThe arrangement of rails, attachment points, and clamps on a roof surface, determined by the engineered drawing and the structural attachment schedule.
Home-Run ConductorThe DC wiring run that carries the combined output of one string from the array to the inverter or combiner box.
MC4 ConnectorA weatherproof, touch-safe, single-contact connector widely used in PV arrays to join module leads and string wiring; must be from a listed manufacturer and installed with the correct crimping tool.
Lockout/Tagout (LOTO)An OSHA-required safety procedure that uses physical locks and warning tags to isolate energy sources—electrical, mechanical, or stored—before work begins on equipment.
Rapid ShutdownAn NEC 690-required function that reduces DC voltage in the PV system to a safe level within a specified time when the building fire service control is activated, protecting first responders.
CommissioningThe structured startup sequence for a completed PV system, including functional testing, measurement of output parameters, and verification that all safety systems operate correctly.
AC DisconnectA listed, lockable switch installed between the inverter AC output and the utility interconnection point, allowing the system to be de-energized for maintenance or emergency response.
Single-Line DiagramA simplified electrical drawing included in the permit set that shows the components of the PV system, conductor sizes, overcurrent protection ratings, and the path from the array to the utility meter.

Worked examples4

1. A string of 10 modules each has a labeled Voc of 40.2 V at Standard Test Conditions (STC). The temperature coefficient of Voc is −0.29 %/°C. At the time of commissioning the module temperature is estimated at 15 °C, compared to the STC reference of 25 °C. What is the expected string Voc at that module temperature, and is a multimeter reading of 418 V within 2% of expected?
  1. Step 1 – Find the temperature difference: ΔT = 15 °C − 25 °C = −10 °C (modules are cooler than STC).
  2. Step 2 – Calculate the per-module voltage correction: ΔV per module = 40.2 V × (−0.29/100) × (−10) = 40.2 × 0.029 = +1.166 V (voltage rises as temperature drops below STC).
  3. Step 3 – Corrected single-module Voc = 40.2 + 1.166 = 41.366 V.
  4. Step 4 – String Voc = 41.366 V × 10 modules = 413.66 V, rounded to 413.7 V.
  5. Step 5 – Check the 2% tolerance band: 413.7 V × 1.02 = 421.97 V upper limit; 413.7 V × 0.98 = 405.43 V lower limit.
  6. Step 6 – The measured 418 V falls between 405.4 V and 422.0 V, so it is within 2% of expected.
Answer: Expected string Voc ≈ 413.7 V; a reading of 418 V is within the 2% tolerance band (405.4 V – 422.0 V), so the string passes the Voc check.
2. The racking manufacturer specifies lag screws to be torqued to 20 ft-lb. Your calibrated torque wrench is marked in inch-pounds. What value do you set on the wrench?
  1. Step 1 – Recall the conversion: 1 ft-lb = 12 in-lb.
  2. Step 2 – Multiply: 20 ft-lb × 12 in-lb/ft-lb = 240 in-lb.
  3. Step 3 – Set the torque wrench to 240 in-lb.
Answer: Set the torque wrench to 240 in-lb.
3. You are checking conduit fill for three 10 AWG THWN-2 conductors in a ¾-inch EMT conduit. Each 10 AWG THWN-2 conductor has a cross-sectional area of 0.0211 in². The allowable fill for three or more conductors in ¾-inch EMT is 40% of the conduit's internal area of 0.533 in². Does this fill comply?
  1. Step 1 – Calculate the total conductor area: 3 conductors × 0.0211 in² = 0.0633 in².
  2. Step 2 – Calculate the maximum allowable fill area: 0.533 in² × 0.40 = 0.2132 in².
  3. Step 3 – Compare: 0.0633 in² < 0.2132 in².
  4. Step 4 – The fill percentage used: 0.0633 / 0.533 = 0.1187, or about 11.9%—well under the 40% limit.
Answer: The installation complies; the conductors occupy approximately 11.9% of the conduit, which is below the 40% maximum allowable fill.
4. Before energizing a two-string combiner, you measure Voc on String 1 and get 402 V. You then connect String 1 to the combiner and go to measure String 2. A coworker says it is fine to measure String 2 at the combiner terminals with String 1 already connected. Is this correct? Explain the correct procedure.
  1. Step 1 – Identify the hazard: String 1 is already connected and live at the combiner terminals. Probing the same bus bar exposes the technician to the full live voltage of String 1 while they attempt to verify String 2.
  2. Step 2 – Correct procedure: Measure each string Voc and polarity at the module end of the home-run conductors—before connecting them to the combiner—so you are never probing live bus bars.
  3. Step 3 – After all strings are verified individually, connect them to the combiner one at a time while wearing insulated gloves rated for the DC voltage present.
  4. Step 4 – If re-measurement at the combiner is required, open and lock out all string fuses or disconnects first, following the site LOTO procedure.
Answer: The coworker's suggestion is incorrect and unsafe. Measure each string at its source conductors before connecting to the combiner; if measurement at the energized combiner is necessary, apply LOTO to isolate all connected strings first.

The math in this lesson2

Learners check conduit fill percent and convert ft-lb torque to in-lb.
7.RP.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: On each assigned job, lay out and install the racking system per the engineered drawings, torque fasteners to specification, and record torque values and roof-penetration waterproofing steps in the daily field log · Pull and terminate DC home-run conductors, install combiners or optimizers as designed, verify polarity with a multimeter before energizing, and document conductor colors, connector types, and measured open-circuit voltages in the field log · Mount and wire the inverter and AC disconnect, apply all required NEC 690 arc-flash and rapid-shutdown labels, and assist the supervising installer during the utility interconnection and system startup sequence, recording final commissioning readings
Materials: Employer-supplied tools (drills, impact drivers, torque wrenches, conduit benders, fish tape), multimeter, clamp meter, racking components, PV modules, inverter, AC disconnect, wire management hardware, NEC-compliant labels, approved permit set and single-line diagram, daily field-log forms
Check: Supervising installer evaluates each task on the employer evaluation rubric (mechanical quality, electrical correctness, code compliance, safety behavior, and documentation accuracy); pass = a rating of 'meets expectations' or higher on all five rubric categories averaged across the logged installations

Practice check8

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

PV System Commissioning and Troubleshooting
1. After mounting modules and before connecting DC home-run conductors to the inverter, what measurement should you take and why?
PV System Mechanical and Structural Installation
2. A racking manufacturer specifies a flashing lag to be torqued to 15 ft-lb. Your torque wrench reads in inch-pounds only. What is the correct setting?
Codes, Standards, and Safety
3. Which of the following is the primary purpose of the rapid-shutdown requirement found in NEC Article 690?
Safety, PPE, and Hazard Identification
4. While working on the AC disconnect wiring at the end of the day, you discover the inverter's AC breaker was left closed. What is the correct immediate action?
Codes, Standards, and Safety
5. You are installing labels on a newly completed residential PV system. Which of the following label materials is acceptable for a permanent outdoor installation?
PV System Mechanical and Structural Installation
6. During racking installation you notice one lag-screw flashing does not have sealant applied before a coworker installs the rail over it. What should you do?
PV System Design and Electrical Theory
7. When measuring DC string Voc on a cold winter morning (module temperature well below 25 °C STC), compared to the STC-rated Voc, you would expect the measured string voltage to be:
Professional Practices and Documentation
8. The supervising installer's quality rubric rates each task on mechanical quality, electrical correctness, code compliance, safety behavior, and documentation accuracy. A student receives 'meets expectations' on four categories but 'below expectations' on documentation accuracy for the second installation. What does this mean for the student's externship evaluation?
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