🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Building & Construction › Solar / PV Installation › Electrical Fundamentals for PV Systems › Lesson 6
BLD-SOLA09-03 · Lesson 6 · 12 clock hours · practice

Reading Electrical Schematics and Wiring a PV Sub-Circuit

Objective. Students will interpret a residential PV one-line diagram and wiring schematic, then correctly wire a three-component sub-circuit (module simulator, DC disconnect, and combiner) on a training board, verified by a continuity and polarity check.

⚠ 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.

Reading Electrical Schematics and Wiring a PV Sub-Circuit

Being able to read a schematic and translate it into a physical wiring job is one of the most important skills you will use on every PV installation. This lesson walks you through how electrical drawings communicate information, how to follow them safely, and how to verify your work before energizing anything.

Types of Electrical Drawings Used in PV Work

Two drawing types appear on nearly every residential PV permit set.

One-line diagram (single-line diagram): This is a simplified diagram that shows the major components and how they connect in sequence, using a single line to represent conductors rather than drawing each wire individually. It is used to communicate the system overview — modules, combiner box, DC disconnect, inverter, AC disconnect, utility meter, and service panel — and the relationships between them. Inspectors and utility engineers review one-line diagrams at permitting.

Wiring schematic (wiring diagram): This drawing shows individual conductors, their color codes, wire gauges, and how each terminal is connected. It is the drawing you follow when you are physically making connections on the job or on a training board.

When you receive a set of drawings, read the legend and title block first. Confirm the system voltage, maximum current values, and any notes about conductor specifications before touching any wire.

Tracing Current Flow on a One-Line Diagram

On a residential PV system, current flows in one general path:

1. PV modules generate DC power.

2. Strings of modules connect to the combiner box, where multiple strings join on a common bus and overcurrent protection (fuses or breakers) is provided for each string.

3. The combined DC output travels through the DC disconnect, which allows the array to be safely isolated from the inverter.

4. The inverter converts DC to AC and provides additional protection functions.

5. AC power moves through the AC disconnect to the service panel.

6. The utility meter measures energy flow before the grid connection.

When you trace this path in writing during the schematic exercise, note the polarity at each stage. Positive and negative conductors must be kept correctly identified throughout the DC side.

DC Conductor Color Coding

The National Electrical Code (NEC) addresses DC conductor identification in PV systems. For DC circuits, the common industry practice — and one supported by NEC requirements for field identification — is:

If you use a conductor color for a purpose other than its standard identification, you must re-identify it with colored tape or heat-shrink at every termination point and at every accessible point along the run. Do not guess on polarity; a reversal can damage equipment and create a shock hazard.

Termination and Torque

Loose terminations cause arcing, heat, and fires. Every terminal in a PV system has a manufacturer-specified torque value, usually printed on the terminal block or in the product data sheet. You must use a calibrated torque screwdriver or torque wrench set to that specification.

Steps for a correct termination:

1. Strip the conductor to the length shown on the terminal's strip-gauge window — no more, no less.

2. Insert the conductor fully so no bare copper is exposed outside the terminal.

3. Tighten to the specified torque value in a single smooth motion.

4. Tug the conductor gently to confirm it is seated; it should not move.

Over-torquing can crack terminal bodies or damage conductor strands. Under-torquing leaves a high-resistance joint that will heat under load.

Lockout/Tagout (LOTO) Before Testing

Before you perform any continuity or polarity check on the training board, you must apply LOTO:

1. Identify all energy sources (the module simulator output in this case).

2. Shut off or disconnect the energy source.

3. Apply a lockout device and your personal lock.

4. Attach a tag stating your name and the reason for the lockout.

5. Verify the circuit is de-energized using your multimeter before touching any conductors.

Never assume a circuit is dead. Always verify. This habit protects you from stored energy in capacitors, backfeed from the grid side, and human error by others working nearby.

Using a Multimeter for Continuity and Polarity Checks

Continuity check: Set the meter to the continuity or resistance (Ω) function. With the circuit de-energized, place one probe at each end of the conductor run. A tone or a reading near zero ohms confirms the conductor is intact and connected. An open reading (OL or infinite resistance) means a broken or disconnected conductor.

Polarity check: With the circuit energized at low voltage from the module simulator, set the meter to DC volts. Place the red probe on the terminal you believe is positive and the black probe on the terminal you believe is negative. A positive voltage reading confirms correct polarity. A negative reading means the leads are reversed — stop and identify which terminal is actually positive before proceeding.

Safety Equipment for This Lesson

Never bypass a step to save time. The habits you build in the lab are the habits you carry to the roof.

Peer Review and Final Inspection

After you complete your LOTO and testing, a classmate reviews your recorded readings against the wiring diagram and signs the verification sheet. This peer review step mirrors real job-site quality control and helps catch errors before the instructor's final check. Your instructor will inspect against a four-point checklist: conductor routing, polarity at the combiner output, torque at all terminations, and continuity on all runs. All four points must pass before your completion is recorded. If any item fails, correct it and request re-inspection.

Key terms9

One-line diagramA simplified electrical drawing that uses a single line to represent all conductors between major system components, showing the sequence and relationship of equipment in a PV system.
Wiring schematicA detailed electrical drawing that shows each individual conductor, its color code, gauge, and exact terminal connections, used as the direct guide for making physical wiring connections.
Combiner boxAn enclosure that joins multiple PV string conductors onto a common bus and provides overcurrent protection (fuses or breakers) for each string.
DC disconnectA listed switch or breaker installed between the PV array and the inverter that allows the DC circuit to be safely isolated for maintenance or emergency.
PolarityThe designation of conductors and terminals as positive (+) or negative (−) in a DC circuit; reversing polarity can damage equipment and create hazardous conditions.
Torque specificationThe manufacturer-required tightening force for a terminal connection, measured in inch-pounds or newton-meters, that ensures a low-resistance, mechanically secure joint without damaging the terminal.
Lockout/Tagout (LOTO)An OSHA-required energy control procedure in which a worker de-energizes a circuit, applies a physical lockout device and a personal lock, and attaches a tag before performing work on electrical equipment.
Continuity checkA test performed with a multimeter on a de-energized circuit to verify that a conductor or connection path is complete and unbroken, indicated by a near-zero resistance reading or audible tone.
Conductor identificationThe use of insulation color, colored tape, or heat-shrink markers to indicate the function of a conductor (positive, negative, grounded, equipment ground) so it can be correctly identified at every accessible point.

Worked examples4

1. A multimeter set to DC volts reads −12.4 V when the red probe is placed on the terminal labeled 'Output +' of the combiner box and the black probe is on 'Output −'. What does this reading tell you, and what should you do?
  1. Understand the reading: a negative DC voltage when the red probe is on the terminal marked positive means the meter probes are seeing the positive terminal as more negative than expected — the conductors at that terminal are reversed.
  2. De-energize the module simulator and apply LOTO before touching any conductors.
  3. Trace the positive (red) and negative (black or white) conductors back to the combiner bus to find where the polarity reversal occurred.
  4. Correct the reversal by moving the conductors to their correct terminals, then torque to specification.
  5. Remove LOTO, re-energize the simulator, and repeat the polarity check; you should now read approximately +12.4 V with the red probe on 'Output +' and the black probe on 'Output −'.
Answer: The negative reading indicates reversed polarity at the combiner output. After LOTO, correct the conductor connections, re-torque, and re-verify; the meter should read a positive voltage confirming correct polarity.
2. You are performing a continuity check on a 6-foot conductor run between the combiner output and the DC disconnect input. The multimeter is set to resistance (Ω). One probe is at one end of the conductor, the other probe is at the far terminal. The meter reads OL (overload / open circuit). What does this mean and what are your next troubleshooting steps?
  1. OL on the resistance function means the circuit is open — current cannot pass through the conductor path. This indicates a broken conductor, a loose termination, or a missing connection.
  2. Confirm LOTO is in place and the circuit is de-energized.
  3. Visually inspect both termination points: check that the conductor is fully seated in the terminal and that the set screw or clamp is tight.
  4. If terminations look correct, flex the conductor slightly while monitoring the meter; intermittent contact may cause a flicker from OL to a resistance value, pointing to a broken conductor inside the insulation.
  5. If a broken conductor is found, replace the conductor with a properly rated and sized replacement, strip to the correct length, terminate, and torque to specification.
  6. Repeat the continuity check; the meter should now read near 0 Ω (a small value reflecting the conductor's inherent resistance over 6 feet) and may sound a continuity tone.
Answer: OL means an open (broken or disconnected) circuit. After confirming LOTO, inspect and correct the termination or replace the conductor, then re-test until the meter reads near 0 Ω.
3. The torque specification label inside a mini combiner box terminal block reads 35 in-lb for the 10 AWG string conductor terminals. Your torque screwdriver is graduated in in-lb. Describe the correct procedure to set and apply this torque, and explain what happens if you significantly exceed it.
  1. Locate the torque adjustment ring or collar on the torque screwdriver and rotate it until the indicator aligns with the 35 in-lb mark on the scale.
  2. Insert the correct bit (matching the terminal screw head) and seat it fully in the screw head.
  3. Strip the 10 AWG conductor to the length shown in the terminal's strip-gauge window, insert it fully, and begin tightening with steady, even pressure.
  4. Continue tightening until you feel or hear the torque screwdriver click or slip — this indicates the set torque has been reached. Stop immediately; do not continue turning.
  5. If you significantly exceed 35 in-lb: the terminal screw threads can strip, the terminal body (often made of nylon or phenolic resin) can crack, or conductor strands can be cut, all of which create a high-resistance joint or a damaged terminal that must be replaced.
Answer: Set the torque screwdriver to 35 in-lb, tighten until the tool clicks or slips, then stop. Over-torquing risks stripping threads, cracking the terminal body, or damaging conductor strands — all of which require component replacement.
4. You need to re-identify a white-insulated conductor that will be used as the positive DC conductor from the combiner to the DC disconnect because no red wire of the correct gauge is available. How do you properly re-identify it, and where must the identification be applied?
  1. Obtain red colored electrical tape or red heat-shrink tubing appropriate for the conductor size.
  2. Apply the red tape or heat-shrink at the terminal end inside the combiner box, wrapping or shrinking it so the white insulation is fully covered for several inches at that end.
  3. Apply the same red identification at the terminal end inside the DC disconnect enclosure.
  4. Apply red identification at every other accessible point along the run (e.g., any conduit pull point or junction where the conductor is visible).
  5. Note the re-identification on the as-built wiring diagram so future workers understand the conductor's function.
Answer: Wrap red tape or install red heat-shrink over the white insulation at every termination point and every accessible point along the run to clearly identify the conductor as the positive DC conductor.
In class: Schematic reading exercise: instructor distributes a simplified residential PV one-line diagram; students identify and label every major component (modules, combiner box, DC disconnect, inverter, AC disconnect, utility meter, service panel), then trace the current path from module to grid in writing. · Wiring practice on training board: using module simulators, a mini combiner, and a DC disconnect mounted on a training board, students follow the provided wiring diagram to make all connections with correct conductor color coding (red/positive, black or white/negative for DC as per NEC), proper torque using a torque screwdriver, and correct polarity. · Verification and peer review: student applies LOTO, then uses a multimeter to perform continuity checks on each conductor run and a polarity check at the combiner output; a peer reviews the results against the wiring diagram and signs off before the instructor performs a final check.
Materials: PV training boards with module simulators, mini combiner box, DC disconnect, torque screwdrivers with specified torque settings, digital multimeters, residential PV one-line diagram and wiring diagram (instructor-prepared at residential scale), appropriate wire (10 AWG THWN-2 or training equivalent), LOTO kit, safety glasses, insulated gloves, student workbook Chapter 6.
Check: Instructor wiring inspection checklist: correct conductor routing (pass/fail), correct polarity at combiner output (pass/fail), all terminations at specified torque (pass/fail), continuity on all runs (pass/fail); student must pass all four checklist items; any fail requires correction and re-inspection before course completion is recorded.

Practice check8

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

PV System Design and Installation
1. On a residential PV one-line diagram, which component is typically shown immediately between the PV array and the inverter on the DC side?
PV System Design and Installation
2. What is the primary purpose of a one-line diagram in a residential PV permit set?
Commissioning, Troubleshooting, and Maintenance
3. When performing a continuity check on a de-energized conductor run with a digital multimeter, which reading indicates the conductor is intact and properly connected?
Commissioning, Troubleshooting, and Maintenance
4. A student places the red meter probe on the positive terminal and the black probe on the negative terminal of the combiner output and reads −14.2 V DC. What does this most likely indicate?
Electrical Theory and Wiring
5. According to common NEC-aligned DC conductor identification practice for PV systems, which color is used to identify the positive DC conductor?
Safety and Health
6. Which of the following best describes the correct LOTO sequence before performing a polarity check on a training board sub-circuit?
Electrical Theory and Wiring
7. A terminal block inside a combiner box has a torque specification of 40 in-lb. A student tightens the terminal screw well past the point where the torque screwdriver clicks. What is the most likely consequence?
Electrical Theory and Wiring
8. A white-insulated 10 AWG conductor must be used as the positive DC string conductor because no red wire of the correct gauge is available. What is the correct way to handle this situation before making the termination?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram plan