Photovoltaic Systems
Learning Objectives
By the end of this chapter, you should be able to:
4.Identify and classify PV system circuits (source, output, and inverter input/output) per NEC Article 100 and 690.
5.Calculate PV circuit currents and conductor ampacity requirements per 690.8.
6.Apply overcurrent protection and disconnect requirements per 690.9 and 690.15.
7.Understand rapid shutdown requirements under 690.12 as enforced in Massachusetts.
8.Distinguish between grounded, ungrounded, and bipolar PV systems for equipment grounding and ground-fault protection.
9.Locate marking, labeling, and interconnection requirements, including Massachusetts-specific provisions.
1.1 System Overview and Circuit Classification
A photovoltaic system converts sunlight directly into DC electricity. The NEC divides PV systems into distinct circuit types, each with its own rules for ampacity, overcurrent protection, and disconnecting means.
PV source circuit: Conductors and equipment from the PV module (or panel) output terminals up to the point where they combine with other source circuits (typically at a combiner box or the inverter input).
PV output circuit: Conductors from the point of combining source circuits (combiner output) to the inverter input (for a DC-to-AC inverter) or to the charge controller input.
Inverter input circuit: Conductors between the inverter and the battery or other DC source when the inverter is not directly connected to PV modules.
Inverter output circuit: Conductors from the inverter output to the service or distribution equipment (AC side).
Key distinction: For a string inverter system, the PV source circuit is each individual string; the PV output circuit is the combined DC feeder to the inverter. For microinverters or DC-to-DC optimizers, the circuit definitions shift—each module's output may be an inverter output circuit (AC) rather than a PV source circuit.
1.2 Circuit Current Calculations (690.8)
This is the most calculation-heavy section for PV work. You must compute maximum circuit current before sizing conductors, overcurrent devices, and disconnects.
PV Source Circuit Maximum Current (690.8(A)(1)): Use the sum of parallel module short-circuit currents, multiplied by 1.25. For a single string, this is simply Isc × 1.25. For parallel strings, sum the individual module Isc values first, then multiply by 1.25.
PV Output Circuit Maximum Current (690.8(A)(2)): Use the sum of the source circuit maximum currents (as calculated above) that feed the output circuit, multiplied by an additional 1.25. In practice, this is often expressed as: (sum of Isc of all parallel strings) × 1.25 × 1.25 = × 1.56 total.
Inverter Output Circuit Maximum Current (690.8(A)(3)): Use the inverter's continuous output current rating. If the inverter is listed and marked with a continuous output current, use that value. Otherwise, multiply the nominal output current by 1.25.
Conductor Ampacity (690.8(B)): Conductors must have an ampacity not less than the greater of:
The maximum circuit current (as above) after applying correction/adjustment factors, OR
The maximum circuit current before correction factors, but with a 1.25 factor already included (i.e., you cannot reduce conductor size below the raw computed current).
Practical approach: For DC conductors, size them at 156% of Isc (for output circuits) before temperature corrections. For AC conductors from the inverter, size at 125% of continuous output current.
Exam trap: Do not apply temperature correction factors to the 1.25 multiplier itself. The 1.25 is a continuous-duty factor, not an ambient correction. Apply ambient corrections separately to the conductor's base ampacity.
1.3 Overcurrent Protection (690.9)
PV source and output circuits require overcurrent protection unless the conductors are already protected by the module's internal fusing or the circuit is inherently limited.
Location: Overcurrent devices must be placed at the point where the circuit receives its supply (the source), except where the conductor is protected by a device rated at or below its ampacity.
Sizing: Overcurrent devices must be rated not less than 125% of the maximum circuit current. Standard sizes per 240.6 apply.
Module internal fusing: Many modules have series fuses inside the junction box. If the module is listed with a maximum series fuse rating (e.g., 15 A), you must ensure that the total fault current from parallel strings does not exceed that rating. If it does, you must add fuses at the combiner.
Ground-fault protection: For grounded DC systems, ground-fault protection is required per 690.41(B). This is typically integrated into the inverter. Ungrounded systems require ground-fault detection per 690.41(C).
Exam trap: Overcurrent devices on the DC side must be rated for DC voltage and must be listed for use in PV systems. Standard AC breakers are not acceptable.
1.4 Disconnecting Means (690.15)
Each PV system must have disconnecting means to isolate all current-carrying conductors from all sources of power.
DC side: A disconnecting means must be provided for the PV output circuit (or inverter input circuit) and for each PV source circuit if more than one source circuit is present. This is typically a DC disconnect or a combiner with an integral disconnect.
AC side: A disconnecting means must be provided for the inverter output circuit, located within sight of the inverter or lockable in the open position.
Load-side disconnects: For battery systems, a disconnect must be provided for the battery circuit.
Requirements: Disconnects must be rated for the maximum circuit voltage and current, must be load-break rated (able to interrupt rated current), and must be suitable for the environment (outdoor-rated if exposed).
Practical field point: The DC disconnect is often a separate enclosure between the combiner and inverter, or it may be integrated into the inverter. On site, verify that the disconnect blade visibly opens all ungrounded conductors.
1.5 Rapid Shutdown (690.12) — Massachusetts Enforcement
Massachusetts enforces 690.12 in full. This section requires that, upon initiation of rapid shutdown, conductors within the array boundary (and within 1 foot of the array) be reduced to a safe voltage within 30 seconds.
Voltage limits: Within the array boundary, voltage must be reduced to ≤ 80 V within 30 seconds. Outside the array boundary (i.e., in the DC conductors running to the inverter), voltage must be reduced to ≤ 30 V within 30 seconds.
Controlled conductors: The requirement applies to conductors more than 1 foot inside the array boundary. Conductors within 1 foot of the array (inside the boundary) must also be controlled.
Equipment: Rapid shutdown is typically achieved using module-level power electronics (MLPE) such as microinverters or DC optimizers, or with rapid shutdown switches that short or open the PV source circuits.
Initiation device: The rapid shutdown initiation must be a listed device, clearly labeled, and located at a readily accessible point. The "service disconnecting means" is the typical initiation point for utility-interactive systems.
Labeling: The rapid shutdown switch must be labeled with the wording "PV SYSTEM RAPID SHUTDOWN" or similar. Additional labels must indicate the location of the initiation device.
Exam trap: The 80 V limit applies within the array boundary; the 30 V limit applies outside the boundary but still on the DC side. Many candidates confuse these two values.
Massachusetts note: 527 CMR 12.00 adopts the NEC with Massachusetts amendments. Massachusetts does not waive or relax 690.12; it is fully enforceable. The state also requires that rapid shutdown equipment be listed and that the system be installed per manufacturer instructions.
1.6 Grounding and Bonding (690.41–690.47)
PV systems may be grounded (one DC conductor bonded to ground) or ungrounded (both DC conductors isolated from ground). The system design determines the requirements.
Grounded systems: One DC current-carrying conductor (typically the negative in a positive-grounded system, or positive in a negative-grounded system) is bonded to the equipment grounding conductor at a single point. A ground-fault detector is required to sense a fault to ground and either interrupt the circuit or signal an alarm.
Ungrounded systems: Neither DC conductor is intentionally grounded. Ground-fault detection is still required to sense an inadvertent ground fault. Ungrounded systems require a ground-fault detector that interrupts the fault or provides an audible/visual alarm.
Equipment grounding: All exposed non-current-carrying metal parts (module frames, racks, inverter enclosures, conduit) must be bonded to the equipment grounding conductor. Use listed grounding lugs, washers, and bonding jumpers.
Grounding electrode: The PV system equipment grounding conductor must terminate at a grounding electrode (typically the building grounding electrode system) per 690.47. A separate ground rod is not required if the building electrode is accessible.
Practical field point: Module frames often have pre-drilled grounding holes. Use listed stainless steel grounding washers that bite into the frame. Do not rely on the racking system alone unless it is listed as a bonding path.
1.7 Ampacity Adjustments and Conductor Sizing
DC conductors in PV systems are often in exposed sunlight on rooftops. Ambient temperature adjustments per 310.15(B) are critical.
Ambient temperature: Use the ambient temperature for the installation location. For rooftop conduit within 1 inch of the roof, add 33 °C (60 °F) to the ambient temperature per Table 310.15(B)(3)(c) (formerly Table 310.15(B)(2)(c)).
Conductor type: PV wire (listed as PV or USE-2) is rated for 90 °C and sunlight resistance. THWN-2 is also acceptable for wet locations.
Voltage drop: Not a code requirement for ampacity, but a practical design consideration. For long DC runs (over 100 ft), voltage drop reduces system efficiency.
Exam trap: When applying temperature corrections, use the 90 °C column for ampacity if the terminals are rated for 90 °C. However, if the conductor terminates on a 75 °C terminal (common for breakers and disconnects), you must use the 75 °C column for the final ampacity check.
1.8 Marking and Labeling Requirements
PV system label: A label must be placed at the service disconnecting means indicating the presence of a PV system and the location of the PV disconnects.
DC disconnect label: Each DC disconnect must be labeled with the operating voltage, current, and "PV SYSTEM" wording.
Rapid shutdown label: As noted in 1.5.
Inverter label: The inverter must be labeled with its AC output rating and DC input rating.
Point of interconnection: If the PV system connects to the load side of the service, a label must indicate the maximum available fault current.
1.9 Interconnection and Massachusetts SMART Program Context
Utility-interactive inverters must be listed and labeled as such (UL 1741). The inverter must be connected to the utility through a dedicated AC disconnect or through the service disconnecting means.
Load-side connection: The PV AC output can connect to the load side of the service disconnect, provided the busbar rating is not exceeded (per 705.12(B)). The sum of the main breaker and the PV breaker must not exceed 120% of the busbar rating.
Supply-side connection: The PV output can connect to the supply side of the service disconnect via a tap, per 230.82(6).
Massachusetts SMART program: While not a code requirement, the SMART (Solar Massachusetts Renewable Target) program requires that systems be installed per the NEC and that the utility be notified. The interconnection agreement typically requires a visible, lockable AC disconnect.
1.10 Code Navigation
Use this quick-reference to find the exact sections during the exam:
| Topic | NEC 2026 Reference |
|---|
| Definitions (PV source/output circuit) | Article 100 |
| Scope of PV systems | 690.1 |
| Circuit current calculations | 690.8 |
| Overcurrent protection | 690.9 |
| Disconnecting means | 690.15 |
| Rapid shutdown | 690.12 |
| Ground-fault protection | 690.41 |
| Equipment grounding | 690.43 |
| Grounding electrode | 690.47 |
| Marking and labeling | 690.56 |
| Interconnection (load side) | 705.12(B) |
| Wiring methods (exposed) | 690.31 |
| Battery systems | 690.71–690.74 |
| Massachusetts amendments | 527 CMR 12.00 (adopts NEC with state-specific rules) |
| Fire alarm interface (if PV on fire alarm building) | NFPA 72, Chapter 23 (signaling) — rarely direct, but be aware |
| Utility interconnection | MGL c.164, 220 CMR (DPU rules) — not on exam directly |
1.11 Common Exam Traps
89.Forgetting the second 1.25 for PV output circuits. Source circuits get one 1.25; output circuits get two (1.56 total).
90.Using AC overcurrent devices on DC circuits. Always check DC rating.
91.Confusing rapid shutdown voltage limits (80 V inside, 30 V outside the array).
92.Applying temperature corrections to the 1.25 factor. Correct the conductor ampacity, not the load.
93.Omitting ground-fault protection on grounded systems. It is mandatory, not optional.
94.Sizing the AC disconnect at 100% of inverter output. It must be at least 125% of continuous current.
95.Forgetting that module frames require equipment grounding even if the system is ungrounded. Equipment grounding is always required.
96.Not checking the maximum series fuse rating on the module label. If parallel strings exceed this, add fuses.
1.12 Practical Field Points
Always verify the module Isc and Voc from the nameplate; do not rely on memory.
Use a torque wrench for all lug connections—DC terminals are often under-torqued.
Install DC conduit with expansion fittings on long rooftop runs to accommodate thermal movement.
Verify rapid shutdown initiation device is accessible to first responders (often at the meter or main panel).
Confirm that the inverter's DC input voltage does not exceed the maximum system voltage (typically 600 V for residential, 1000 V for commercial in MA).
Before energizing, measure DC voltage at the inverter input to confirm correct polarity.
Summary
PV systems require careful attention to circuit classification, current calculations, and the interaction between DC and AC requirements. Massachusetts enforces 690.12 rapid shutdown strictly, so be prepared to identify MLPE or rapid shutdown switches. Use Article 690 as your primary reference, but cross-check with Article 705 for interconnection and Article 240 for overcurrent device standards. On the exam, read each question carefully to identify whether you are dealing with a source circuit or an output circuit—the 1.25 vs. 1.56 multiplier is the most common point of failure.
