13.3 Solar Photovoltaic (PV) Systems & Energy Storage Systems (Articles 690 & 706)
Key Takeaways
- Maximum PV DC voltage calculation under NEC 690.7 requires adjusting module open-circuit voltage (Voc) for the lowest expected ambient temperature using manufacturer temperature coefficients or NEC Table 690.7(A) correction factors.
- PV circuit conductor ampacity and overcurrent protection must be sized to at least 156% of module short-circuit current (Isc) under NEC 690.8 (125% maximum circuit current x 125% continuous load factor = 1.5625).
- Rapid Shutdown of PV systems on buildings under NEC 690.12 mandates reducing controlled conductors outside the array boundary to 30V or less within 30 seconds, and conductors inside the array boundary to 80V or less within 30 seconds.
- Under the NEC 705.12(B) 120% Busbar Rule, the sum of the main service overcurrent device and all inverter backfeed breakers cannot exceed 120% of the busbar ampere rating: Max PV Breaker = (Busbar Rating x 1.20) - Main Breaker Rating.
- Energy Storage Systems (ESS) governed by NEC Article 706 mandate readily accessible, lockable disconnecting means (706.15), 110.26 working space clearances, and adequate ventilation to mitigate thermal runaway off-gas hazards.
13.3 Solar Photovoltaic (PV) Systems & Energy Storage Systems (Articles 690 & 706)
Renewable energy installations require specialized engineering knowledge bridging direct-current (DC) power generation, solid-state power electronic inverters, grid interconnection rules, and electrochemical storage chemistry. NEC Article 690 (Solar Photovoltaic Systems), Article 705 (Interconnected Power Production Sources), and Article 706 (Energy Storage Systems) govern these complex systems.
On the Idaho Journeyman Electrician examination, candidates must execute temperature-adjusted DC voltage calculations, determine conductor ampacities with double 125% multipliers, verify Rapid Shutdown timing and boundary voltage limits, and calculate maximum solar backfeed breakers under the 120% Busbar Rule.
1. Maximum PV System DC Voltage Calculation (NEC 690.7)
Photovoltaic crystalline silicon cells exhibit a negative temperature coefficient: as ambient temperature decreases, the open-circuit voltage (Voc) of the solar module increases significantly. If an array is designed only for standard test condition (STC) temperatures (25°C / 77°F), cold winter mornings in Idaho (where temperatures frequently drop to -20°C / -4°F or lower) will produce dangerously elevated DC voltages, destroying inverters and exceeding the 600V dwelling limit or 1000V commercial limit.
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| PV TEMPERATURE VOLTAGE RELATIONSHIP |
| |
| HIGHER AMBIENT TEMP (e.g. +45°C) ---> LOWER DC VOLTAGE (Module V drops) |
| STC RATING BENCHMARK (25°C / 77°F) ---> RATED Voc (Nameplate Benchmark) |
| LOWER AMBIENT TEMP (e.g. -20°C) ---> HIGHER DC VOLTAGE (Voltage rises!)|
| |
| * NEC 690.7 MANDATE: Sizing MUST use the lowest expected ambient temp! |
+-----------------------------------------------------------------------------+
Calculation Method 1: Manufacturer Temperature Coefficient Formula
Where manufacturer coefficient data is available, NEC 690.7(A) authorizes the exact formula:
Voc_max = Voc_stc * [1 + (Temp_Coeff_(%/°C) * (Tmin - 25°C))]
Comprehensive Calculation Example:
- PV Module STC Open-Circuit Voltage (Voc): 45.0 Volts
- Temperature Coefficient of Voc: -0.28% / °C (-0.0028 / °C)
- Lowest Expected Ambient Temperature in Boise, ID (Tmin): -15°C
- Number of Modules in Series per String: 10 modules
- Calculate Temperature Delta: ΔT = -15°C - 25°C = -40°C
- Calculate Voltage Multiplier: 1 + (-0.0028 * -40) = 1 + 0.112 = 1.112 (11.2% voltage increase)
- Calculate Single Module Voc_max: 45.0 V * 1.112 = 50.04 Volts
- Calculate 10-Module Series String Voltage: 10 * 50.04 V = 500.4 Volts DC
- Code Compliance Check: 500.4 V ≤ 600 V limit for residential dwellings (NEC 690.7(C)) -> COMPLIANT.
Calculation Method 2: NEC Table 690.7(A) Correction Factors (Crystalline Silicon)
| Lowest Expected Ambient Temperature (°C) | Lowest Expected Ambient Temperature (°F) | Table 690.7(A) Correction Factor |
|---|---|---|
| 20°C to 24°C | 68°F to 76°F | 1.02 |
| 15°C to 19°C | 59°F to 67°F | 1.04 |
| 10°C to 14°C | 50°F to 58°F | 1.06 |
| 5°C to 9°C | 41°F to 49°F | 1.08 |
| 0°C to 4°C | 32°F to 40°F | 1.10 |
| -1°C to -5°C | 23°F to 31°F | 1.12 |
| -6°C to -10°C | 14°F to 22°F | 1.14 |
| -11°C to -15°C | 5°F to 13°F | 1.18 |
| -16°C to -20°C | -4°F to 4°F | 1.20 |
| -21°C to -25°C | -13°F to -5°F | 1.23 |
| -26°C to -30°C | -22°F to -14°F | 1.25 |
2. PV Circuit Sizing & The 156% Sizing Rule (NEC 690.8)
Photovoltaic source and output circuits are subject to two compounding current factors:
- Edge-of-Cloud Effect (Irradiance Factor): Solar irradiance can momentarily exceed the 1000 W/m² standard test benchmark due to atmospheric cloud reflections, generating currents up to 125% of module rated short-circuit current (Isc). Thus, Maximum Circuit Current = Isc * 1.25.
- Continuous Load Multiplier: PV systems operate at peak capacity for hours continuously, requiring a standard 125% continuous load factor per NEC 690.8(B).
Total Sizing Multiplier = 1.25 * 1.25 = 1.5625 (156.25% of Isc)
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| PV CONDUCTOR & OCPD SIZING CALCULATION |
| |
| [ PV MODULE NAMEPLATE: Isc = 10.0 Amperes ] |
| | |
| Step 1: Calculate Max Circuit Current (NEC 690.8(A)) |
| I_max = 10.0A x 1.25 = 12.5 Amperes |
| | |
| Step 2: Apply Continuous Load Factor (NEC 690.8(B)) |
| Minimum Conductor & OCPD = 12.5A x 1.25 = 15.625 Amperes |
| | |
| Step 3: Direct Formula Check |
| 10.0A x 1.5625 = 15.625 Amperes |
| | |
| Step 4: Select Standard OCPD (NEC 240.6) |
| Next standard fuse/breaker = 20 Amperes |
| Conductor = #12 AWG Copper THHN (Ampacity >= 15.63A) |
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3. Rapid Shutdown of PV Systems on Buildings (NEC 690.12)
When firefighters respond to structural fires, rooftop PV arrays remain energized by sunlight even after the main utility service is disconnected. NEC 690.12 mandates Rapid Shutdown systems to protect first responders from high-voltage DC electrocution.
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| NEC 690.12 RAPID SHUTDOWN BOUNDARIES & LIMITS |
| |
| +=====================================================================+ |
| | PV ARRAY BOUNDARY (1 ft around array) | |
| | +-------------------------------------------------------------+ | |
| | | [PV MODULE] --- [PV MODULE] --- [PV MODULE] --- [PV MODULE] | | |
| | | | | |
| | | INSIDE ARRAY BOUNDARY: <= 80 VOLTS within 30 SECONDS | | |
| | | (Achieved via MLPE / Module-Level Microinverters/Optimizers)| | |
| | +-------------------------------------------------------------+ | |
| +=================================||==================================+ |
| || 1 ft (305 mm) Boundary Limit |
| || |
| OUTSIDE ARRAY BOUNDARY: || 3 ft from building penetration |
| <= 30 VOLTS within 30 SECONDS || |
| vv |
| [ RAPID SHUTDOWN INITIATOR ] |
| (External Service Switch / Inverter Switch) |
+-----------------------------------------------------------------------------+
Rapid Shutdown Voltage & Timing Thresholds (NEC 690.12(B))
| Controlled Conductor Location | Maximum Voltage Threshold | Time Limit | Primary Engineering Method |
|---|---|---|---|
| Outside Array Boundary (conductors located more than 1 ft from array or entering building) | ≤ 30 Volts | 30 Seconds | Automatic line-commutated contactors or string disconnect at array edge |
| Inside Array Boundary (conductors within 1 ft of array or between modules) | ≤ 80 Volts | 30 Seconds | Module-Level Power Electronics (MLPE): DC optimizers with keep-alive signal or microinverters |
[!NOTE] Rapid Shutdown Initiation (NEC 690.12(C)): The rapid shutdown initiation device must be installed in a readily accessible location outside the building (such as the main service disconnect, a dedicated labeled rapid shutdown switch, or a PV system disconnect) and marked with a permanent plaque per NEC 690.56(C).
4. Point of Connection & The 120% Busbar Rule (NEC 705.12(B))
When interconnecting solar inverters on the load side of a service panelboard, current feeds into the busbar from two opposing directions: the utility main breaker and the solar backfeed breaker. To prevent busbar thermal overheating while avoiding a costly main panel upgrade, NEC 705.12(B)(2)(3)(b) establishes the famous 120% Busbar Rule.
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| THE 120% BUSBAR RULE FORMULA |
| |
| (Busbar Rating x 1.20) >= (Main Breaker Rating + Total Solar Breakers) |
| |
| Max Solar Backfeed Breaker = (Busbar Rating x 1.20) - Main Breaker |
+-----------------------------------------------------------------------------+
+-----------------------------------------------------------------------------+
| 120% BUSBAR RULE PANELBOARD ARCHITECTURE |
| |
| +-------------------------------------------------------+ |
| | [ 200A MAIN SERVICE BREAKER ] <--- Utility Input | |
| +-------------------------------------------------------+ |
| | || | |
| | 200A RATED BUSBAR || (200A x 1.20 = 240A Max) | |
| | || | |
| | [Branch Loads] || [Branch Loads] | |
| | [Branch Loads] || [Branch Loads] | |
| | [Branch Loads] || [Branch Loads] | |
| | || | |
| +-------------------------------------------------------+ |
| | [ 40A PV BACKFEED BREAKER ] <--- Solar Inverter | |
| | (Installed at OPPOSITE END of busbar from Main!) | |
| +-------------------------------------------------------+ |
+-----------------------------------------------------------------------------+
Step-by-Step 120% Busbar Calculation Examples
-
Case A: Standard 200A Panel with 200A Main Breaker
- Busbar Rating = 200A
- Apply 120% Multiplier: 200A * 1.20 = 240A
- Subtract Main Breaker: 240A - 200A = 40A maximum PV breaker
-
Case B: 200A Panel derated with 150A Main Breaker ("Main Breaker Derate")
- Busbar Rating = 200A
- Apply 120% Multiplier: 200A * 1.20 = 240A
- Subtract Derated Main Breaker: 240A - 150A = 90A maximum PV breaker
-
Case C: Standard 100A Panel with 100A Main Breaker
- Busbar Rating = 100A
- Apply 120% Multiplier: 100A * 1.20 = 120A
- Subtract Main Breaker: 120A - 100A = 20A maximum PV breaker
[!IMPORTANT] Mandatory Opposite-End Placement & Labeling: Under NEC 705.12(B)(2)(3)(b), the PV backfeed breaker MUST be positioned at the opposite (bottom) end of the busbar from the utility main breaker. This guarantees that current distributed along the busbar never exceeds 200A at any point. The panel must have a permanent warning label: "WARNING: INVERTER OUTPUT CONNECTION; DO NOT RELOCATE THIS OVERCURRENT DEVICE."
Under NEC 690.8, what total combined multiplier must be applied to a PV module's rated short-circuit current (Isc) to determine the minimum ampacity of circuit conductors and overcurrent devices?
According to NEC 690.12, what are the maximum allowable voltage and timing limits for controlled conductors located OUTSIDE the array boundary upon initiation of rapid shutdown?
A residential electrical service panel has a busbar rating of 200 amperes and a 200-ampere main circuit breaker. Under the NEC 705.12(B) 120% rule, what is the maximum rating of the solar PV inverter backfeed circuit breaker that can be installed at the opposite end of the busbar?