NEC Guidelines for Solar Design
Understanding the electrical codes and safety principles behind professional solar system design tools. Each calculator incorporates specific NEC requirements to ensure code-compliant, safe, and efficient solar installations.
Understanding Article 690
The National Electrical Code (NEC) Article 690 specifically addresses solar photovoltaic systems, providing comprehensive requirements for safe installation and operation. These guidelines ensure that solar installations protect people, property, and electrical equipment from hazards such as overcurrent, ground faults, and arc faults.
Each design tool incorporates specific NEC requirements to ensure code-compliant system design. Understanding these electrical principles is essential for safe, legal, and efficient solar installations.
String Sizing Calculator
NEC 690.7 & 690.8Primary NEC Requirements
String sizing must comply with voltage limitations to protect inverters and ensure system safety under all operating conditions.
Requires that the maximum PV system voltage be calculated using the open-circuit voltage (Voc) of the modules at the lowest expected ambient temperature. This prevents overvoltage conditions that could damage equipment or create safety hazards.
PV circuit currents must be calculated at 125% of the sum of parallel source circuit maximum currents. This ensures adequate conductor sizing and prevents overheating under sustained peak conditions.
Electrical Principles
Solar modules produce higher voltages in cold temperatures due to the temperature coefficient of voltage. The string sizing calculator applies temperature correction factors based on the coldest expected ambient temperature using NEC Table 690.7. For example, a module with 48V Voc might reach 54V at -10°C.
Exceeding inverter MPPT voltage limits can cause permanent damage to power electronics. The calculator ensures strings remain within safe operating ranges across all temperature conditions, protecting expensive equipment and maintaining warranty compliance.
Wire Sizing Calculator
NEC 690.8 & Article 310Primary NEC Requirements
Conductor sizing ensures safe current-carrying capacity with appropriate derating for environmental conditions.
PV system conductors must be sized for at least 125% of the circuit’s maximum current. This provides a safety margin for continuous operation in full sunlight conditions.
Conductor ampacity must be adjusted for ambient temperature using correction factors from NEC Table 310.15(B)(1). Additionally, conductors carrying continuous loads require an additional 125% multiplier per NEC 210.19(A)(1).
OCPD (breakers/fuses) must be sized between 100% and 125% of the calculated circuit current. This ensures protection activates before conductors overheat while avoiding nuisance tripping.
Electrical Principles
Wire resistance causes heat generation proportional to I²R. As ambient temperature increases, conductors have less capacity to dissipate this heat. Rooftop installations commonly experience 60-75°C ambient temperatures, reducing ampacity by 30-40%. The calculator applies these derating factors to ensure wires never exceed their thermal limits.
Undersized conductors are a fire hazard. PV systems operate continuously in hot environments, and module currents can exceed nameplate ratings under certain conditions. Proper wire sizing with temperature derating ensures conductors remain cool even during peak summer operation.
Voltage Drop Calculator
NEC 690.8 & Informative Annex DPrimary NEC Requirements
While not a strict requirement, the NEC provides voltage drop recommendations to ensure efficient system operation.
The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total for combined feeder and branch circuits. These recommendations prevent efficiency losses and ensure equipment operates within rated parameters.
Electrical Principles
Voltage drop occurs due to conductor resistance: Vdrop = I × R × 2 (for round-trip distance). For a 30A circuit over 100 feet using 10 AWG copper (1.018Ω per 1000ft), the voltage drop is: 30A × 1.018Ω × 0.2 = 6.1V, or 2.5% on a 240V system.
Excessive voltage drop reduces power transfer efficiency. For DC PV circuits, this directly reduces energy harvest. For AC circuits, voltage drop can cause inverters to trip on undervoltage conditions or reduce power output.
Every volt dropped is energy lost as heat in the conductors. A 3% voltage drop represents 3% of system production permanently lost. Over 25 years, this compounds to significant revenue loss. While oversizing conductors costs more upfront, the energy savings typically justify the investment within 5-7 years.
Conduit Size Calculator
NEC Chapter 9, Tables 4 & 5Primary NEC Requirements
Conduit fill limits prevent overheating and ensure conductors can be safely installed and maintained.
Limits conductor fill to 40% for three or more conductors, 31% for two conductors, and 53% for one conductor. These limits ensure adequate air circulation for heat dissipation and allow conductors to be pulled without damage.
Table 4 provides conduit dimensions and allowable fill areas. Table 5 provides conductor cross-sectional areas including insulation. These tables enable precise conduit sizing calculations.
Electrical Principles
Current-carrying conductors generate heat (P = I²R). In conduit, this heat must dissipate through the conduit walls. Overfilled conduits trap heat, causing insulation degradation and increasing fire risk. The 40% fill limit ensures adequate spacing for convective cooling.
Overfilled conduits are difficult to work with and can damage conductor insulation during installation. More critically, inadequate spacing prevents heat dissipation, effectively derating the conductors beyond NEC Table 310.15 adjustment factors. Always verify conduit fill before pulling wire.
Equipment Grounding Conductor
NEC 250.122 & 690.43Primary NEC Requirements
Equipment grounding conductors provide a low-impedance path for fault currents, enabling rapid overcurrent device operation.
EGC sizing is based on the rating of the overcurrent protective device (OCPD) ahead of the circuit, not the conductor size. Table 250.122 specifies minimum EGC sizes for different OCPD ratings.
PV systems require both equipment grounding conductors and system grounding conductors. All exposed non-current-carrying metal parts must be grounded to ensure personnel safety.
Electrical Principles
During a ground fault, the EGC must carry fault current back to the source without excessive voltage rise. The impedance of this path determines fault current magnitude, which must be sufficient to trip the OCPD within code-specified timeframes. For a 30A breaker protecting a 240V circuit, fault current typically needs to exceed 150A (5× rating) to ensure magnetic trip within 0.4 seconds.
An undersized or high-resistance EGC may not allow sufficient fault current to flow, preventing OCPD operation. This leaves energized equipment in a dangerous state, creating electrocution and fire hazards. The EGC is the critical safety conductor that ensures faults clear quickly and safely.
Expansion Fittings Calculator
NEC 352.44 & 358.44Primary NEC Requirements
Expansion fittings accommodate thermal expansion and contraction in conduit systems exposed to temperature variations.
PVC conduit systems must include expansion fittings where necessary to compensate for thermal expansion and contraction. The need is determined by expected temperature change and conduit length.
EMT systems require expansion fittings when installed where temperature changes exceed the conduit’s ability to expand without damage to the system or building structure.
Electrical Principles
Materials expand and contract with temperature according to their coefficient of thermal expansion. PVC expands significantly (about 3.38 inches per 100 feet per 100°F), while metal conduits expand less. Rooftop PV conduits can experience 150°F temperature swings between winter nights and summer days.
Without expansion fittings, thermal stress causes conduit to bow, pull apart at joints, or damage building attachments. The calculator determines maximum spacing based on material properties and expected temperature range.
Rooftop conduit runs are particularly vulnerable to thermal cycling. Failed joints can expose conductors to moisture and physical damage. Expansion fittings prevent these failures, ensuring long-term system integrity and preventing costly repairs or safety hazards.
Modules in Area Calculator
NEC 690.12 & Building/Fire CodesPrimary NEC Requirements
Array layout must provide adequate access pathways for emergency responders and comply with rapid shutdown requirements.
PV arrays must include rapid shutdown capability to de-energize conductors within the array boundary and within 1 foot of the array. This requires specific array layouts that accommodate shutdown equipment and maintain accessibility.
Building and fire codes typically require 3-foot pathways around roof perimeter and 4-foot pathways through arrays. These ensure firefighter access and ventilation during emergencies.
Electrical Principles
Array configuration affects voltage and current characteristics, string count, wire routing, and DC optimizer/microinverter placement. Proper spacing ensures conductors can be routed safely while maintaining required clearances from combustible materials per NEC 690.31(E).
Firefighters need safe access to roofs even when arrays are energized. Rapid shutdown requirements mean array layouts must accommodate module-level shutdown devices. Planning for these requirements during initial layout prevents costly redesigns and ensures code compliance.
Battery Sizing Calculator
NEC 706 & 690.71Primary NEC Requirements
Energy storage systems must comply with specific requirements for disconnects, overcurrent protection, and system grounding.
Covers requirements for battery storage systems including disconnecting means, overcurrent protection, ventilation, and working space clearances. Battery circuits must be sized for maximum charge and discharge currents with appropriate safety factors.
When batteries are part of a PV system, they must comply with both Article 690 and Article 706. This includes proper location, ventilation, and integration with PV circuits.
Electrical Principles
Battery sizing involves calculating amp-hours needed based on daily energy consumption, days of autonomy, depth of discharge limits, and temperature effects. Lead-acid batteries lose capacity at low temperatures and suffer reduced lifespan if deeply discharged. Lithium batteries have different charge/discharge characteristics and thermal management requirements.
Conductor sizing for battery circuits must account for high currents during charging and discharging. A 48V, 10kWh battery bank can deliver 200A+ surge currents, requiring substantial conductors and properly rated overcurrent protection.
Undersized battery banks won’t meet backup power requirements, while oversized banks waste money and space. Battery chemistry selection affects NEC compliance—lithium systems may require fire suppression per Article 706.12. Proper sizing ensures reliable backup power while maintaining code compliance and safety.