Use this PV conductor ampacity calculator to calculate maximum PV circuit current and compare the NEC 690.8(B)(1) and 690.8(B)(2) ampacity checks. Enter module Isc, parallel string count, and applicable correction and adjustment factors to find the governing minimum base conductor ampacity before selecting a conductor.
Who this is for: solar installers, PV designers, engineers, and electricians checking PV DC conductor ampacity for permit plan sets and electrical design.
Calculator note: This tool is for preliminary design checks only and does not replace a stamped plan set or AHJ review. It determines the required minimum base conductor ampacity under the standard NEC 690.8(B) method modeled here; it does not select a wire size. Use the governing result with the applicable NEC ampacity table, conductor characteristics, installation conditions, and termination limits to choose a conductor. Confirm all values and the NEC edition and local amendments adopted by the project AHJ before submitting for permitting.
How to Use the NEC 690.8 Calculator
1
Enter the module's rated short-circuit current, Isc.
2
Enter the number of parallel strings contributing current. Use 1 for an individual string.
3
Enter the applicable ambient temperature correction factor from NEC Table 310.15(B)(1). Use 1.00 if no correction applies.
4
Enter the applicable current-carrying conductor adjustment factor from NEC 310.15(C)(1). Use 1.00 if no adjustment applies.
5
Review the governing result and confirm which check, 690.8(B)(1) or 690.8(B)(2), controls the required minimum base ampacity.
What the NEC 690.8 Calculator Checks
For the standard module Isc method modeled by this calculator, the tool determines maximum PV circuit current from module Isc and parallel string count, then runs two ampacity checks: one without adjustment or correction factors, and one with the entered adjustment and correction factors applied. The larger of the two results governs as the required minimum base conductor ampacity, subject to applicable NEC exceptions and project-specific conditions.
Maximum PV circuit current = module Isc × parallel strings × 1.25
NEC 690.8 Example Calculation
Module Isc: 11.80 A
Parallel strings: 2
Temperature correction factor: 0.80
Conductor adjustment factor: 0.70
Maximum PV circuit current: 11.80 A × 1.25 × 2 = 29.50 A
690.8(B)(1) check: 29.50 A × 1.25 = 36.88 A
690.8(B)(2) check: 29.50 A ÷ (0.80 × 0.70) = 52.68 A
Governing minimum base ampacity: 52.68 A
NEC 690.8(B)(1) vs. 690.8(B)(2): Which Check Governs?
NEC 690.8(B) requires PV circuit conductors to satisfy applicable ampacity requirements. For the standard method modeled here, this calculator runs both ampacity checks and uses the larger result as the governing minimum base ampacity.
690.8(B)(1) applies a 125% multiplier to maximum PV circuit current, without adjustment or correction factors.
690.8(B)(2) requires the conductor's ampacity after applicable temperature correction and conductor adjustment factors are applied to be sufficient for the maximum PV circuit current.
With the standard calculation modeled by this tool, the combined correction and adjustment factor determines which check is more restrictive. If the combined factor is greater than 0.80, 690.8(B)(1) governs. At exactly 0.80, the two checks are equal. Below 0.80, 690.8(B)(2) governs. In practice, conduit runs with multiple current-carrying conductors or high ambient temperatures often push the combined factor below 0.80, which is why 690.8(B)(2) frequently governs on real installations.
Temperature Correction vs. Conductor Adjustment Factors
Temperature correction and current-carrying conductor adjustment are separate conditions-of-use factors. NEC 690.8(B)(2) evaluates conductor ampacity using the applicable adjustment and correction factors.
Temperature correction factor accounts for ambient temperature and conductor insulation rating, from NEC Table 310.15(B)(1). Depending on the applicable table and conductor rating, this factor can exceed 1.00 at ambient temperatures below the table's reference temperature.
Conductor adjustment factor accounts for the number of current-carrying conductors bundled in the same raceway or cable, from NEC 310.15(C)(1). This factor only reduces ampacity, so it cannot exceed 1.00.
Entering these separately, rather than as one combined derating number, makes it clear which factor is driving the governing result.
Where PV Conductor Ampacity Appears on Solar Plans
PV conductor ampacity calculations typically appear on the electrical portion of a solar plan set alongside conductor sizes, circuit information, and one-line diagram details. During plan review, the selected conductor may be checked against the required ampacity, applicable ampacity table, insulation rating, conditions of use, and termination temperature limitations under NEC 110.14(C).
Showing the calculation clearly helps reviewers verify how the selected conductor was sized and can reduce avoidable questions during permitting.
NEC 690.8 Calculator FAQs
How do you calculate maximum PV circuit current under NEC 690.8?
Maximum PV circuit current equals module Isc multiplied by the number of parallel strings, multiplied by 1.25. This is the standard module Isc calculation method modeled by this calculator.
What is the NEC 690.8 conductor ampacity formula?
For the standard module Isc method modeled by this calculator, maximum PV circuit current equals module Isc × parallel strings × 1.25. The 690.8(B)(1) check then multiplies that maximum current by another 1.25. The 690.8(B)(2) check evaluates conductor ampacity using the maximum current with the applicable adjustment and correction factors. The larger required ampacity governs.
What is the difference between NEC 690.8(B)(1) and 690.8(B)(2)?
690.8(B)(1) applies a 125% multiplier to maximum PV circuit current without adjustment or correction factors. 690.8(B)(2) evaluates maximum PV circuit current using the applicable temperature correction and current-carrying conductor adjustment factors. Both checks apply for the standard method, and the larger result governs.
Which NEC 690.8 ampacity check governs?
For the standard calculation modeled here, 690.8(B)(1) governs when the combined adjustment and correction factor is greater than 0.80. The two checks are equal at 0.80, and 690.8(B)(2) governs when the combined factor is below 0.80.
How do temperature correction and adjustment factors affect PV conductor ampacity?
Temperature correction accounts for ambient temperature based on the conductor's temperature rating, while conductor adjustment accounts for the number of current-carrying conductors installed together. These factors modify allowable ampacity. A lower combined factor increases the required base conductor ampacity under 690.8(B)(2).
What is 156% of Isc, and how does it relate to NEC 690.8?
Under the standard module Isc method and 690.8(B)(1) calculation modeled by this tool, the two 125% multipliers produce 156.25% of module Isc for a single string with no exceptions applied: 1.25 × 1.25 = 1.5625. Other permitted 690.8(A) calculation methods and applicable 690.8(B)(1) exceptions can change the calculation. This calculator shows the two steps separately so installers can see the maximum circuit current calculation and the subsequent conductor ampacity check.
Does this calculator select a wire size?
No. This calculator determines the governing minimum base conductor ampacity under the standard NEC 690.8(B) method. Final conductor selection still requires the applicable NEC ampacity table, conductor material and insulation rating, installation conditions, and termination temperature limits under NEC 110.14(C).