How to Calculate Solar Cable Size — NEC and IS Methods Compared

Cable sizing is one of those solar design tasks that looks simple until you do it wrong. An undersized cable overheats and degrades over 25 years, increasing resistance and reducing yield. An oversized cable wastes capital. The correct cable size is the smallest conductor that satisfies all three constraints simultaneously: ampacity (current-carrying capacity), voltage drop (resistive loss), and short-circuit withstand capacity. Both the National Electrical Code (NEC 2023) for the USA and Indian Standards (IS 732 + CEA regulations for India) provide the calculation methodology — and they differ in important ways.

Direct answer. Solar cable sizing requires three sequential calculations: (1) ampacity — the conductor must carry 125% of the module’s STC short-circuit current (Isc) per NEC 690.8 for DC, or 125% of the inverter’s rated AC output current per NEC 310.15 for AC; (2) voltage drop — the DC string cable voltage drop must not exceed 1–2% of system voltage (PVsyst uses 1% DC wiring loss as a design target); (3) short-circuit withstand — the cable must handle the fault current without insulation damage for the protection device clearing time. IS 732 uses the same three-constraint framework with different correction factors for Indian ambient temperatures and installation methods. The Cable Sizing Trifecta Framework ensures all three constraints are checked before specifying any conductor.

This tutorial is for all five ICPs: Rohan (Indian EPC calculating BOQ cable quantities), Mike (US residential installer sizing string cables for a NEC permit packet), Jennifer (US C&I developer reviewing a design firm’s cable sizing methodology), Suresh (Indian utility-scale developer verifying a BOQ before SECI bid), and any field engineer who needs to verify that the installed cables match the design specification.

The Cable Sizing Trifecta Framework — Three Constraints, One Cable Size

The Cable Sizing Trifecta Framework establishes the correct sequence for any solar cable sizing calculation. The final cable size is the largest size produced by any of the three constraints — the most conservative answer wins.

DC String Cable Sizing — Step-by-Step NEC Method

Example system: Residential rooftop, California (NEC 2023 jurisdiction)

• Module: 400W, Isc = 10.2A, Voc = 49.5V
• String configuration: 10 modules in series = 1 string
• 4 strings connected to the inverter
• String cable length (module to combiner/inverter): 25 meters one-way

Step 1: Calculate the NEC design current (NEC 690.8)

NEC 690.8(A) states that the maximum current for a PV source circuit (one string) is 125% of the module’s STC Isc. NREL’s photovoltaic wiring and code compliance guide provides worked examples and explains the 1.25× rationale in detail.

Design current = 1.25 × Isc = 1.25 × 10.2A = 12.75A

Step 2: Find the base ampacity for the design current

For a 90 degrees C-rated PV cable (USE-2 or PV wire in NEC 310.15 terminology), the ampacity from NEC Table 310.15(B)(16) for a 10 AWG conductor at 30 degrees C ambient is 40A. This is well above 12.75A, suggesting 10 AWG is ampacity-adequate without derating.

Step 3: Apply temperature derating

California rooftop in summer: ambient temperature in conduit on roof can reach 60 degrees C. For a 90 degrees C insulation cable at 60 degrees C ambient, the temperature derating factor from NEC Table 310.15(B)(2)(a) is 0.71.

Derated ampacity = 40A × 0.71 = 28.4A

Still above 12.75A — 10 AWG remains adequate on ampacity.

Step 4: Calculate voltage drop

One-way cable length = 25m. Total conductor length (2-way for voltage drop) = 50m = 0.05 km.

For 10 AWG (5.26 mm2) copper conductor, DC resistance from manufacturer data: approximately 3.4 Ohm/km at 20 degrees C.

At 60 degrees C operating temperature, resistance increases by approximately 15%: 3.4 × 1.15 = 3.91 Ohm/km.

Voltage drop = I × R × L = 12.75A × 3.91 Ohm/km × 0.05 km = 2.49V

System DC voltage (10 modules × 49.5V Voc) = 495V.

Voltage drop % = 2.49 / 495 = 0.50%

This is below the 1% target — 10 AWG is acceptable for voltage drop.

Step 5: Verify short-circuit withstand

For a string cable with one string, the fault current is Isc = 10.2A — the same as the operating current. The protection device (fuse or OCPD) is sized at 15A or 20A. A 10 AWG copper conductor in PVC conduit has a short-circuit withstand capacity far exceeding the fault current at this level. This constraint does not govern.

Result: 10 AWG (or 6 mm2 metric equivalent) copper USE-2 or PV wire is correct for this string cable.

DC String Cable Sizing — Step-by-Step IS 732 Method

Example system: C&I rooftop, Gujarat (IS 732 + CEA regulations)

• Module: 545W bifacial, Isc = 13.8A, Voc = 49.5V
• String configuration: 16 modules in series = 1 string
• 6 strings to one inverter MPPT
• String cable length: 35 meters one-way

Step 1: Calculate the IS 732 design current

The design approach under IS 732 for solar DC circuits aligns with NEC methodology: the design current is 1.25 × Isc. IS 732:2019 published by the Bureau of Indian Standards is the applicable code for wiring installations, complemented by IS 694 and IS 7098 for cable specifications and CEA Regulations 2010 (amended 2023) for solar-specific requirements.

Design current = 1.25 × 13.8A = 17.25A

Step 2: Select cable cross-section from IS 694 ampacity table

IS 694 (PVC insulated cables) and IS 7098 (XLPE cables) provide ampacity tables for Indian conditions. For a 6 mm2 copper conductor with XLPE insulation, the base ampacity at 30 degrees C ambient (IS 694 reference condition) is approximately 50A.

Step 3: Apply derating for Indian conditions

Indian ambient temperatures on rooftops in Gujarat reach 45–50 degrees C during June–August. For XLPE insulation (90 degrees C rated) at 50 degrees C ambient, the temperature derating factor (IS 732 Annex B) is approximately 0.75.

Derated ampacity = 50A × 0.75 = 37.5A

37.5A > 17.25A — 6 mm2 is ampacity-adequate.

Step 4: Voltage drop calculation

Two-way cable length = 70m = 0.07 km.

6 mm2 copper conductor DC resistance: approximately 3.08 Ohm/km at 20 degrees C.

At 60 degrees C: 3.08 × 1.15 = 3.54 Ohm/km.

Voltage drop = 17.25 × 3.54 × 0.07 = 4.28V

System DC voltage (16 × 49.5V) = 792V.

Voltage drop % = 4.28 / 792 = 0.54% — within the 1% target.

Result: 6 mm2 copper XLPE cable is correct for this string cable.

NEC vs IS Method — Key Differences and When They Matter

Design Element	NEC Method	IS 732 Method	Practical Difference
Design current multiplier	1.25 × Isc	1.25 × Isc	Same
Reference ambient temperature	30 degrees C (NEC Table)	30 degrees C (IS table)	Same base
Derating for high ambient	From NEC Table 310.15(B)(2)(a)	From IS 732 Annex B	Similar factors; minor numerical differences
Cable insulation standard	USE-2 / PV Wire / THHN	IS 694 (PVC) / IS 7098 (XLPE)	Different product standards; verify cross-section match
Voltage drop target	1-3% per NEC guidelines	1% per IEC 62548 and CEA	NEC allows slightly more flexibility
Minimum string cable size	10 AWG (5.26 mm2) typical	4 mm2 per CEA guidelines	Metric vs AWG size mapping needed
Earth conductor sizing	Per NEC 690.45	Per IS 3043 and CEA	Different methodologies; see IS 3043

AC Cable Sizing — From Inverter Output to the Grid Connection

AC cable sizing uses the same three-constraint framework, but the design current basis changes. For AC cables from the inverter to the service panel (residential) or step-up transformer (commercial):

Design current = 1.25 × Inverter rated AC output current

For a 10 kW single-phase inverter at 240V AC output: AC current = 10,000W / 240V = 41.7A. Design current = 1.25 × 41.7 = 52A.

For a 100 kW three-phase inverter at 415V AC output (India): Phase current = 100,000 / (1.732 × 415) = 139A. Design current = 1.25 × 139 = 174A.

Voltage drop for AC cables:

The voltage drop calculation for AC cables uses the same formula but adds the reactive component (which matters at power factors below 0.95):

Voltage drop (V) = I × (R cos phi + X sin phi) × L

Where R is resistance (Ohm/km), X is reactance (Ohm/km), phi is the power factor angle, and L is one-way length in km. For most solar applications with unity or near-unity power factor, the reactive component is negligible and the formula simplifies to the DC form.

AC voltage drop target: NEC allows up to 3% voltage drop on branch circuits; IEC and IS guidelines suggest 2% maximum for power-quality-sensitive applications. A 1% maximum is conservative and appropriate for high-value projects.

Earthing Conductor Sizing — IS 3043 vs NEC 690.45

Earthing (grounding) conductors protect equipment and personnel by providing a low-impedance path for fault currents. The sizing methods differ significantly between India and the USA. IEC 62548 — Design requirements for photovoltaic installations — provides an internationally recognized framework that harmonizes elements of both the NEC and IS approaches for designers working across markets.

Indian method (IS 3043):

IS 3043 requires the earthing conductor cross-section to be at least 50% of the phase conductor cross-section, with a minimum of 6 mm2 copper for exposed earth conductors and 16 mm2 for buried earth conductors. The wire sizing glossary entry covers the IS 3043 calculation methodology.

For a 35 mm2 AC main cable: minimum earth conductor = 50% × 35 = 17.5 mm2 → use 25 mm2 (next standard size).

US method (NEC 690.45 and NEC 250):

NEC uses an equipment grounding conductor (EGC) sizing table (NEC Table 250.122) based on the overcurrent protection device (OCPD) rating. For a 60A OCPD protecting the DC circuit, the minimum EGC is 10 AWG copper. For a 200A OCPD on the AC main, the minimum EGC is 6 AWG copper.

The Earthing Conductor Comparison:

System Parameter	IS 3043 Minimum	NEC 690.45 Minimum
String cable 6 mm2	6 mm2 earth (minimum)	10 AWG (5.26 mm2)
AC main 35 mm2	25 mm2 earth	Based on OCPD; see NEC Table 250.122
Structure earthing	16 mm2 buried	10 AWG or per NEC Table 250.122
Buried earth electrode	6m minimum rod depth or mat	8-foot electrode per NEC 250.52

Common Cable Sizing Mistakes in Solar BOQs

Solar BOQs consistently show three types of cable sizing errors, all in the direction of under-specification:

Error 1: Using cable catalog ampacity without derating. The ampacity in a cable catalog is the base value at 30 degrees C ambient in free air. Without applying the temperature derating factor (0.71–0.82 for Indian rooftop temperatures), the cable appears oversized in the catalog but is undersized in actual operating conditions.

Error 2: Voltage drop calculated at rated current, not at design current. Some designers calculate voltage drop using the module’s Imp (maximum power current) rather than 1.25 × Isc. This can produce a cable size that meets voltage drop limits at normal operation but fails the ampacity check at the correct design current.

Error 3: Using a single cable run length for all strings. In a realistic rooftop layout, string cable lengths vary from 5m (nearest row to inverter) to 60m (farthest row). A BOQ that uses a single average length for all strings will underspecify cable for the longest runs and overspecify for the shortest. The correct approach is to calculate voltage drop for the worst-case (longest) run and apply that cable size to all strings, or to use two cable sizes — one for short runs and one for long runs.

The voltage drop glossary entry and the wire sizing glossary entry provide the calculation references for both NEC and IS methods.

How Heaven Designs Helps

Heaven Designs’ BOQ documents include a complete cable sizing schedule with the Cable Sizing Trifecta calculations for every conductor type — DC string cables, home-run cables, AC cables, and earthing conductors. The schedule shows the design current, derated ampacity, voltage drop percentage, and the final specified cable size for each run type.

• Solar Rooftop Detailed Engineering Design — Complete BOQ with cable sizing schedule, DISCOM format SLD, and structural drawing.
• Electrical CEIG Drawings — CEIG-ready electrical drawings with cable sizing documented for state electrical inspector review.
• Solar Permit Design — NEC 2023-compliant permit packets with conductor sizing per NEC 690.8 and NEC 310.15 documented in the SLD.
• Solar Ground Mount Design — Utility-scale cable BOQ with IS 732 method documented and auditable.
• Download a sample deliverable — Sample BOQ with cable sizing schedule included.

Contact us for a cable sizing review on an existing project or a full BOQ with sizing calculations for a new project.

FAQ

What is the correct design current for solar DC string cables under NEC 2023?

Under NEC 690.8, the maximum current for a PV source circuit is 125% of the module’s STC short-circuit current (Isc). This 1.25x factor accounts for the continuous operation of solar systems. If you have a module with Isc = 10.2A, the design current for the string cable is 12.75A. This design current must be less than the cable’s derated ampacity (base ampacity from NEC Table 310.15(B)(16), derated for ambient temperature and conduit fill).

What cable cross-section is standard for solar string cables in India?

The most common string cable cross-section in Indian C&I solar projects is 4 mm2 or 6 mm2 XLPE copper cable (meeting IS 7098 or IS 694). For modules with Isc around 10–12A (350–450W modules), 4 mm2 is adequate for short runs (under 20m one-way). For modules with Isc of 13–15A (500–600W bifacial modules) or for cable runs above 25–30m, 6 mm2 is required to meet both ampacity and voltage drop criteria. The minimum acceptable string cable per CEA guidelines is 4 mm2 copper.

How do I calculate voltage drop for a solar DC string cable?

Voltage drop (V) = Design current (A) × DC resistance of cable (Ohm/km) × two-way cable length (km). The DC resistance of the cable must be corrected for operating temperature: multiply the 20 degrees C resistance by [1 + 0.00393 × (T - 20)], where T is the operating temperature in degrees C. For a 6 mm2 copper cable at 60 degrees C, the DC resistance is approximately 3.08 Ohm/km × 1.157 = 3.56 Ohm/km. The voltage drop percentage is the calculated voltage drop divided by the system DC voltage, expressed as a percentage. The target is below 1–2%.

Does cable sizing change for bifacial modules compared to monofacial?

Yes, in one important way. Bifacial modules can produce more current than their STC Isc rating under certain conditions — high albedo ground, clear sky diffuse light on the rear surface. The bifacial gain in current is typically 3–10% above Isc. For cable sizing purposes, if you use bifacial modules with expected bifacial gain, the safe practice is to apply the full bifacial current as the basis for the 1.25x design current calculation. In practice, most designers use the STC Isc without bifacial correction, which provides an implicit additional safety margin. For high-albedo sites (desert, snow), explicitly accounting for bifacial current gain in cable sizing is the more conservative and technically correct approach.

What is the minimum earthing conductor size for a solar rooftop in India?

Per IS 3043 and CEA guidelines, the minimum earthing conductor for a solar rooftop is 6 mm2 copper for above-ground conductors and 16 mm2 copper for buried conductors. For the main earthing conductor from the inverter to the earth electrode, the minimum is the larger of 50% of the phase conductor cross-section or 16 mm2. For module frame bonding conductors, 6 mm2 is the minimum. These are minimum values — the final earthing system design must also satisfy fault current calculations for the project’s OCPD rating.

How does NEC 690.8 handle parallel strings for home-run cable sizing?

When multiple strings are combined at a combiner box, the home-run cable from the combiner box to the inverter carries the combined current of all parallel strings. Per NEC 690.8(A), the design current for this home-run cable is 1.25 × (number of parallel strings × Isc per string). For a combiner box with 4 parallel strings of Isc = 10.2A each: design current = 1.25 × 4 × 10.2 = 51A. This design current then drives the cable sizing for the home-run cable — typically 25 mm2 or 35 mm2 for 4–6 string combiners, depending on run length.

What standard governs solar cable selection in India?

The primary standards for solar cable selection in India are: IS 694 (PVC-insulated cables for general use — covers the base conductor sizing tables), IS 7098 (XLPE-insulated cables — preferred for solar applications due to higher temperature rating), and IS 732 (Code of practice for electrical wiring installations — provides installation methods and derating factors). CEA’s “Measures Relating to Safety and Electric Supply Regulations” provides the regulatory framework. For solar-specific cable (UV-resistant, halogen-free string cables), there is no dedicated Indian standard — these products are typically imported and tested against IEC 62930 or equivalent European standards.