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) in the USA and Indian practice (IS 732, IEC 62548 and the CEA safety regulations) give you a method for calculating solar cable size, and they differ in important ways.
Quick answer. Solar cable sizing requires three sequential calculations: (1) ampacity: under NEC 690.8 the maximum PV circuit current is 125% of the module’s STC short-circuit current (Isc), and the conductor must then carry 125% of that (156% of Isc) before adjustment; IEC 62548 practice in India uses 1.25 × Isc; (2) voltage drop: DC string cable drop is usually held to about 1-2% at the operating voltage; (3) short-circuit withstand, the cable must handle the fault current without insulation damage for the protection device clearing time. Indian practice 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.
TL;DR
- Cable size is set by whichever of three constraints, ampacity, voltage drop, short-circuit withstand, demands the largest conductor.
- NEC 690.8 sets the maximum PV circuit current at 1.25 × Isc and sizes conductors at 125% of that (1.56 × Isc); IEC 62548 practice in India uses 1.25 × Isc. AC conductors are sized at 1.25 × the inverter's continuous output current.
- DC voltage drop is usually held to about 1-2%; the NEC's 3% figure for AC is a non-mandatory informational note.
- Base ampacity from a cable table must be derated for ambient temperature and installation method, Indian rooftop derating factors typically run 0.71-0.82.
- The most common BOQ mistakes are skipping temperature derating, using operating current (Imp) for the ampacity check, and applying one cable length to every string run.
- Earthing conductor sizing differs by code: Indian practice follows IS 3043 and the IEC 60364-5-54 protective-conductor table, while NEC 690.45 uses Table 250.122 based on the OCPD rating.
This tutorial is for Indian EPCs calculating BOQ cable quantities, US installers sizing string cables for an NEC permit packet, developers reviewing a design firm’s cable sizing, and field engineers checking that installed cables match the design.
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, so the most conservative answer wins.
Constraint 1: Ampacity (Current-Carrying Capacity)
The cable must carry the maximum continuous operating current without exceeding the insulation's temperature rating. For solar DC circuits, NEC 690.8 sets the maximum circuit current at 125% of STC Isc (for irradiance above STC), then requires conductors rated for 125% of that (continuous operation). IEC 62548 practice, used in India, sizes string cable for at least 1.25 × Isc. The ampacity from the cable table must then be derated for ambient temperature and installation method.
Constraint 2: Voltage Drop
The resistive voltage drop across the cable reduces the power delivered to the inverter. A common design target is about 1-1.5% DC wiring loss at STC (PVsyst's default ohmic loss is 1.5%). The annual energy loss is lower, because the array runs below STC current most of the time. For a 900V DC system, 1% is 9V across all DC wiring combined. Longer cable runs require larger conductors to maintain this voltage drop limit. The voltage drop calculation uses the cable's DC resistance (Ohms/km) from the manufacturer's datasheet.
Constraint 3: Short-Circuit Withstand Capacity
During a fault, the cable must carry the fault current for the time it takes the protection device (fuse, circuit breaker) to clear the fault without the insulation degrading. This constraint is usually checked for AC cables connected to grid protection devices with high clearing energy. For DC string cables, the fault current (parallel strings × Isc) is usually modest enough that ampacity-driven sizing naturally satisfies this constraint. The check matters most for home-run cables and AC cables.
Definition. Ampacity is the maximum continuous current a conductor can carry without exceeding the temperature rating of its insulation. Ampacity is not a fixed number for a given cable cross-section, it depends on ambient temperature, installation method (exposed, conduit, direct burial), number of bundled cables, and the insulation temperature rating (90 degrees C for XLPE/PV cable; 70 degrees C for PVC cable).
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, Vmp ≈ 41.5V
- 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.
Maximum circuit current = 1.25 × Isc = 1.25 × 10.2A = 12.75A
Conductor ampacity required before adjustment (NEC 690.8(B)) = 1.25 × 12.75A = 15.9A
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 310.16 (numbered 310.15(B)(16) before NEC 2020) for a 10 AWG conductor at 30 degrees C ambient is 40A. This is well above 15.9A, 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 correction factor from NEC Table 310.15(B)(1) is 0.71.
Derated ampacity = 40A × 0.71 = 28.4A
Still above the 12.75A maximum circuit current, so 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
String operating voltage (10 modules × 41.5V Vmp) = 415V.
Voltage drop % = 2.49 / 415 = 0.60% (conservative, since it uses 12.75A rather than Imp)
This is below the 1% target, so 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 Indian (IS/IEC) Method
Example system: C&I rooftop, Gujarat (IS 732, IEC 62548 and CEA regulations)
- Module: 545W bifacial, Isc = 13.8A, Voc = 49.5V, Vmp ≈ 41.8V
- 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 design current
Indian practice for solar DC circuits follows IEC 62548: 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 the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023.
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 approximately 50A (check the manufacturer’s table for the exact installation method).
Step 3: Apply derating for Indian conditions
Rooftop ambient temperatures in Gujarat can reach 45-50 degrees C from April to June. 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
String operating voltage (16 × 41.8V Vmp) = 669V.
Voltage drop % = 4.28 / 669 = 0.64%, within the 1% target.
Result: 6 mm2 copper XLPE cable is correct for this string cable.
1.25x
Isc multiplier for maximum current
NEC adds a second 1.25 for conductors
1%
Maximum DC voltage drop target
Common design target at STC
0.75
Temperature derating, 50 degrees C, XLPE
IS 732 Annex B; Indian rooftop conditions
4 mm2
Common minimum string cable in India
Typical in MNRE/DISCOM technical specs
Solar Cable Size: NEC vs Indian Method, Key Differences
| Design Element | NEC Method | IS 732 Method | Practical Difference |
|---|---|---|---|
| Design current multiplier | 1.25 × Isc, then 1.25 again for conductor ampacity | 1.25 × Isc (IEC 62548) | NEC is more conservative |
| 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)(1) | 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 | 3% branch / 5% total (non-mandatory NEC notes) | Project specification, typically 1-2% DC | Set it in your design basis |
| Minimum string cable size | 10 AWG (5.26 mm2) typical | 4 mm2 typical in tender specifications | Metric vs AWG size mapping needed |
| Earth conductor sizing | Per NEC 690.45 and Table 250.122 | Per IS 3043, IEC 60364-5-54 and CEA | Different methodologies |
Field tip. When converting between AWG and metric cross-sections, note that AWG sizes do not exactly correspond to metric sizes. 10 AWG (5.26 mm2) is closest to 6 mm2 metric, so if you are specifying metric cable for a NEC project, use 6 mm2 where 10 AWG is called out, not 4 mm2 (which is undersized). Always confirm with the cable manufacturer's datasheet that the specific product meets the applicable standard.
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 maximum continuous AC output current (NEC 690.8(A)(3) and 690.8(B))
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 informational notes suggest 3% on branch circuits and 5% overall; they are not mandatory. Many Indian project specifications set around 2%, and 1% is a conservative target for high-value projects.
Watch out. AC cable sizing for central inverter projects often uses the inverter's nominal current rather than its maximum continuous output current. Many inverters can deliver more than nominal current at low grid voltage or have an overload rating. Always size AC cables for the maximum continuous output current on the datasheet, not the nominal or typical output current.
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 and IEC 60364-5-54):
Protective conductors are commonly sized from the IEC 60364-5-54 table: equal to the phase conductor up to 16 mm2, 16 mm2 for phase conductors of 16-35 mm2, and half the phase conductor above 35 mm2. IS 3043 then sets the earthing system design and the fault-current (adiabatic) check. The wire sizing glossary entry covers the IS 3043 calculation methodology.
For a 35 mm2 AC main cable: minimum protective conductor = 16 mm2 copper, subject to the fault-current check.
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 | 16 mm2 (IEC 60364-5-54 table) | Based on OCPD; see NEC Table 250.122 |
| Structure earthing | 16 mm2 buried | 10 AWG or per NEC Table 250.122 |
| Buried earth electrode | Per IS 3043 design (pipe/rod electrodes, commonly 2.5-3 m) | 8-foot electrode per NEC 250.52/250.53 |
Download sample engineering deliverables from Heaven Designs
Redacted residential, commercial and utility engineering samples, including SLDs and cable layouts. See how we document cable sizing.
Get the sample pack ->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 correction factor (around 0.71-0.82 for Indian rooftop temperatures), the cable appears oversized in the catalog but is undersized in actual operating conditions.
Error 2: Mixing up the two currents. Ampacity must be checked at the code design current (1.25 × Isc, plus the second 125% under NEC). Voltage drop and wiring loss are normally estimated at operating current (Imp). Designers who run the ampacity check at Imp end up with cables that look adequate but are undersized.
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, Redacted SLDs and a 2 MWp utility package with cable layout.
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. 4 mm2 copper is the usual minimum in Indian tender specifications.
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 usual 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?
Common Indian practice uses at least 6 mm2 copper for above-ground protective conductors and 16 mm2 copper (or equivalent GI strip) for buried conductors, in line with IS 3043. For the main earthing conductor, size it from the IEC 60364-5-54 protective-conductor table and confirm it with the fault-current check.
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 maximum current for this home-run cable is 1.25 × (number of parallel strings × Isc per string), and 690.8(B) requires conductors rated for 125% of that.
For a combiner box with 4 parallel strings of Isc = 10.2A each: maximum current = 1.25 × 4 × 10.2 = 51A, and required ampacity before adjustment = 63.75A. After temperature correction and a voltage drop check on the actual run length, that typically lands at 6 AWG to 4 AWG copper (roughly 16-25 mm2).
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), 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, 2023 provide the regulatory framework. Solar-specific string cable (UV-resistant, halogen-free) is normally specified to IEC 62930 or EN 50618; check whether a BIS standard or quality control order applies to the product you buy.