String Sizing

Quick Facts

What is String Sizing?

String sizing is the design step of determining how many modules to wire in series within a PV string. It’s bounded by two hard constraints and several practical considerations.

Hard constraints

1. Upper bound — NEC 690.7 max system voltage. String Voc at site’s coldest expected ambient × number of modules in series ≤ 1,000 V (or 600 V residential, 1,500 V utility).
2. Upper bound — Inverter MPPT max. String Voc at coldest temperature ≤ inverter’s MPPT upper bound (sometimes lower than NEC limit).
3. Lower bound — Inverter MPPT min. String Vmp at hottest expected cell temperature ≥ inverter’s MPPT lower bound.

Practical considerations

• Inverter per-MPPT current limit (must accommodate Imp × parallel strings).
• Cable length and resulting voltage drop.
• Bifacial current uplift.
• Mismatch — keep strings same length on the same MPPT.

How to Calculate String Length

Step 1: Cold-day Voc

Voc_cold = Voc(STC) × [1 + β_Voc × (T_cold − 25)]

where β_Voc is in per-°C (typically −0.0025 to −0.0030).

Step 2: Maximum series modules

N_max = floor(V_limit / Voc_cold)

where V_limit = min(NEC 690.7 limit, MPPT_max).

Step 3: Hot-day Vmp

Vmp_hot = Vmp(STC) × [1 + β_Vmp × (T_hot − 25)]

T_hot is the effective cell temperature, typically 65–75°C.

Step 4: Minimum series modules

N_min = ceil(MPPT_min / Vmp_hot)

Step 5: Choose N

Pick N within [N_min, N_max]. Target middle to upper end for higher specific yield with margin.

Worked Example

Module: Trina Vertex S+ 440 W bifacial.

• Voc(STC) = 38.7 V, Vmp(STC) = 32.5 V.
• β_Voc = −0.0025/°C, β_Vmp = −0.0030/°C.
• Imp = 13.55 A.

Inverter: SMA Sunny Tripower CORE2 110-US.

• MPPT range: 500–800 V.
• Vmax_system: 1,000 V.
• Per-MPPT current limit: 24 A.

Site: Albuquerque NM. ASHRAE 99.6% min = −12°C. T_hot = 65°C.

Calculations

Voc_cold = 38.7 × [1 + (−0.0025)(−12 − 25)] = 38.7 × 1.0925 = 42.28 V

N_max (NEC) = floor(1000 / 42.28) = 23
N_max (MPPT) = floor(800 / 42.28) = 18

Binding constraint: MPPT max → N_max = 18

Vmp_hot = 32.5 × [1 + (−0.003)(65 − 25)] = 32.5 × 0.88 = 28.6 V

N_min = ceil(500 / 28.6) = 18

Result

Legal range: exactly 18 modules. Strings ≤ 17 fail the MPPT min; ≥ 19 fail the MPPT max.

Design choice: 18 modules per string. With per-MPPT current limit 24 A and Imp 13.55 A → 1 string per MPPT (parallel would exceed 24 A).

For a 110 kW inverter with 6 MPPTs × 1 string × 18 modules × 440 W = 47.5 kW_DC per inverter. Too little — designer should select an inverter with a wider MPPT range or different module.

Per-MPPT Current Check

I_per_MPPT = N_parallel × Imp_module × (1 + bifacial uplift)

Must satisfy I_per_MPPT ≤ inverter per-MPPT current limit.

Cable Voltage Drop

Length-induced Vd reduces voltage at the inverter:

Vmp_effective = Vmp_string − I × R_cable

Include Vd when checking Vmp ≥ MPPT_min.

Common Mistakes

1. Using STC Voc without temperature correction → NEC violation.
2. Using average annual min temp instead of ASHRAE extreme.
3. Forgetting bifacial Imp uplift in current limit check.
4. Mixing string lengths on a single MPPT.
5. Not checking per-MPPT current limit for high-Imp bifacial modules.

Best Practices

• Always document T_cold source on the SLD (ASHRAE table citation or local code).
• Provide string sizing worksheet attached to permit submission.
• Cross-check PVsyst against manufacturer string sizing tool.
• Target N at 85% of N_max for thermal and measurement margin.
• For mixed orientations, use separate MPPTs.

Standards & Certifications

• NEC 690.7 — Maximum system voltage.
• NEC 690.8 — Conductor sizing.
• IEC 62548 — PV array design.

Key Takeaways

• String sizing is bounded by NEC 690.7 max voltage (cold Voc) and inverter MPPT min (hot Vmp).
• Always temperature-correct Voc using site-specific cold temperatures (ASHRAE extreme).
• Per-MPPT current limit constrains parallel string count, especially for bifacial modules.
• Cross-validate PVsyst against manufacturer string sizing tools before permit submission.
• Document calculations on the SLD or in an attached worksheet.