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 gain adding current uplift beyond STC ratings.
• 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 ampacity and 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.

Related Reading

Once string lengths are locked in, the numbers still have to survive contact with the rest of the design. The Solar SLD software a designer uses to document the string count also auto-checks NEC 2023 voltage and conductor rules, which is where a lot of string sizing errors actually get caught before permit submission. Because the cold Voc and hot Vmp inputs used here come from a yield simulation, it’s worth reading how those same tools are scored in PV yield simulation software: the bankable yield 5 — the string sizing module inside PVsyst or Helioscope is only as good as the weather file and temperature model feeding it. For bifacial arrays specifically, QBits Energy’s own string sizing reference and MPPT glossary entry are useful cross-checks on the same voltage-window math from a different modeling stack.