Course: Solar PV Balance-of-System Specification — CT-201
CT TECHNICAL LIBRARY • ONLINE COURSE CT-201
Solar PV Balance-of-System Specification
From module leads to the point of interconnection: string protection sizing, combiner architecture, isolation and rapid shutdown, MC4 termination quality, and AC-side protection — built around NEC 690 and the CT-31 catalog. For solar installers and PV designers. Approx. 2 hours, self-paced.
GET CT-31 (PDF) ← TECHNICAL LIBRARYMODULE 1 — THE DC SIDE IS DIFFERENT
1.1 No zero crossing: why AC gear cannot go on strings
DC current never passes through zero, so an opening contact must stretch and cool the arc to extinction — AC devices rely on the zero crossing and can sustain an internal arc on a DC fault. Every protective device on the string side must carry a V DC rating: DC-rated MCBs build voltage by series poles (typically 250 V per pole — 1P/250, 2P/600, 3P/800, 4P/1000 on the CT-B1Z-63), and polarity-sensitive frames steer the arc magnetically, so + / − markings are installation-critical.
1.2 System voltage classes: 600 / 1000 / 1500
Residential runs 600 V-class strings; commercial 1000 V; utility 1500 V. The class flows through every component choice: combiner rating, fuse format (10×38 mm to 1000 V, 10×65/85 mm at 1500 V), SPD Uc, isolator pole count, and connector rating (MC4 1000 V vs 1500 V bodies). Mixing classes is the most common takeoff error — lock the class first, then spec everything to it.
Knowledge check: Why does a 4-pole DC MCB carry a higher voltage rating than a 2-pole of the same family?
Answer
Series-connected poles add arc-chute length; each pole contributes ~250 V of interruption capability, so 4 poles in series interrupt ~1000 V DC.
MODULE 2 — STRING PROTECTION SIZING (NEC 690)
2.1 The 1.56 rule, worked
Fuse rating ≥ 1.56 × Isc(STC), and ≤ the module's max series fuse rating (690.9(B)). Worked example on a 580 W bifacial: Isc 13.9 A × 1.56 = 21.7 → next standard gPV size = 25 A; module series-fuse rating 30 A → 25 A passes. Conductor ampacity ≥ 1.56 × Isc before conditions-of-use derates (690.8) → ≥21.7 A → 4 mm² PV wire. Fusing is required when 3+ strings parallel (combined backfeed exceeds module fuse rating).
2.2 Voltage and the cold-day correction
Max system voltage = Voc(STC) corrected for the record low ambient (690.7) — use the module's temperature coefficient or Table 690.7(A). This is the number that decides your 1000 V vs 1500 V class and every downstream device rating. Design shops that skip the correction discover it during plan review; crews discover it in winter.
2.3 SPD selection and lead length
Type 2 (CT-SP) in every combiner and at the inverter DC input when home runs exceed ~10 m. Type 1+2 at the building DC entry when a lightning protection system is present. Uc ≥ max system voltage. Keep total lead length to bus and PE under 0.5 m — every extra meter adds roughly a kilovolt of let-through during a surge event.
Knowledge check: Two strings in parallel, module series-fuse rating 20 A, Isc 11.2 A. Are string fuses required?
Answer
With only two strings, the maximum backfeed into a faulted string is one string's current (≤module fuse rating with margin per 690.9(A) exception logic) — fusing generally not required at two strings; required at three or more. Verify against the module datasheet and AHJ.
MODULE 3 — COMBINER ARCHITECTURE
3.1 What lives inside, in order
String in → touch-safe gPV fuse holder per polarity (CT-PV-30/32) → copper bus → SPD across +/−/PE (CT-SP) → output isolator or breaker (CT-IS-40 / CT-B1Z / CT-M1Z) → home run out. Monitored steel combiners (CT-XLD-16/24) add per-string CTs reporting over RS-485 — spec monitoring wherever an O&M contract or performance guarantee rides on string availability.
3.2 Polymer vs. steel; sizing the box
PC+ABS (CT-JB series) for residential and light commercial — light, non-conductive, UV-stabilized. Powder-coated steel (CT-XLD) for commercial/utility — higher string counts, padlockable, conduit-friendly. Size for 20% spare positions; a full combiner on day one is a change order waiting to happen.
MODULE 4 — ISOLATION & RAPID SHUTDOWN
4.1 Where disconnects go and why they're rotary
Code and utilities want lockable, visible-break disconnection at the array (CT-IS-40MD, IP66) and at the inverter AC output (CT-F1). Quality DC isolators use an energy-storage spring so contact speed is independent of the operator — sub-3 ms break regardless of how slowly the knob turns — plus self-cleaning contacts for 10k+ cycles.
4.2 Rapid shutdown at the array level
Rapid shutdown drops conductors outside the array boundary to a safe voltage on initiation (AC loss, E-stop, or command). Array-level units (CT-RSD150, 1–10 strings) price per array instead of per panel; feedback signal confirms the open state and a 70°C thermal trip acts with no command at all. Verify your AHJ's adopted code cycle — boundary and voltage limits vary by edition.
Knowledge check: Why does sub-3 ms switching matter on a DC isolator?
Answer
DC arcs don't self-extinguish; a slow, operator-dependent contact separation sustains the arc and erodes contacts. Spring-assisted fast break minimizes arc time regardless of hand speed.
MODULE 5 — CONNECTOR QUALITY: THE CRIMP IS THE RATING
5.1 Five gates per termination
1) Strip with a PV stripper — zero nicked strands. 2) Full-cycle ratchet crimp with the MC4 die — pull-test ≥310 N. 3) Seat until the pin clicks; tug-check. 4) Torque the gland with the connector spanner — pliers destroy the IP67 seal. 5) Mate same-pattern connectors only; cross-brand mating voids listings and is the documented root cause of a large share of rooftop connector fires.
5.2 1000 V vs 1500 V bodies, diode & fused variants
Match connector class to system class. Inline fuse connectors (CT-MC4-F) put gPV protection in the home run — the cheapest code-compliant fusing on 2–4 string arrays without a fused combiner. Diode connectors (CT-MC4-D) block reverse current in parallel architectures; budget 0.4–0.7 V forward drop in string calcs.
MODULE 6 — AC SIDE & CERTIFICATION
6.1 AC protection stack, briefly
Inverter AC out → CT-F1 isolator → AC distribution with MCB/RCBO (30 mA on receptacle/EV circuits, 100/300 mA fire tiers upstream) → AC SPD at the board → point of interconnection (CT-X grid boxes give you SPD + main breaker + visible-break knife switch in one inspected assembly). Full detail in the CT-31 Section 03 reference pages.
6.2 Complete the course
Email sales@conversionstech.com, subject “CT-201 Certification”, with your four knowledge-check answers plus one line describing your typical system class. The engineering desk returns your certificate and a BOS takeoff template pre-mapped to CT part numbers. Design teams: book a free 1-hour plan-set review against NEC 690 sizing — one project per account.
Spec the whole array from one book.
CT-31 carries every family in this course with ratings matrices, ordering codes and live SKUs.
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