Every grid-tied or off-grid system is the same chain: modules wired into series strings, strings landed in a combiner or directly on the inverter, DC and AC disconnects, an inverter (string, hybrid or microinverter), AC overcurrent protection at the point of interconnection, and, increasingly, a battery energy storage system (BESS) on the DC bus or on the AC side.

What changes job to job is voltage class and current. Residential strings run to 600 V, commercial to 1000 V, and utility and large C&I to 1500 V. Those classes drive every connector, fuse, disconnect and wire rating you will buy. A 1000 V MC4 on a 1500 V string is a rejected inspection and a fire risk.

Think of the BOM in zones: module zone (PV wire, MC4 connectors, branch adapters, grounding clips), combiner zone (fuses, fuse holders, DC disconnects, lugs), inverter zone (AC disconnect, EMT and fittings, compression lugs), and storage zone (DLO or welding-class battery cable, battery lugs, busbar, Class T or MRBF fusing).

Module-to-module and module-to-combiner wiring in the array is almost always PV wire (UL 4703, 1000 V or 1500 V, 90 °C wet, sunlight resistant) or USE-2 where permitted. PV wire has thicker insulation and is required where the conductors are exposed and not in a raceway on ungrounded (transformerless) systems, which is nearly every modern inverter.

The MC4-style connector is a locking, touch-safe, IP67 connector rated for the string voltage. Three things determine a good MC4 termination:

• Correct strip and crimp. Strip to the contact's gauge window (10 AWG is standard, 12 and 8 AWG are common), crimp with a ratcheting MC4 crimper so the contact closes on the strands, not the insulation, and pull-test.
• Gland nut torque. Hand-tight plus the wrench flat. Under-torqued glands admit water; over-torqued crack the housing.
• Same brand mating. NEC 690.33(C) requires mating connectors to be of the same type and brand, or listed and identified as intermateable. A Stäubli MC4 mated to a generic MC4 is a code violation and the most common cause of connector fires. Keep one brand per string, and if a customer runs a specific brand, build the leads in that brand.

Branch adapters (Y connectors) let you parallel strings at the array, but they carry the sum of the string currents: check the adapter rating and remember that paralleling more than two strings generally requires fusing per 690.9.

String design is two limits. The maximum string voltage is the module's open-circuit voltage (Voc) corrected for the lowest expected ambient temperature (NEC 690.7, using the module's temperature coefficient or Table 690.7(A) for crystalline silicon) multiplied by the number of modules in series. It must stay under the inverter's maximum DC input voltage and the system voltage class. Cold mornings push Voc up 10 to 20 percent over the datasheet STC value.

The maximum circuit current is 125 percent of the module short-circuit current (Isc) per 690.8(A). For continuous-duty sizing, conductors and overcurrent devices are then sized at 125 percent of that value again (690.8(B)), giving the familiar 1.56 × Isc.

Conductor ampacity comes from NEC 310.16 (formerly 310.15(B)(16)), using the 90 °C column for PV wire, then corrected for ambient temperature (Table 310.15(B)(1)) and for more than three current-carrying conductors in a raceway or cable (310.15(C)). Rooftop conduit in sunlight adds a temperature adder (310.15(B)(2)). The terminal temperature rating (110.14(C)) still limits the final value to the 75 °C column for most equipment.

Voltage drop is not a Code requirement but it is a revenue requirement: keep DC runs under 2 percent. Long home runs are the usual reason to step from 10 to 8 AWG.

PV source circuits need overcurrent protection when more than two strings are paralleled, or when the sum of the other strings' currents can exceed the conductor or module series fuse rating (690.9). The device is a listed PV fuse (gPV class, 1000 or 1500 V DC) in a touch-safe holder inside a combiner box, sized at 1.56 × Isc rounded up to a standard size, and never above the module's series fuse rating.

Every PV system needs a DC disconnect (690.13) within sight of the inverter or at the inverter, rated for the DC voltage and current and listed for the purpose; AC disconnects for the interactive inverter output follow 690.15 and the utility's interconnection requirements. Rapid shutdown (690.12) applies to PV systems on buildings: conductors inside the array boundary must be reduced to 80 V within 30 seconds, which in practice means module-level shutdown devices or an inverter with listed rapid-shutdown control.

Enclosures in the array and on the roof are wet locations: NEMA 3R minimum, with listed cable glands or weatherproof fittings for every penetration, and sunlight-resistant conductors or raceway.

Modules and racking have exposed metal that must be bonded to the equipment grounding conductor (690.43). There are three accepted ways: a listed grounding lug on each module frame (AL9CU dual-rated lay-in lugs with stainless hardware), listed bonding clips or washers between module frames and rails (WEEB-style), or racking systems listed as a bonding means where each module and splice is listed to carry the bond.

The equipment grounding conductor for PV circuits is sized by 250.122 based on the overcurrent device, with a 14 AWG copper minimum, and must be run with the circuit conductors when in a raceway. Where it is exposed to damage it must be 6 AWG or protected (250.120(C)).

The grounding electrode system is the building's (250.50). A separate PV array electrode is permitted (690.47) but it is bonded, not isolated: an isolated "clean ground" is a lightning path through your equipment, not away from it. Ground-mount arrays get an auxiliary electrode at the array bonded back to the system.

Use dual-rated connectors anywhere aluminum racking or aluminum conductors meet copper: AL9CU lugs, oxide inhibitor on aluminum, stainless hardware. Copper touching aluminum in a wet location is galvanic corrosion by the second year.

Storage is where currents get large. A 48 V, 10 kW inverter pulls over 200 A continuously; a 48 V BESS rack feeding a 100 kW inverter is over 2000 A across its strings. Article 706 (energy storage) and 480 (storage batteries) apply, along with the manufacturer's listing instructions, which usually dictate lug style and torque.

Conductor. Battery and inverter jumpers are built from DLO (diesel locomotive cable: 2 kV, Class I flexible tinned copper, EPR insulation, UL 44 RHH/RHW-2, 90 °C) or welding-class cable where a listing permits it. DLO's fine stranding bends into tight battery compartments and its tinned strands resist corrosion. Size from 310.16 for the continuous current × 1.25, then check voltage drop: a 48 V bus cannot afford 2 percent across a jumper.

Termination. Flexible Class I conductors need flex-rated lugs: the barrel bore is sized for the larger diameter of fine-strand cable and the lug is listed for it (UL 486A). A standard Class B lug on DLO is an under-filled crimp. Use long-barrel lugs with two crimps on larger sizes, color-keyed dies, and heat shrink over the barrel. NEMA two-hole pads (1/2 in on 1-3/4 in) are standard on UPS and switchgear; telecom uses 1/4 in on 5/8 in.

Protection and distribution. Each battery string gets a Class T or MRBF fuse at the battery terminal; distribution goes through a busbar rated for the total current. Keep jumpers the same length per string so strings share current evenly.

Mechanical work decides how long the electrical work lasts. Z-brackets and rail systems carry wind and snow load; the installer's job is the manufacturer's torque spec and stainless fasteners. Cable entry through a roof, wall or enclosure uses a listed, gasketed cable entry or an NPT cable gland with the right cord range; a bare drilled hole with sealant is a leak with a schedule.

In the array, PV wire is supported and secured so it does not touch the roof, is protected from the sharp edges of module frames, and is kept out of standing water. UV-rated stainless or polymer clips every 18 to 24 in, with a drip loop before every entry. Zip ties need to be UV-rated black nylon or stainless; standard white ties fail in two seasons.

Label the DC system: source and output circuit labels, disconnect labels, rapid shutdown placards (690.56) and the directory at the service. Inspectors look at labels first.