Beginner's guide to wiring a string inverter

Plan the DC and AC sides before you mount anything

Before you pull the inverter out of the box, sketch the system end to end. PV array, DC conductors, inverter, AC disconnect, point of interconnection. Confirm the inverter's MPPT voltage window, max DC input current per channel, and continuous AC output current against the array string sizing and the service. If any of those numbers fight each other, fix it on paper, not on the roof.

Pull the manufacturer's installation manual and the listed conductor sizes. NEC 110.3(B) is not optional, the listing dictates torque values, conduit entries, and disconnect placement. Match string voltage at record low ambient temperature to the inverter's max DC input per 690.7. Going over by even a few volts trips the unit and voids the listing.

• Verify Voc cold using 690.7 and the local ASHRAE extreme low.
• Confirm Isc x 1.25 x 1.25 for DC conductor ampacity per 690.8(A) and 690.8(B).
• Check AC continuous output current and breaker sizing per 690.9.
• Confirm the inverter has integrated rapid shutdown initiation or plan a separate device per 690.12.

Mount and ground the inverter correctly

Outdoor string inverters need clearance for airflow and service. Read the manual for minimum side and top clearance, usually 12 to 18 inches, and keep it off the ground per the listed mounting orientation. Direct sun on a south wall in summer will derate the unit, shade it where you can.

Bond the inverter chassis with an equipment grounding conductor sized per 250.122 based on the AC overcurrent device. The DC side equipment grounding conductor is sized per 690.45. If the inverter is transformerless, the DC and AC grounded conductors are not bonded inside the unit, the system is functionally ungrounded and 690.35 applies. Mark conductors accordingly.

Tip: torque every lug twice. Once when you land the conductor, again after you've wiggled the conduit and tightened the locknuts. Loose lugs are the number one warranty call.

Land the DC strings

PV wire from the array transitions to THWN-2 or equivalent inside the building or inside metallic conduit per 690.31. Keep DC homeruns in metallic raceway anywhere they run inside a building, that is a hard rule from 690.31(B), and it exists because firefighters cannot kill the array from the street.

Polarity matters. Reversed strings will not damage a modern inverter, but they will refuse to start and you will spend an hour chasing a fault code. Meter every string open circuit before landing it, and label positive and negative at both ends. Combine strings only as the inverter allows, parallel strings on the same MPPT need matching module count and orientation.

1. Open the DC disconnect and verify zero volts at the inverter terminals.
2. Meter each string for expected Voc, within 3 percent of calculated.
3. Check string to ground for leakage, anything above a few hundred kilo-ohms is suspect.
4. Land positive and negative on the correct MPPT input, torque to spec.

Wire the AC output and interconnection

The AC output conductor is sized for 125 percent of the inverter's continuous output current per 690.8(B). The OCPD protecting the inverter output is sized per 690.9(B), again at 125 percent, and rounded up to the next standard size. Use the inverter's listed AC breaker if the manufacturer specifies one, do not substitute.

For a load side connection at the main panel, apply the 120 percent rule from 705.12(B)(3)(2). Bus rating times 1.2 must be greater than or equal to main breaker plus inverter breaker, and the inverter breaker lands at the opposite end of the bus from the main. Line side taps under 705.12(A) are cleaner on tight services but require a fused disconnect ahead of the tap.

Labels, signs, and the rapid shutdown story

Labels are not garnish, they are part of the install. The AHJ will reject a system with missing or wrong placards faster than they will reject bad wiring. Use UV rated, engraved or laminated labels, not Sharpie on duct tape.

• PV system disconnect with max voltage and current per 690.13(B) and 690.53.
• Rapid shutdown sign at the service per 690.56(C), matching the 2017 or 2020 NEC version your jurisdiction has adopted.
• Dual power source warning on the main service per 705.10.
• Interconnection point label per 705.12(B)(2)(3)(b) where applicable.

Rapid shutdown per 690.12 means conductors more than three feet from the array or more than one foot inside the array boundary drop to 30 volts within 30 seconds of the initiation device opening. Most modern string systems use module level rapid shutdown devices to meet this, confirm the inverter and the MLPE are a listed combination per 690.12(B)(2).

Commissioning checklist

Once everything is landed and torqued, commissioning is a sequence, not a vibe. Megger the DC conductors before you energize, verify polarity one more time, then close the DC disconnect, then the AC. Watch the inverter walk through its startup, log the firmware version, and connect to the monitoring portal before you leave the site.

Tip: take a photo of every torqued lug, every label, and every meter reading. When the customer calls in eight months saying production is down, that folder is your fastest diagnostic tool.

Walk the homeowner through the AC and DC disconnects, the rapid shutdown initiator, and the monitoring app. Hand off the manual, the as-built one-line, and the inspection card. Then come back in 30 days and check the production numbers against the design model, anything more than 5 percent low is worth a second look.