Why Two Solar Inverters cannot be connected in parallel on the DC Solar Array side.

Why connecting two standard (non-parallel-compatible) string inverters to the same DC solar array is dangerous and violates electrical codes:

The Core Problem: Standard string inverters are designed to operate independently and assume they have exclusive control over the DC array they are connected to. Connecting two to the same DC source breaks this fundamental assumption, leading to multiple failure modes:

1. Voltage Conflict & “Fighting”:
   • Each inverter has its own Maximum Power Point Tracker (MPPT) algorithm constantly searching for the optimal voltage/current (the MPP) to draw maximum power from the array.
   • Two MPPTs pulling from the same array will inevitably “fight” – one tries to pull the array voltage down to its calculated MPP, while the other tries to pull it up to its calculated MPP.
   • Result: Constant instability. The inverters oscillate wildly, drastically reducing efficiency, causing frequent shutdowns/errors, and putting immense stress on their internal components (like capacitors and switching devices), leading to premature failure.
2. Backfeeding & Reverse Current Flow (Major Damage/Fire Risk):
   • This is the most critical danger. If one inverter shuts down (due to grid loss, internal error, nightfall, etc.) while the other is still operating:
     • The operating inverter pushes DC current into the array.
     • The shutdown inverter’s DC input circuitry now acts like a load (a path to ground or a lower voltage point) instead of a source.
   • Result: DC current from the operating inverter flows backwards through the shutdown inverter’s DC input terminals and wiring.
   • Consequence: This reverse current can severely damage the shutdown inverter’s sensitive DC input components (diodes, transistors, capacitors). Worst case, it can cause overheating, melting of wiring/connectors, arcing, and potentially fire. This violates fundamental safety standards (like UL 1741).
3. Compromised Anti-Islanding Protection (Life Safety Hazard):
   • Grid-tied inverters have a critical safety feature called “anti-islanding.” They must shut down instantly if the grid power fails. This prevents them from “islanding” – continuing to energize the grid lines, which would endanger utility workers trying to fix the outage.
   • With two inverters connected to the same DC array and AC point:
     • If one inverter shuts down due to detecting a grid failure, the other inverter might not detect the failure correctly because it still “sees” voltage/power from the first inverter or the unstable conditions created by the conflict.
     • Result: One inverter might keep running, feeding power onto what should be a dead grid circuit. This creates a potentially lethal hazard for utility workers.
4. Unpredictable & Unsafe Operation:
   • The chaotic interactions described above make the entire system unpredictable. Inverters may:
     • Shut down randomly with obscure error codes.
     • Overheat even under moderate load.
     • Produce distorted AC power that could damage connected appliances.
     • Trip breakers frequently.
   • This instability makes the system inherently unreliable and unsafe.
5. Electrical Code Violations (NEC/UL):
   • The National Electrical Code (NEC) and safety standards (UL 1741) explicitly require inverters to be listed (certified) for their intended use.
   • Standard string inverters are NOT listed or certified to be connected in parallel on the DC side. Doing so:
     • Violates NEC Article 690.64(B)(2): Requires inverters connected in parallel to be “listed and identified” for such use.
     • Violates UL 1741: The safety standard inverters are tested to assumes single-inverter operation per DC source. The reverse current scenario described in point 2 is a direct violation of its safety requirements.
     • Result: The installation will fail inspection, cannot be legally connected to the grid, and voids all equipment warranties. It also creates significant liability issues.
6. Warranty Voidance:
   • All major inverter manufacturers explicitly state in their warranties that connecting the inverter in a way not specified in their installation manual (which prohibits DC-parallel connection for standard models) voids the warranty. Any damage caused will not be covered.

In Simple Terms: Imagine two drivers trying to steer the same car at the same time, each pulling the wheel in a different direction. They fight, the car swerves uncontrollably, and eventually crashes. Connecting two standard inverters to one DC array is like that – they fight for control, create chaos, and something gets broken (or catches fire). It bypasses all the safety mechanisms built into each individual inverter.

The Solution (Reiterated): Only use inverters explicitly designed and certified for parallel operation (with their required communication kits), microinverters, or a single inverter with multiple MPPTs. Never improvise a DC-parallel connection for standard string inverters. Always involve a qualified solar professional.