200-Amp Split-Phase Whole-Home Solar Backup Design (MID, ATS & Neutral Sizing)
How to engineer a true 200-Amp Whole-Home Backup system using a Microgrid Interconnect Device (MID) or 200A Pass-Through Hybrid Inverter—including Service Entrance Rated (SE) disconnect rules and Neutral-Ground bonding switching.
1. NEC Code Rules & Electrical Engineering Breakdown
Why Whole-Home 200A Interception Beats Moving 16 Circuits to a Subpanel: Instead of spending 8 hours of electrician labor pulling 16 branch circuits out of your main panel into a cramped subpanel, a 200A Service-Entrance-Rated Microgrid Interconnect Device (or 200A pass-through inverter) installs directly between the utility meter and your existing 200A panel.
The Single Neutral-to-Ground Bond Rule (NEC 250.24): In any residential electrical system, AC Neutral (Grounded Conductor) and Ground (Equipment Grounding Conductor) may be bonded at ONLY ONE point—the First Means of Disconnect (Service Entrance). Once you install a 200A Service-Entrance Automatic Transfer Switch (ATS) or MID ahead of your main panel, that new switch becomes your Service Disconnect!
Unbonding the Existing Main Panel (Floating Subpanel Conversion): Because the new 200A Service-Entrance MID/ATS now houses the main Neutral-to-Ground bond, you MUST remove the green bonding screw/strap inside your existing 200A panel and separate the neutral wires from the bare copper ground wires onto an isolated ground bar (running 4-wire SER: L1, L2, N, G).
2. Field Calculation Worksheet & Amperage Table
200A WHOLE-HOME SERVICE ENTRANCE CONDUCTOR SIZING (NEC TABLE 310.12) ------------------------------------------------------------------ • 200A Dwelling Service/Feeder (83% Rule, NEC 310.12): - Copper (75°C THHN/THWN-2 or XHHW-2): 2/0 AWG Copper (L1, L2, Neutral) - Aluminum (75°C SER / XHHW-2): 4/0 AWG Aluminum (L1, L2, Neutral) - Equipment Grounding Conductor (250.122): 6 AWG Copper or 4 AWG Aluminum • Conduit Size for 3x 2/0 Cu + 1x 6 AWG Cu: 2.0-Inch Schedule 80 PVC or EMT
3. Recommended UL-Listed Hybrid Hardware
Run Interactive NEC 220 Critical Loads Sizer →Inspector & NEC Compliance FAQ
What happens if I forget to remove the green Neutral-Ground bonding screw in my old main panel after adding a Service-Entrance ATS?
Leaving Neutral and Ground bonded in two places creates a parallel neutral return path over the bare copper ground wires and metal conduit—violating NEC 250.24, energizing metal enclosures with neutral return current, and failing electrical inspection.
Can a single 200A pass-through inverter pass 200A from the grid when the grid is up?
Yes! While the grid is online, the internal 200A bypass contactor passes up to 200A (48 kW) straight from the utility meter to your main panel. When the grid fails, the relay opens in <16ms and the inverter supplies 50A–100A (12kW–24kW) from solar and batteries.