Solar PV System Design & Net Metering: The Ontario Engineer's Guide
Ontario's net metering program lets building owners offset their hydro bill with rooftop solar — but the electrical design has to satisfy CEC Section 64, ESA inspection, and utility interconnection requirements simultaneously. Here's the engineering behind a code-compliant PV installation.
System Architecture: Grid-Tied PV
The vast majority of Ontario commercial PV installations are grid-tied without battery storage. The system components:
| Component | Function | Key Specification |
|---|---|---|
| PV Modules | Convert sunlight to DC electricity | Typically 400–550W monocrystalline, Voc 40–50V per module |
| String Inverter | Convert DC to AC (grid-synchronous) | Must be CSA/UL listed, anti-islanding per IEEE 1547 |
| AC Disconnect | Visible, lockable disconnect for utility access | Required by utility — must be within sight of meter |
| DC Disconnect | Isolate PV array from inverter | CEC Rule 64-112 — required for maintenance |
| Revenue Meter | Bidirectional meter tracking import/export | Provided by LDC (Local Distribution Company) |
| Rapid Shutdown | De-energize rooftop conductors for firefighter safety | CEC Rule 64-218 — within 30 seconds, to ≤30V within array |
CEC Section 64 — Solar Photovoltaic Systems
Section 64 is the dedicated CEC section for PV installations. Key rules:
| CEC Rule | Requirement |
|---|---|
| 64-060 | PV source circuit conductors shall be sized at 125% of Isc (short circuit current) of the module string |
| 64-062 | Overcurrent protection for PV source circuits — maximum fuse rating per module manufacturer datasheet |
| 64-076 | PV system output connected to the supply side of the service disconnect requires utility approval |
| 64-112 | DC disconnect means required — must be accessible and lockable |
| 64-210 | The 120% rule — PV backfeed breaker + main breaker ≤ 120% of busbar rating |
| 64-218 | Rapid shutdown — array conductors must be reduced to ≤30V within 30 seconds of initiation |
The 120% Rule (CEC Rule 64-210)
The sum of the PV backfeed breaker rating and the main breaker rating shall not exceed 120% of the busbar rating.
Example: 200A busbar panel → 200A × 1.2 = 240A maximum → 240A − 200A main = 40A maximum PV backfeed breaker
At 240V single-phase: 40A × 240V = 9.6 kW maximum inverter output
If the PV system exceeds this limit, options include:
- Upgrade the busbar — install a higher-rated panel
- Supply-side connection — connect PV between meter and main breaker (requires utility approval per Rule 64-076)
- Line-side tap — connect directly to service entrance conductors (complex, P.Eng design required)
Net Metering in Ontario
| Parameter | Detail |
|---|---|
| Program | Ontario Net Metering (O. Reg. 541/05) |
| Maximum size | 500 kW (must be sized to offset own consumption) |
| Meter type | Bidirectional — credits excess generation at same rate as consumption |
| Credit rollover | Excess credits roll forward for 12 months, then reset to zero |
| Eligible LDCs | Hydro One, Toronto Hydro, Alectra, and all Ontario LDCs |
| Application | Submit to your LDC with single-line diagram, P.Eng stamp, ESA notification |
Utility Interconnection Process
- Step 1: Pre-application — confirm service capacity and transformer sizing with LDC
- Step 2: Engineering design — single-line diagram with P.Eng seal
- Step 3: ESA permit — file notification with Electrical Safety Authority
- Step 4: Installation and ESA inspection
- Step 5: LDC meter exchange — install bidirectional meter
- Step 6: Authorization to connect (ATC) — LDC approves grid connection
Inverter Sizing
| Parameter | Rule |
|---|---|
| DC/AC ratio | Typically 1.1–1.3 (e.g., 13 kW DC array → 10 kW inverter) |
| Input voltage window | String Voc (at lowest temp) must be within inverter max Vdc rating |
| MPPT range | String Vmp must fall within inverter MPPT voltage window at all operating temperatures |
| Anti-islanding | Inverter must cease export within 2 seconds of grid loss (IEEE 1547 / CSA C22.2 No. 107.1) |
Disclaimer: This article provides general engineering guidance for educational purposes. Always verify requirements against the current edition of the Canadian Electrical Code (CEC), Ontario Electrical Safety Code (OESC), and your Local Distribution Company's interconnection standards. Consult a licensed Professional Engineer (P.Eng) for project-specific applications.
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