Rogowski Coils (CTs) for the Lunar System
Application Note | lunarenergy.com
Summary
Rogowski Coils are flexible, air-core sensors that measure AC current by detecting the rate of change of current in a conductor. These current transformers (CTs) provide accurate, non-intrusive current measurements and transmit telemetry to the Lunar System.
Rogowski Coils enable two key functionalities:
Partial Home Backup: Monitoring energy usage of non-backup loads connected to an upstream panelboard.
Power Control System (PCS): The Lunar System (Battery, Bridge, and optional Rogowski Coils) is certified to both UL 1741 PCS CRD and UL 3141.
Installation Requirements
Compatible Coils: Only install Rogowski Coils sourced directly from Lunar Energy. The Lunar System uses the Magnelab RCT-120L Series Coils. Two Rogowski Coils and field-applied labels are included in the Rogowski Coils kit (90-06789A).
Coils Installation at Main Panel
The latch end of the Coils are typically installed on the service disconnect. The Coil must be installed on the conductors in series between the Lunar Bridge and the grid. Ensure the Coil clasp is fully engaged — perform a gentle tug test to confirm it is secure.
Coil Installation at Lunar Bridge
Terminate Coil leads to the labeled terminals Coil 1A/1B and Coil 2A/2B in the Bridge. If a drain wire is present, do not connect it to any terminal. Maximum splice wire extension: 100 feet (using 12-24 AWG shielded, twisted or untwisted wires).
Coil Phase & Polarity Verification
The Installer App uses factory-installed Bridge CTs to compare measured current from Coils with Bridge CTs. The installer must adjust which loads are on/off in the home during this test.
Requirements: Both AC L1 and L2 must have at least 2A measured current; at least 2A of imbalanced current between L1 and L2.
Scenario | Installer Action |
Phasing Check fails (less than 2A difference between Bridge and Coils) | Ensure loads draw at least 2A per phase with at least 2A/240W imbalance between phases |
Polarity Check fails | Swap terminations of Coil 1A with Coil 1B (or 2A with 2B) depending on error shown |
Both checks fail with correct load imbalance | Swap Coil 1A/1B with Coil 2A/2B (phase flip); then also swap within each pair (polarity flip) |
Checks fail and "-" shown on Coil 1 or 2 | Check 2A per phase, verify clasp is secure, verify all wire terminations at Bridge and splice points |
Adjusting Coil Phase & Polarity
The easiest method is to adjust wiring terminations at the Bridge.
Flip Coil Phase: Swap terminations of Coil 1A and 1B with Coil 2A and 2B.
Flip Coil Polarity: Swap terminations of Coil 1A with 1B; swap Coil 2A with 2B.
Coil Indicator Arrow & Wire Termination
Coil Arrow Direction | Wiring Terminations |
Arrow points toward the Load | Black on Coil A, White on Coil B |
Arrow points toward the Grid | White on Coil A, Black on Coil B |
Installing Coils on Branch Circuit Wires
When direct Coil installation on service conductors is not feasible (e.g. due to a 4-wire main breaker), bundle branch circuit wires instead. Bundle all non-backup load feeders AND Lunar Bridge feeders through the Coils:
All L1 conductors bundled together through Coil 1.
All L2 conductors bundled together through Coil 2.
All wires must be oriented so current flow direction to/from the grid is the same.