The Problem

A 50-amp RV shore power supply is a split-phase system: two 120-volt legs sharing a common neutral conductor. When the neutral connection is healthy, both legs track each other closely. When it's not — when the neutral has resistance from a worn receptacle, a loose lug, or years of oxidation — the two legs start to drift apart.

A 2-volt imbalance. A 3-volt imbalance. These numbers won't trip your surge protector. But they tell a story about what's happening inside your air conditioner compressors.

A compressor running at 112 volts instead of 120 volts works harder, draws more current, and runs hotter. The damage accumulates silently over days and weeks. There's no event log entry. There's just a compressor that fails two years earlier than it should have, and no way to trace it back.

The Neutral Drift Index (NDI) is the core metric this project introduces: a continuous measure of load-dependent voltage differential that quantifies neutral path resistance without requiring a fault event to occur.

Hardware

• Shelly Pro 3EM — three-phase energy monitor measuring both 120V legs and neutral current at the shore power inlet
• Raspberry Pi running Node-RED for data ingestion and processing
• PostgreSQL for time-series storage and analytics
• Shore power inlet tap — passive measurement, no interference with park infrastructure

The system runs entirely aboard the coach, publishing data via MQTT to a local broker. No cloud dependency.