An automatic changeover switch (ATS) works by using a voltage-sensing control module to monitor the primary utility feed. When utility voltage drops below a programmed threshold (typically 85% of nominal) for a set time delay (usually 1 to 3 seconds to ignore micro-outages), the controller energizes a motorized actuator or a heavy-duty solenoid. This mechanically throws a double-throw contact assembly, physically disconnecting the utility and routing power from the backup generator or inverter. A physical mechanical interlock ensures both sources cannot close simultaneously, preventing catastrophic backfeed.
The Core Specs: Reading an ATS Rating Table
To understand how the switch handles your specific electrical loads, you have to look past the marketing "Amps" and read the utilization categories. An ATS is essentially two mechanically interlocked contactors or a single motorized double-throw switch. The rating plate dictates its survival under different inrush currents.
Below is a representative spec sheet for a standard residential/light-commercial 100A motorized ATS (similar to the Schneider Electric EZT series or Eaton RTS series).
| Parameter | Utility (Source 1) | Generator (Source 2) | Control / Coil Circuit |
|---|---|---|---|
| Nominal Operating Voltage (Ue) | 240V AC, 50/60Hz | 240V AC, 50/60Hz | 24V DC / 120V AC |
| Continuous Thermal Current (Ith) - AC-1 | 100A | 100A | N/A |
| Motor Load Rating (AC-3) at 240V | 40 HP (approx. 52A FLC) | 30 HP (approx. 40A FLC) | N/A |
| Making Capacity (Icm) at 240V | 1050A peak | 840A peak | N/A |
| Breaking Capacity (Icu) at 240V | 500A RMS | 400A RMS | N/A |
| Coil Pull-in / Hold VA | N/A | N/A | 250 VA / 15 VA |
| Transfer Time (Motorized) | < 150 milliseconds | N/A | |
Which Rating Column Governs Your Load?
The governing column depends entirely on the physics of the connected load. If you are switching space heaters or incandescent lighting, the AC-1 (Continuous Thermal) column governs; the switch can safely carry its full 100A rating. However, if you are switching an HVAC compressor or a well pump, the AC-3 (Motor Load) and Making Capacity (Icm) columns govern. A 100A ATS might only be rated for 40A of full-load motor current because starting a motor generates a Locked Rotor Amp (LRA) inrush that can reach 6 to 8 times the running current. If the Icm making capacity is exceeded, the magnetic forces can physically weld the silver-tin oxide contacts shut.
Coil Wiring, Contact Routing, and DC Flyback Protection
Wiring an ATS requires strict separation between the high-current contact side and the low-voltage coil/control side.
- Contact Side (Power): Labeled L1/L2 (Utility input), L3/L4 (Generator input), and T1/T2 (Load output). These require torque-specific terminations. For a 100A switch using 3 AWG copper THHN, typical terminal torque is 45-50 in-lbs. Always use a calibrated inch-pound torque screwdriver; loose connections cause high-resistance arcing that mimics a contact failure.
- Coil/Control Side: Labeled A1/A2 for the transfer solenoid, or specific PCB terminal blocks for the sensing module. The sensing module monitors L1/L2 and L3/L4 to decide when to trigger the A1/A2 coil.
Load Selection Decision Path: Resistive, Inductive, and Motor
Matching the ATS to the load profile prevents premature contact degradation. Use the decision tree below to select the correct utilization category and upstream protection.
| Load Type | Examples | Governing ATS Rating | Upstream Protection Strategy |
|---|---|---|---|
| Resistive (AC-1) | Electric heat strips, water heaters, incandescent lighting | Continuous Thermal Current (Ith) | Standard thermal-magnetic breaker (B or C curve) sized to 125% of continuous load. |
| Inductive (AC-3) | Transformers, solenoid banks, HID lighting ballasts | Making/Breaking Capacity (Icm/Icu) | Breaker with C-curve or D-curve to tolerate moderate inrush without nuisance tripping. |
| Motor (AC-3/AC-4) | HVAC compressors, well pumps, elevator hoists | Motor HP Rating & Peak Making Capacity (Icm) | Motor-rated breaker (D-curve) or Class RK1/J time-delay fuses. Fuses clear high fault currents faster than standard breakers. |
| Capacitive | Large UPS banks, VFD input stages, power factor correction | Peak Making Capacity (Icm) | Current-limiting fuses to handle the instantaneous capacitive charging inrush. |
The Fuse vs. Breaker Curve Reality
A common mistake is treating upstream fuses and circuit breakers as interchangeable without considering the trip curve. If your ATS feeds a 5-ton AC compressor, the LRA inrush can hit 120A for a few hundred milliseconds. A standard B-curve breaker will interpret this as a short circuit and trip (nuisance trip). You must use a D-curve breaker, which has a higher magnetic trip threshold. Alternatively, NEC Article 430 permits the use of Class RK1 or Class J time-delay fuses, which provide superior high-fault clearing speeds (protecting the ATS contacts from explosive failure) while safely riding through the motor starting inrush.
Diagnostics: Testing Dead/Live and the Repair vs. Replace Verdict
When an ATS fails to transfer, or transfers but drops the load, you need a systematic diagnostic approach. Never guess; measure.
How to Test an ATS Dead (De-energized)
- Lockout/Tagout: Disconnect both utility and generator sources. Verify zero voltage at L1-L4 with a Category III multimeter.
- Coil Resistance Check: Set your meter to Ohms. Measure across the transfer coil terminals (A1/A2). A healthy 120VAC coil typically reads between 10 and 50 ohms. An infinite reading (OL) means the coil wire is burnt open. A reading near 0 ohms indicates a shorted coil.
- Contact Continuity: Manually actuate the mechanism (most motorized switches have a manual override handle). Measure resistance across L1-to-T1 and L2-to-T2 in the Utility position. It must read less than 0.1 ohms. Repeat for the Generator position.
- Insulation Resistance (Megger): For industrial units, apply 500VDC between the phases and the grounded chassis. Acceptable insulation resistance is >1 Megohm. Anything lower indicates carbon tracking from internal arcing.
How to Test an ATS Live (Energized)
- Voltage Sensing Verification: With utility power on, measure voltage at the control board sensing terminals. It should read nominal (e.g., 240V). Drop the utility voltage using a variac (if bench testing) or simulate an outage; verify the controller initiates the transfer delay timer.
- The Millivolt Drop Test: This is the ultimate test of contact health. With the ATS fully loaded and running, set your multimeter to DC millivolts (mV). Place the probes directly on the metal bus bars on either side of the closed contacts (e.g., L1 and T1). A healthy contact yields a voltage drop of less than 50mV. If you read >150mV, the contacts are pitted, oxidized, or losing spring pressure, generating excess heat.
- Coil Pull-in Voltage: Measure voltage at the coil terminals during the exact moment of transfer. If the voltage dips below 85% of nominal due to undersized control wiring, the solenoid will chatter and fail to latch.
When to Repair vs. Replace
The decision to repair or replace hinges on the unit's class and the failure mode.
- Replace the entire unit if: The main power contacts show visible pitting, arc burns, or missing silver-alloy surfacing. You cannot sand down ATS contacts; removing the silver-tin oxide layer exposes the base copper, which will rapidly oxidize and weld shut on the next high-inrush motor start. Also, replace if the mechanical interlock is cracked or if it is a sealed residential unit (under 200A).
- Repair (Component Swap) if: You are working with a modular industrial ATS (e.g., ASCO 7000 series). In these units, you can safely replace a burnt control logic board, a failed transfer solenoid, or a worn motor drive gear, provided the main copper busbars and contact arms are structurally sound and pass the millivolt drop test.
