An FLA (Full Load Amps) relay—more accurately termed a motor overload relay or motor-rated contactor block—is an electromechanical device sized to carry a motor's continuous running current and trip during sustained overloads. If you are sizing one, the FLA column on the motor nameplate governs your overload relay selection, while the LRA (Locked Rotor Amps) or Horsepower (HP) rating governs the contactor’s breaking and making capacity. Standard ice-cube control relays will weld their contacts shut under motor inrush; you must use a dedicated motor-protection component.
Decoding the Nameplate: Which Rating Column Governs?
When selecting a motor protection assembly (contactor + overload block), beginners often look at the wrong column on the component's rating table. Here is how the governing ratings break down for a standard IEC-rated assembly like the Schneider Electric TeSys D line:
| Parameter | Governing Motor Nameplate Value | Typical Component Rating (TeSys D Example) | Function in Circuit |
|---|---|---|---|
| Overload Trip Setting | FLA (Full Load Amps) | Adjustable 4A – 6A | Protects wire and motor windings from sustained thermal overloads. |
| Contact Making/Breaking | LRA (Locked Rotor Amps) / HP | AC-3: 9A (4 HP @ 480V) | Ensures contacts can survive the massive inrush current during startup without welding. |
| Coil Voltage | Control Circuit Transformer Rating | 120V AC (50/60Hz) or 24V DC | Energizes the electromagnet to pull in the main power contacts. |
| Short Circuit Protection | Available Fault Current (AIC) | Requires external Class CC/J fuses or MCP breaker | Protects against catastrophic short circuits (overloads do not protect against shorts). |
The Golden Rule: Set your overload dial to exactly 100% of the motor's nameplate FLA (unless the motor has a 1.15 service factor, in which case you may set it to 115% of FLA per NEC 430.32). Do not size the overload to the breaker size; size it to the motor.
Coil vs. Contact Side Wiring (and DC Flyback Rules)
An electromechanical relay or contactor has two entirely isolated circuits: the high-power contact side and the low-power coil side.
The Contact Side (Power and Control)
On a 3-phase motor starter, the main power terminals are labeled L1/T1, L2/T2, and L3/T3. Line voltage enters the L terminals, passes through the magnetic contacts and the bimetallic overload heaters, and exits the T terminals to the motor. Auxiliary contacts used for control logic (like holding circuits or PLC feedback) are numbered: 13/14 for Normally Open (NO) and 21/22 for Normally Closed (NC).
The Coil Side and DC Flyback Protection
The coil terminals are universally labeled A1 (positive/line) and A2 (negative/neutral). When wiring a DC coil (e.g., 24VDC) driven by a PLC transistor output or a microcontroller, you must install a reverse-biased flyback diode (like a 1N4007) across A1 and A2. The cathode (stripe) goes to A1. When the DC coil de-energizes, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback). Without the diode, this spike will instantly destroy your PLC's solid-state output transistor. AC coils do not strictly require flyback diodes, though RC snubber networks are sometimes used to reduce electromagnetic interference (EMI).
Selection Decision Path by Load Type
Misapplying a relay based on load type is the leading cause of welded contacts and premature failure. Use this decision tree to select the right contactor and short-circuit protection.
| Load Type | Inrush Characteristic | Required Contactor Rating (IEC Category) | Short-Circuit Protection Strategy |
|---|---|---|---|
| Resistive (Heaters, Ovens) | None (Inrush = FLA) | AC-1 (Non-inductive or slightly inductive) | Standard thermal-magnetic breaker or fast-acting fuse. |
| Inductive (Transformers, Solenoids) | Moderate (Up to 10x FLA for milliseconds) | AC-15 (Control of electromagnetic loads >72VA) | Standard breaker; use RC snubbers across coils to suppress arcing. |
| Motor / Compressor | Extreme (LRA is 6x to 8x FLA for seconds) | AC-3 (Squirrel cage motors: starting, switching off during run) | Time-delay fuses (Class CC/RK5) or Motor Circuit Protectors (Magnetic only). See note below. |
Dead and Live Testing: When to Repair vs. Replace
When a motor fails to start or the overload trips prematurely, you need a systematic diagnostic approach. Grab your multimeter and follow these thresholds.
Dead Testing (Power Off & LOTO Applied)
- Coil Continuity: Set your meter to Ohms (Ω). Measure across A1 and A2. A healthy 120VAC coil typically reads between 15Ω and 50Ω. A 24VDC coil will read much lower (often 2Ω to 10Ω). If you read "OL" (Open Line), the coil is burnt out. Verdict: Replace the contactor/relay.
- Contact Resistance: Manually depress the contactor armature (or use the manual test button on the overload). Measure across L1 and T1. You should read less than 0.5Ω. If it reads higher, the contacts are carbon-tracked or pitted. Verdict: Replace. Do not attempt to sand or file modern silver-alloy contacts; you will remove the protective plating and cause rapid future failure.
- Overload Heaters: Measure across the line and load terminals of the overload block. You should see a very low resistance (usually < 1Ω). If it's open, the internal heater element has melted. Verdict: Replace the overload block.
Live Testing (Power On, Extreme Caution)
- Coil Voltage: Measure AC or DC voltage across A1 and A2 while the system is calling for operation. It must be within ±10% of the coil's nominal rating. A 120V coil dropping to 95V due to a bad control transformer or long wire run will cause the contactor to chatter and burn out.
- Voltage Drop Across Contacts: With the contactor pulled in and the motor running, measure the AC voltage from L1 to T1, L2 to T2, and L3 to T3. A good contact drops less than 0.1V. If you read 2V to 5V across a closed contact, it is generating massive heat and is on the verge of single-phasing the motor. Verdict: Replace immediately.
For more advanced thermal imaging and voltage drop techniques, Fluke's motor troubleshooting guides provide excellent field methodologies.
Frequently Asked Questions
How do I set the FLA dial on an adjustable overload relay?
Locate the motor nameplate and find the "FLA" or "Amps" value at the specific voltage you are supplying (e.g., 12.4A at 230V). Using a small flathead screwdriver, turn the adjustment dial on the overload relay until the red indicator line points exactly to 12.4 on the printed scale. If your motor has a Service Factor (SF) of 1.15 or higher, NEC Article 430.32(A)(1) permits you to increase the trip setting to 115% of the nameplate FLA (12.4 x 1.15 = 14.26A) to prevent nuisance tripping during heavy ambient temperature days.
Why did my FLA relay trip immediately on startup?
Overload relays operate on thermal principles (either bimetallic strips or electronic thermal modeling). They are designed to ignore the brief, massive inrush of LRA during startup. If it trips immediately, check three things: 1) Incorrect Trip Class: Standard motors use Class 10 or 20. High-inertia loads (like large fans or centrifuges) take longer to spin up and require a Class 30 overload. 2) Missing Phase: If one leg of the 3-phase power is dead, the motor will draw massive current on the remaining two legs to compensate, tripping the overload in seconds. 3) Thermal Memory: If the motor just tripped and was immediately reset, the bimetallic strip hasn't cooled down. Wait 5 to 10 minutes for the thermal memory to reset before attempting a restart.
Can I use a standard ice-cube relay for a 10A motor FLA load?
No. A standard 10A ice-cube relay (like an Omron MY2 or similar) is rated for 10A resistive load. A 10A motor has an LRA (inrush) of roughly 60A. When the ice-cube relay attempts to break that inductive circuit, the resulting arc will instantly weld the small silver contacts together, meaning the motor will not turn off when the coil de-energizes. Always use a contactor rated for AC-3 motor loads, paired with a dedicated FLA overload block, for any inductive motor circuit.
