Across the Line Motor Starter: Sizing, Wiring, and Load Matching

The Direct Answer: When to Deploy an Across the Line Motor Starter

An across the line motor starter (often called Direct-On-Line or DOL) applies full line voltage directly to the motor terminals the moment the contactor closes. You should use this starting method for 3-phase AC squirrel-cage induction motors rated under 10 HP (or up to 50 HP if your local utility permits the inrush current), provided the driven load can tolerate high starting torque and you do not require variable speed control.

The primary advantage is simplicity and cost: an across the line starter delivers 100% of the motor's locked-rotor torque at startup. The trade-off is inrush current, which typically spikes to 600% of the Full Load Current (FLC) for the first few seconds. If your facility has a weak power grid or the driven mechanics (like a fragile conveyor belt) cannot withstand sudden torque shocks, you must step up to a soft starter or Variable Frequency Drive (VFD).

Motor Type Match and Load Profiles

Across the line starters are not universal. They are engineered specifically for the torque curves and starting characteristics of specific motor topologies. Treating different motor types as interchangeable will result in immediate component failure or nuisance tripping.

For 95% of industrial and commercial applications—such as centrifugal pumps, exhaust fans, and basic compressors—the AC squirrel cage induction motor paired with an across the line starter is the default, most economical choice.

Sizing the Starter: Rules of Thumb and a Worked 10HP Example

Sizing a motor starter requires matching both the continuous running current and the inrush current. The National Electrical Code (NEC) provides baseline tables, but manufacturer IEC or NEMA ratings dictate the physical frame size.

The Sizing Rule of Thumb

• Contactor Frame Size: Must be rated for at least 115% to 125% of the motor's Full Load Amps (FLA) for continuous duty (AC-3 utilization category for IEC, or NEMA Size classifications).
• Overload Relay Setting: Must be dialed exactly to the motor nameplate FLA (not the NEC table value) to protect against thermal damage. Typically set between 100% and 115% of nameplate FLA depending on service factor.
• Short Circuit Protection: The upstream breaker or fuses must be sized per NEC Table 430.52 (usually 250% of FLA for inverse-time breakers) to allow the inrush spike without tripping, while still clearing dead shorts.

Worked Load Example: 10 HP Centrifugal Pump

Let's size a starter for a 10 HP, 460V, 3-phase, 60Hz squirrel cage motor driving a water pump. The motor nameplate reads: FLA 13.2A, Service Factor 1.15.

1. Verify NEC Baseline: NEC Table 430.250 lists 14A for a 10HP/460V motor. However, we always use the nameplate FLA (13.2A) for overload sizing.
2. Select Contactor: We need an IEC contactor rated for at least 13.2A in the AC-3 category. A standard 18A or 25A frame is appropriate. We will select the Schneider Electric TeSys D LC1D18 (rated 18A AC-3 at 440V). Retail price is approximately $85.
3. Select Overload Relay: The LRD series thermal overload relay must bracket 13.2A. The LR2D1422 covers the 12A to 18A range. We set the physical dial precisely to 13.2A. Retail price: ~$65.
4. Upstream Breaker: 13.2A x 2.5 (NEC max for inverse time) = 33A. We step down to the nearest standard size: a 30A 3-pole molded case circuit breaker (MCCB).

Wiring and Terminal Identification

Proper termination ensures the control circuit safely interrupts power during a fault. Here is the standard terminal mapping for a 3-phase IEC/NEMA across the line starter assembly:

Failure Signatures: Hum, Overheat, and Stall

Across the line starters are robust, but they fail in predictable ways when subjected to grid anomalies or mechanical abuse. Recognizing these signatures prevents catastrophic motor burnout.

1. The 'Hum' (Single-Phasing)

Symptom: The motor emits a loud 120Hz hum and refuses to start, or if running, it vibrates heavily and overheats rapidly.
Cause: One of the three phases is lost. This could be a blown upstream fuse, a loose terminal on T2, or a pitted internal contactor pad failing to pass current on one pole.
Fix: Measure voltage phase-to-phase at T1-T2, T2-T3, and T1-T3 with the contactor pulled in. If one reads 0V, replace the contactor. Standard thermal overloads will eventually trip on single-phasing, but electronic overloads (like the TeSys LRD series with phase-loss detection) trip in under 2 seconds, saving the windings.

2. Overheat and Nuisance Tripping

Symptom: The overload relay trips randomly, but the motor feels cool to the touch and the ammeter shows current well below FLA.
Cause: High ambient temperature inside the control panel. Bimetallic thermal overload relays are ambient-compensated, but if the panel interior exceeds 60°C (140°F) due to nearby VFD heat sinks or lack of ventilation, the relay will derate and trip prematurely.
Fix: Install a panel exhaust fan with a thermostat, or relocate the overload relay outside the high-heat zone using remote mounting adapters.

3. Mechanical Stall

Symptom: The contactor pulls in, the motor draws 500%+ FLA, but the shaft does not turn. The overload trips after 5-10 seconds.
Cause: The driven load is physically jammed (e.g., a seized pump bearing or a rock in a conveyor), or the line voltage has sagged so severely during startup that the motor cannot develop enough torque to break static friction (Torque = Voltage²).
Fix: Disconnect the motor from the load and spin the shaft by hand. If the load spins freely, measure line voltage at L1-L3 during the start attempt. If it drops below 85% of nominal, you have a voltage dip issue requiring utility intervention or a reduced-voltage soft starter.

The Final Decision Tree

Do not default to an across the line starter out of habit. Use this decision matrix to lock in the correct starting method and specific hardware for your application.

For standard, low-inertia fractional and small integral horsepower loads, the across the line motor starter remains the undisputed, most cost-effective baseline. Lock in your IEC frame size based on the exact nameplate FLA, wire your control circuit through the 95/96 NC overload contacts, and verify phase rotation before coupling the load.