Castair 25 HP 460 Volt. 3 Phase Magnetic Starter S254 With Overload

Castair S254 Magnetic Motor Starter — 25 HP, 460V, 3-Phase with Overload

The Castair S254 is a magnetic motor starter sized for 25 HP air compressor motors running on 460V three-phase power. It includes a built-in overload relay for motor protection. The correct overload heater element for this starter is B45 — three heater elements are required for proper operation. This is the OEM-spec starter Castair uses in their industrial and commercial compressor builds at this HP and voltage rating.

At 208/230V 3-phase, this starter handles higher line current than the 460V equivalent at the same 25 HP rating, which is reflected in higher pricing for the upper HP models in this voltage class.

Magnetic motor starters handle two distinct jobs on an air compressor: switching the motor on and off in response to the pressure switch signal (the contactor function), and protecting the motor from overload conditions that would otherwise burn out the windings (the overload relay function). On a compressor that starts and stops repeatedly throughout a workday, a correctly sized starter with properly rated heater elements is not optional — it is what keeps a 25 HP motor running for years instead of failing prematurely.

How Magnetic Motor Starters Work on Air Compressors

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• Contactor function: The pressure switch sends a low-voltage control signal to the magnetic coil. The coil energizes and closes the high-current power contacts, connecting line voltage to the motor. When tank pressure reaches the cutout setpoint, the pressure switch opens, the coil de-energizes, and the contacts open to stop the motor.
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• Overload relay function: Heater elements in the overload relay sense motor current. If the motor draws excessive current — from mechanical binding, low supply voltage, locked rotor, or genuine overload — the heaters trip the overload relay and disconnect the motor before windings overheat. The relay resets manually after the fault is corrected.
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• Why correct sizing matters: A heater element that is too small will trip on normal starting current. An element that is too large won't trip in time to prevent motor damage. The B45 heater is sized specifically for 25 HP at 460V three-phase — do not substitute a different heater number.
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Technical Specifications

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• Part Number: S254
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• Manufacturer: Castair, Inc. (Spicer, MN)
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• Motor HP Rating: 25 HP
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• Voltage Rating: 460 Volt
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• Phase: 3-Phase
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• Overload Protection: Included (overload relay with heater element socket)
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• Required Heater Element: B45 (three required for this 3-Phase starter)
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Heater Element Requirement: B45

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• Heater Number: B45
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• Quantity Required: Three (3-Phase)
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• Heater elements must match the motor's full-load amp draw at operating voltage. The B45 heater is sized for 25 HP at 460V three-phase. Using an incorrectly sized heater will cause either nuisance tripping (undersized) or inadequate motor protection (oversized).
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• If replacing a starter where the original heater elements are missing or unknown, use B45 for this application. Have motor nameplate FLA available when ordering to confirm.
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Full Castair Magnetic Motor Starter Lineup

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• S51 — 5 HP, 208/230V, 1-Phase, Heater: B36 ($267.96)
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• S71 — 7.5 HP, 208/230V, 1-Phase, Heater: B45 ($309.54)
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• S52 — 5 HP, 208/230V, 3-Phase, Heater: B25 ($237.16)
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• S54 — 5 HP, 460V, 3-Phase, Heater: B11.5 ($237.16)
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• S72 — 7.5 HP, 208/230V, 3-Phase, Heater: B36 ($287.21)
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• S74 — 7.5 HP, 460V, 3-Phase, Heater: B22 ($237.16)
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• S154 — 15 HP, 460V, 3-Phase, Heater: B32 ($287.21)
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• S202 — 20 HP, 208/230V, 3-Phase, Heater: CC87.7 ($592.90)
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• S204 — 20 HP, 460V, 3-Phase, Heater: B40 ($413.49)
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• S252 — 25 HP, 208/230V, 3-Phase, Heater: CC132 ($776.16)
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• S254 — 25 HP, 460V, 3-Phase, Heater: B45 ($616.00)
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• S302 — 30 HP, 208/230V, 3-Phase, Heater: CC143 ($1,135.75)
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• S304 — 30 HP, 460V, 3-Phase, Heater: B56 ($616.00)
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When Does a Shop Need the S254?

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• Order the S254 if your compressor motor is exactly 25 HP at 460V three-phase. Starter HP rating, voltage, and phase must all match the motor nameplate. Do not use a starter rated for a different HP, voltage, or phase on a mismatched motor.
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• Voltage confirmation required: A 460V starter operates on 460V power only. Verify your shop supply voltage before ordering. 208/230V is the most common shop voltage for smaller facilities. If your panel feeds 460V 3-phase, order the 460V variant.
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• Phase confirmation required: This is a 3-phase starter requiring 3-phase electrical service. If your shop only has single-phase service, you need the S51 (5 HP) or S71 (7.5 HP) single-phase starters.
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• Replacement scenario: Replacing a failed starter on an existing compressor? Match the original motor HP, voltage, and phase from the motor nameplate to find the correct replacement part number.
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What’s in the Box

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• 1x Castair S254 magnetic motor starter with overload relay
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• Heater element B45 sold separately (three required) — confirm availability when ordering
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Installation Notes

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• Magnetic motor starters must be installed and wired by a qualified electrician in accordance with NEC requirements and local electrical codes.
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• The starter requires 460V three-phase power supply. Verify panel capacity and circuit breaker sizing for 25 HP motor load before installation.
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• Do not start the compressor without the correct heater elements installed and the overload relay properly set. Running without overload protection risks immediate motor damage on the first overload event.
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• For wiring diagrams and duty cycle guidance, see Castair's installation resources at castair.net/guides/.
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Why Buy From Air Compressor Factory Direct?

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• Genuine Castair OEM starters — matched to Castair compressor motor specifications by the manufacturer
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• Free shipping on qualifying orders
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• Free tech support at 866-217-0063 — call with your motor nameplate data (HP, voltage, phase, FLA) and we will confirm the correct starter and heater element before you order
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• Full manufacturer warranty
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• We stock the full Castair starter lineup from S51 (5 HP 1-phase) through S302/S304 (30 HP 3-phase)
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Call 866-217-0063 for quick answers and help!