Castair 7.5 HP 208/230 Volt. 1 Phase Magnetic Starter S71 With Overload

Castair S71 Magnetic Motor Starter — 7.5 HP, 208/230V, 1-Phase with Overload

The Castair S71 is a magnetic motor starter sized for 7.5 HP air compressor motors running on 208/230V single-phase power. It includes a built-in overload relay for motor protection. The correct overload heater element for this starter is B45 — one heater element is 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.

This single-phase starter is for shops on standard 208/230V single-phase service that cannot run 3-phase power. Single-phase starters are typically used for 5 and 7.5 HP compressors in smaller shops or facilities without 3-phase electrical service.

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 7.5 HP motor running for years instead of failing prematurely.

How Magnetic Motor Starters Work on Air Compressors

\n
• 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.
\n
• 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.
\n
• 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 7.5 HP at 208/230V single-phase — do not substitute a different heater number.
\n

Technical Specifications

\n
• Part Number: S71
\n
• Manufacturer: Castair, Inc. (Spicer, MN)
\n
• Motor HP Rating: 7.5 HP
\n
• Voltage Rating: 208/230 Volt
\n
• Phase: 1-Phase
\n
• Overload Protection: Included (overload relay with heater element socket)
\n
• Required Heater Element: B45 (one required for this 1-Phase starter)
\n

Heater Element Requirement: B45

\n
• Heater Number: B45
\n
• Quantity Required: One (1-Phase)
\n
• Heater elements must match the motor's full-load amp draw at operating voltage. The B45 heater is sized for 7.5 HP at 208/230V single-phase. Using an incorrectly sized heater will cause either nuisance tripping (undersized) or inadequate motor protection (oversized).
\n
• 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.
\n

Full Castair Magnetic Motor Starter Lineup

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

When Does a Shop Need the S71?

\n
• Order the S71 if your compressor motor is exactly 7.5 HP at 208/230V single-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.
\n
• Voltage confirmation required: A 208/230V starter operates on 208/230V 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.
\n
• Phase confirmation required: This is a 1-phase starter. If your shop has 3-phase service available, a 3-phase motor and 3-phase starter is the better choice for motors 5 HP and above — 3-phase motors start smoother and run more efficiently.
\n
• 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.
\n

What’s in the Box

\n
• 1x Castair S71 magnetic motor starter with overload relay
\n
• Heater element B45 sold separately (one required) — confirm availability when ordering
\n

Installation Notes

\n
• Magnetic motor starters must be installed and wired by a qualified electrician in accordance with NEC requirements and local electrical codes.
\n
• The starter requires 208/230V single-phase power supply. Verify panel capacity and circuit breaker sizing for 7.5 HP motor load before installation.
\n
• 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.
\n
• For wiring diagrams and duty cycle guidance, see Castair's installation resources at castair.net/guides/.
\n

Why Buy From Air Compressor Factory Direct?

\n
• Genuine Castair OEM starters — matched to Castair compressor motor specifications by the manufacturer
\n
• Free shipping on qualifying orders
\n
• 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
\n
• Full manufacturer warranty
\n
• We stock the full Castair starter lineup from S51 (5 HP 1-phase) through S302/S304 (30 HP 3-phase)
\n

Call 866-217-0063 for quick answers and help!