The Castair S101 is the OEM magnetic motor starter for 10 HP, 230-Volt, single-phase Castair piston air compressors. This is a fully assembled magnetic starter with overload relay, factory-configured for the S101 application. The associated overload heater element is the B70, which is required for proper overload protection at this motor rating — one heater is used with this single-phase starter. Castair specifies name-brand, off-the-shelf magnetic starters and controls on all their compressors specifically so shops can source replacements without going through proprietary channels.
What a Magnetic Starter Does — and Why It Matters
A magnetic motor starter combines three functions that are all critical for compressor motor protection:
- Motor starting: The contactor closes to apply power to the motor. On a single-phase 10 HP motor at 230V, starting current is significantly higher than running current — the starter is rated to handle that inrush without contact damage.
- Overload protection: The overload relay (with B70 heater element) monitors motor current. If the motor draws excessive current — from a locked rotor, low voltage, bearing failure, or seized pump — the overload trips and disconnects power before the motor winding insulation burns. This is the primary motor protection mechanism.
- Remote control integration: The magnetic coil allows pressure switch control — the pressure switch turns the starter on and off, not the motor directly. This protects the pressure switch contacts (which are rated for low-current coil loads, not motor loads) and provides a safe, controlled start-stop sequence.
Heater Element Specification
The S101 requires one B70 heater element for single-phase overload protection. The heater element must be matched to the actual running current of your specific motor — B70 is the Castair-specified heater for the standard 10 HP 230V single-phase application. If your motor's nameplate FLA differs from the standard specification, verify the heater element sizing against the overload relay's selection chart.
Castair Magnetic Starter Lineup — Single-Phase Models
- S51 — 5 HP, 208/230V, 1-phase, heater: B36
- S71 — 7.5 HP, 208/230V, 1-phase, heater: B45
- S101 (this unit) — 10 HP, 230V, 1-phase, heater: B70
For three-phase 10 HP applications, see S102 (208/230V 3-phase) or S104 (460V 3-phase). Single-phase 10 HP motors draw significantly more current than their 3-phase equivalents at the same horsepower — the S101 is heavier-duty than the S102 despite the same nameplate HP.
Installation Notes
- One B70 heater element required (single-phase — one heater per phase)
- Verify supply voltage matches 230V rating before installation
- Castair compressors ship with starter mounted and wired from the factory — this replacement starter mounts in the same enclosure and connects to the same terminals
- Verify pressure switch wiring reconnects to the magnetic coil terminals, not the load terminals
Technical Specifications
- Part Number: S101
- Motor Horsepower: 10 HP
- Voltage: 230 Volt
- Phase: 1-Phase
- Overload: Included
- Required Heater Element: B70 (1 required for 1-phase)
- Manufacturer: Cast Air
Why Buy From OEM Diagnostic Tools?
OEM Castair magnetic starters — correct contactor rating and overload relay for the S101 application. Free shipping on qualifying orders. Free tech support at 866-217-0063. Full Castair manufacturer warranty. Call us if you need the B70 heater element as well — we can supply the complete assembly.
Call 866-217-0063 for quick answers and help!
