The Castair 20-256-3 is a 20 HP three-phase open drip proof motor rated for 208/230/460V service, designed for industrial air compressor applications. This is the heaviest single-pump motor in the Castair standard lineup — 315 lbs, 256T NEMA frame, 1-5/8 inch shaft. It is the OEM replacement for Castair industrial systems in the 20 HP class. If you are replacing a motor on a Castair 20 HP unit or building a new system at this power level, confirm the 256T frame designation and 1-5/8 inch shaft size before ordering.
Motor Specifications
- Horsepower: 20 HP
- Phase: 3-Phase
- Voltage: 208/230/460V (dual-voltage winding)
- Service Factor: 1.15
- Amp Draw: 53A at 208V / 50A at 230V / 25A at 460V
- Frame: 256T NEMA frame
- RPM: 1725
- Shaft Size: 1-5/8 inch
- Weight: 315 lbs
- Listings: UL and CSA listed
- Enclosure: Open Drip Proof (ODP)
- Part Number: 20-256-3
The 1-5/8 Inch Shaft: Critical Fitment Detail
Note carefully: the 20-256-3 uses a 1-5/8 inch shaft, not the 1-3/8 inch shaft common on 7.5, 10, and 15 HP Castair motors. This requires 1-5/8 inch bore pulleys — Castair SD158 or SK158 hubs for the P3B-series pump pulleys. If you are replacing a 20 HP motor, your existing pulleys should already be 1-5/8 inch bore. If you are building a new system or upgrading from a lower HP class, you will need to source the correct pulleys and potentially a new belt guard (5BG for C3U/C4U pumps, 4BG7 for C3/G30 pumps). Do not try to adapt a 1-3/8 inch bore pulley to a 1-5/8 inch shaft.
Motor Base for 20 HP
- Loose base: 256D adjustable belt tensioner base
- Pre-mounted: No pre-mounted option listed — order 256D loose and mount separately
- The 256D is specific to 20 HP — do not substitute the 213D (7.5-10 HP) or 254D (15 HP) bases
Power Requirements at 20 HP
Running at 460V three-phase, the 20-256-3 draws 25 amps. That is manageable on most commercial electrical services. Running at 230V three-phase, the amp draw doubles to 50A — requiring significantly heavier wiring and larger contactors. Any 20 HP installation requires a magnetic motor starter (not covered by overload protection alone), properly sized fusing, and a dedicated circuit. If your electrical panel is marginal, 460V operation is the only practical choice.
Compatible Pump Configurations
- C3 pump: 15 HP, two-stage, W3 configuration, 485 lbs — sometimes used in 15-20 HP applications
- C4 pump: 15-25 HP, two-stage, V4 configuration, 800 lbs — the heavy industrial pump paired with 20 HP motors in demanding applications
- Pump RPM and pulley sizing must be calculated for the specific pump/motor combination — call us if you need help with the math
What's in the Box
- 1x Castair 20 HP open drip proof motor, 208/230/460V 3-phase, Frame 256T, 1725 RPM
- Standard motor leads and terminal connections
Note: Motor base (256D), pulleys with 1-5/8" bore, belts, belt guard, and magnetic motor starter sold separately.
When Does a Shop Need This vs. Stepping Up or Down
- Step down to 15 HP (15-254-3, $1,677.14): If your CFM demand is 40-50 CFM at 175 PSI and you have adequate storage volume, 15 HP may be sufficient and saves on motor cost and operating power
- This motor (20 HP, $1,773.66): Right for heavy production environments, large body shops, or any application requiring 60+ CFM at working pressure
- Step up to 25 HP (25-284-3) or 30 HP (30-286-3): For very high CFM demands or when a duplex 20 HP system still cannot meet peak demand
Why Buy From Air Compressor Factory Direct?
We have been specializing in industrial air compressor components since 2016. At 315 lbs, the 20-256-3 is a significant piece of equipment — we can help you plan the complete installation including motor base, pulleys, belt guard, and controls. Free shipping, free technical support at 866-217-0063, and full manufacturer warranty on all Castair components.
Call 866-217-0063 for quick answers and help!
