Castair I20312HC4Z-S: 20HP 3-Phase Industrial Air Compressor — 120 Gallon Horizontal with Mounted Starter
The Castair I20312HC4Z-S is a 20-horsepower, 3-phase, 120-gallon horizontal industrial air compressor built on Castair's heavy-duty C4 cast iron pump. The Z designation indicates this unit includes a factory-mounted magnetic motor starter — a meaningful distinction when you are doing a commercial installation and need a code-compliant, ready-to-wire setup rather than sourcing and mounting a separate starter panel.
At 20HP with the C4 pump, this is the entry point into Castair's large-frame compressor lineup. The C4 pump is a two-stage V4 cylinder design — four cylinders arranged in a V configuration — which gives it high volumetric efficiency and the capacity to deliver 72 CFM at 175 PSI. That is enough air to run 5-7 technicians simultaneously on standard shop tools, or to supply a moderate-size spray booth with adequate reserve.
Note that this is the 120-gallon horizontal configuration. Castair also offers the 20HP with a 240-gallon horizontal tank (I20324HC4Z-S). The 120-gallon unit is appropriate when your peak demand is relatively consistent and your installation footprint limits tank size. If you regularly see high-demand spikes — running multiple spray guns or sandblasting while also supplying general shop air — step up to the 240-gallon.
Manufacturer specifications are based on available Castair Industrial Series data. Contact us for current lead times and freight details on this model.
Pump Design — C4 Cast Iron
Construction Details
- Pump designation: C4 — Two-stage, V4 cylinder configuration (four cylinders in a V layout)
- Rated for: 15-25HP applications
- Construction: 100% cast iron throughout — cylinders, heads, valve plates, crankcase
- Flywheel: 13-3B configuration
- Oil capacity: 1 gallon
- Tapered roller bearings at both ends of the crankshaft for superior support and reduced friction
- Precision steel connecting rods with automotive-type insert bearings at crankshaft and wrist pin
- Disc and spring valves for field serviceability and efficiency
- 4-ring pistons for high efficiency and low oil carry-over
- Centrifugal unloader — loadless start-up on every cycle protects the motor and drive system
Tank, Frame, and Controls
- Tank capacity: 120 gallons, horizontal
- Tank construction: Powder-coated ASME-rated steel with channel saddle frame
- Safety valve: ASME-rated, factory installed
- Belt guard: Totally enclosed metal
- Discharge lines: Copper tubing or stainless braided flex line
- Motor starter: Factory-mounted magnetic motor starter (Z designation) — code-compliant, wired and ready for power connection
Motor
- Horsepower: 20HP
- Phase: 3-phase (requires proper 3-phase service and appropriate breaker sizing)
- Motor speed: 1725 RPM industrial
- Service factor: 15% minimum
- Max working pressure: 175 PSI
Performance Specifications
- Motor output: 20 HP
- Electrical: 3-phase with factory-mounted starter
- Tank capacity: 120 gallons, horizontal
- Pump: C4 (Two-stage, V4 cylinder)
- Pump RPM: 519
- Free air delivery (0 PSI): 92.8 CFM
- Delivery at 175 PSI: 72 CFM
- Max pressure: 175 PSI
- Motor starter: Factory mounted magnetic starter
Dimensions and weight: Contact us for current shipping specs on this model — dimensional data for the 120-gallon C4 configuration is pending manufacturer confirmation.
What's in the Box
- Castair I20312HC4Z-S compressor — fully assembled
- Factory-mounted magnetic motor starter (Z designation)
- ASME safety valve (factory installed)
- Pressure switch with unloader
- Totally enclosed metal belt guard
- Copper discharge and unloader tubing (or stainless braided flex line)
- Owner's documentation
Aftercooler is not included in base configuration. Available as factory option (BGAC3M). Electrical connection to 3-phase service by customer/electrician.
Available Configurations — 20HP Industrial Series
- I20312HC4Z-S (this unit): 20HP, 3-phase, 120gal Horizontal, C4 pump, 519 RPM, 72 CFM @ 175 PSI, with mounted starter
- I20324HC4Z-S: 20HP, 3-phase, 240gal Horizontal, C4 pump, 519 RPM, 72 CFM @ 175 PSI, 90x39x70, 1,700 lbs — same pump, double the tank storage
Both units use the identical C4 pump and deliver the same CFM. The difference is tank storage capacity. The 240-gallon tank provides significantly more buffering for high-demand spikes. If your operation has variable demand — busy periods with multiple simultaneous users — the 240-gallon tank lets the compressor run fewer cycles and recover more quickly after heavy draw-down periods.
When Does a Shop Need This?
- You need 72 CFM at 175 PSI for a multi-bay shop, spray booth, or continuous-duty industrial process
- Your shop has 3-phase electrical service available
- You need a factory-mounted starter for code compliance or simplified installation
- Your demand profile is consistent enough that 120 gallons of tank storage is adequate (versus the 240-gallon model)
Step up to the 240-gallon (I20324HC4Z-S) if:
- Your shop sees high-demand spikes with multiple high-CFM tools running simultaneously — larger tank storage reduces compressor cycling and extends motor and pump life during heavy use periods
Step up to the 25HP (I25312HC4Z-S or I25324HC4Z-S) if:
- Your continuous demand regularly exceeds 72 CFM at pressure — the 25HP delivers 90 CFM at 175 PSI from the same C4 platform
Why Buy From Air Compressor Factory Direct?
Since 2016, we have specialized in industrial air compressors for shops, production facilities, and commercial operations. We understand the C4 pump platform, can help you size tank capacity to your actual demand profile, and can advise on electrical requirements before you commit to this purchase.
- Free shipping to commercial addresses in the lower 48 states
- Free tech support at 866-217-0063 before and after your purchase
- Full Castair manufacturer warranty
- Honest configuration guidance — we help you get the right tank size and options
- Authorized dealer with direct Castair factory support
Call 866-217-0063 for quick answers and help!
