A ground power unit (GPU) is a mobile generator that powers a parked aircraft with its engines off. GPUs come in two main output types: 400 Hz AC and 28 VDC. Which one you need depends on the electrical system your aircraft was built around.
Quick Answer: Most airliners and large business jets need 400 Hz, 115/200 volt, three-phase AC power. That usually means a 90 kVA or larger unit. Most piston aircraft, turboprops, helicopters and many light jets need 28 VDC for power and engine starts. Check the aircraft’s external power receptacle and flight manual before you choose.
This guide explains the difference, how to size each type, and which listed Victory GSE units fit. If you are new to ramp equipment, start with our overview of ground support equipment types and uses.
What Is the Difference Between 400 Hz and 28 VDC Ground Power?
The difference is the type of current. A 400 Hz GPU delivers alternating current at 115/200 volts in three phases. A 28 VDC GPU delivers direct current at about 28.5 volts. That matches the electrical bus on a typical light aircraft.
Large transport aircraft use 400 Hz AC because higher frequency allows smaller, lighter generators and electrical parts. The FAA’s Aviation Maintenance Technician Handbook, Airframe covers how aircraft AC and DC systems are built and fed. Smaller aircraft keep things simple with a DC bus and a battery.
A GPU must match what the aircraft expects. AC and DC external power plugs use different pin layouts. That makes the receptacle the fastest way to confirm what you need.
Which Aircraft Need 400 Hz AC Power?
Narrow-body and wide-body airliners, regional jets and most large business jets need 400 Hz power. These aircraft run avionics, cabin systems, lighting and galleys on AC while parked.
Tronair lists its 90 kVA 400 Hz unit for the Airbus A220 through the A380 and the Boeing 707 through the 787. That range shows how standard 400 Hz power is across commercial fleets. The same unit can supply 400 Hz AC and 28.5 VDC at once. Its DC side is rated at 600 amps continuous.
Military aircraft often call for power quality that meets MIL-STD-704. The Arvico 4-90STD-D listed at Victory GSE meets or exceeds MIL-STD-704 for Category B aircraft, per its product page.
Which Aircraft Need 28 VDC Power?
Piston singles and twins, most turboprops, most helicopters and many light jets use 28 VDC. On these aircraft, a DC GPU powers the bus and, more importantly, cranks the engine or starter-generator.
Engine starts are the hard part. A turbine starter-generator pulls a heavy current spike for a few seconds. That is why DC units list both a continuous rating and a peak or starting rating.
The Tronair 112820C provides 800 amps continuous and up to 2,000 amps peak for starting. The Hobart Jet-Ex 5D is rated at 28.5 volts DC and 600 amps. The Hobart Jet-Ex 4D can start any fixed-wing aircraft or helicopter with a 28.5 volt DC receptacle.
How Do You Size a 400 Hz GPU in kVA?
Size a 400 Hz GPU by the aircraft’s ground power demand in kVA, then add margin. A 90 kVA unit covers most narrow-body airliners and regional jets. Wide-body aircraft and heavy hangar loads often call for 120 kVA, 140 kVA or more.
kVA measures apparent power, while kW measures the power that does real work. The Stewart & Stevenson TM4140 shows the gap clearly: its listing rates it at 140 kVA and 112 kW. That 0.8 ratio is a common power factor for aircraft generator sets.
Running a GPU near its limit for long periods adds heat and stress. A unit one size up gives headroom for galley loads, cabin heaters and avionics running at once. Check the aircraft manufacturer’s ground power data before you settle on a rating.
How Do You Size a 28 VDC GPU in Amps?
Size a 28 VDC GPU by two numbers. Continuous amps run the bus, and peak amps handle the engine start. Most buyers focus on peak amps, since a weak start can stall or overheat a starter.
Light piston aircraft need far less current than a turboprop or a medium helicopter. Turbine starts draw far more current than piston starts. A unit like the Tronair 112820C, at 2,000 amps peak, is built with turbine starts in mind.
Many DC units also let you set a peak current limit. Tronair notes adjustable peak current limiting on its combined AC/DC unit. Set that limit to the starter maker’s figure to protect the aircraft.
400 Hz vs. 28 VDC GPUs Compared
The table below compares both output types and the listed Victory GSE units for each. Specs come from each unit’s Victory GSE product page.
| Factor | 400 Hz AC GPU | 28 VDC GPU |
|---|---|---|
| Output | 115/200 V, 400 Hz, three-phase AC | About 28.5 V DC |
| Rated by | kVA (and kW) | Continuous and peak amps |
| Typical aircraft | Airliners, regional jets, large business jets | Piston aircraft, turboprops, helicopters, many light jets |
| Main job | Powering systems while parked | Powering the bus and starting engines |
| Listed units | Hobart 90CU20 / 90CU24-P5 (90 kVA), Hobart 90D20-PLD (90 kVA), Arvico 4-90STD-D (90 kVA), Davco 400-100/120 (90 to 120 kVA), TLD ACE-4120 (120 kVA), TLD GPU-4000 (90 to 180 kVA by option), Stewart & Stevenson TM4140 (140 kVA) | Hobart Jet-Ex 4D (28.5 VDC), Hobart Jet-Ex 5D (28.5 VDC, 600 A), Tronair 112820C (800 A continuous, 2,000 A peak) |
Not sure if you need 400 Hz or 28 VDC?
Tell us your aircraft types and how you plan to use the unit. Victory GSE in Los Angeles can help you buy, rent or lease the right GPU.
What If You Operate Both AC and DC Aircraft?
If your ramp serves both airliners and light aircraft, you need both outputs. You can run two dedicated units or choose a unit that offers both.
Some 400 Hz units offer a 28 VDC option. The Arvico 4-90STD-D listing notes an optional 28 VDC output for aircraft that need DC to start. Dual-output units save ramp space but tie both jobs to one machine.
FBOs with mixed traffic often keep a 90 kVA unit for jets. A separate DC unit then covers turboprops and helicopters. Browse the full range on our ground power units page.
Why Use a GPU Instead of the Aircraft’s APU?
A GPU lets crews shut down the APU while parked. That cuts fuel burn, noise and emissions on the ramp.
Airports have put federal money behind this. The FAA’s Voluntary Airport Low Emissions (VALE) program, established in 2004, funds ground power projects that reduce APU use. A VALE project at Oakland International electrified 18 parking positions. The FAA expects it to avoid over 400 tons of NOx.
Diesel GPUs remain the flexible choice for remote stands, hangars and FBOs without gate power. They go where the aircraft is, not just where the wiring is.
Buying, Renting or Leasing a GPU
Victory GSE sells, rents and leases ground power units. Buying makes sense when GPU demand is steady year round. Renting or leasing fits seasonal peaks, a new contract, or a trial before you commit to a model.
Either way, the sizing rules above still apply. Match the output type and rating to your aircraft first. Then pick the option that fits your budget and timeline. Our team can talk through both.
400 Hz vs. 28 VDC GPU FAQ
Can a 400 Hz GPU power a 28 VDC aircraft?
Not directly. A 400 Hz unit only powers a DC aircraft if it has a built-in 28 VDC output or option. Otherwise, you need a separate 28 VDC GPU.
Is a 90 kVA GPU enough for a Boeing 737 or Airbus A320?
A 90 kVA, 400 Hz unit is the common choice for narrow-body aircraft like these. Confirm the aircraft’s ground power demand in the manufacturer’s data, and size up if you run heavy cabin loads.
How do I tell which power type my aircraft uses?
Look at the external power receptacle and check the flight manual or maintenance manual. AC and DC receptacles use different pin layouts, and the placard near the door usually states the voltage.
What does peak amps mean on a DC ground power unit?
Peak amps is the short burst of current the unit can deliver during an engine start. Continuous amps is what it can supply for long periods to run the electrical bus.
Can I rent or lease a GPU instead of buying?
Yes. Victory GSE offers ground power units to buy, rent or lease. Contact the team with your aircraft types and timeline to talk through the options.
Buy, rent or lease a ground power unit
Victory GSE lists 400 Hz and 28 VDC GPUs from Hobart, Tronair, TLD and other makers. Tell us your aircraft and we will help you pick the right unit.
