28V DC vs. 400Hz AC Ground Power Units: Understanding Aircraft Power Requirements

Walk any ramp and you’ll see the same scene: a crew member hauling a thick cable toward a parked aircraft, a unit humming on the apron, and someone checking a placard beside the receptacle. What most people never notice is the question that decided which cable that was. Does this aircraft want direct current at 28 volts, or three-phase alternating current at 400 hertz?

Getting that wrong isn’t a small slip. This guide explains how aircraft ground power units deliver each type of power, where each one is used, and the practical factors that should drive your decision when you buy, specify or replace a GPU.

Why Aircraft Ground Power Units Matter on Every Ramp

An aircraft sitting at the gate or in the hangar still needs electricity. Crews run avionics checks, load flight plans, power the cabin lighting and keep the environmental systems going. The alternative to a ground power unit is running the aircraft’s own auxiliary power unit or its engines, which burns fuel, adds noise and wears expensive hardware for no reason. A good GPU takes that load off the aircraft. For the bigger picture, our article on why modern ground power units matter covers what reliable ground power does for turnaround times and operating cost.

28.5V DC Ground Power: How It Works and Where It’s Used

Direct current ground power plugs straight into the aircraft’s DC electrical system. There’s no conversion on the aircraft side, which is a large part of why it’s so common on smaller airframes.

Why the Output Is 28.5V and Not 28V

Aircraft electrical systems are described as 28V nominal. A GPU outputs a touch higher, 28.5V, for two reasons. First, there’s always some voltage lost in the output cable, especially when hundreds of amps are flowing. Second, the aircraft’s bus needs to see a healthy supply voltage, not a sagging one. Many DC units add adjustable current limiting and line-drop compensation so the voltage the aircraft sees stays steady even on a long cable run.

Where DC Power Is the Right Choice

• Engine starting. Turbine engines on many turboprops, business jets and helicopters are started electrically, which takes a big surge of current. For scale, Jetall’s rectifier packs are rated around 800 amps continuous with peaks up to 2,200 amps for starting.
• Avionics and maintenance work. Technicians can power a DC bus for troubleshooting without draining the battery.
• Helicopter and light aircraft operations where the electrical architecture is DC from end to end.
• Military and legacy fleets built around 28V DC distribution.

What to Watch With DC

• High current means heavy cables. Long runs lose voltage and get awkward to handle.
• The current limit has to be set correctly. Too low and the engine won’t spin up, too high and you stress the aircraft’s wiring.
• Output quality matters. Modern avionics don’t like noisy or rippled DC.

If DC is your main requirement, our practical guide to Jetall’s 28.5V DC electric GPU lineup walks through the electrically powered options in detail, including the JTL28TR-400 transformer-rectifier kit.

400Hz AC Ground Power: How It Works and Where It’s Used

Larger aircraft generate their own AC power in flight, so on the ground they expect the same thing from the outside. The standard is three-phase, four-wire, 115/200 volts at 400 hertz.

Why 400Hz?

It comes down to weight. Transformers and motors get physically smaller as frequency goes up, and every kilogram saved in an aircraft is a kilogram that can be fuel or payload. Our everyday 50 or 60Hz grid power is a poor fit for that job, which is why a GPU for AC aircraft has to generate or convert its own 400Hz output.

Reading “115/200V”

The 115V figure is measured from any one phase to neutral, and the 200V figure is measured between two phases. Both numbers describe the same three-phase supply, and the aircraft uses whichever it needs. The GPU’s voltage regulation matters here, since aircraft protection systems will trip the connection if voltage or frequency drift out of tolerance.

Where AC Power Is the Right Choice

• Commercial airliners and large transports parked at the gate, where cabin systems, galleys, lighting and air conditioning run off ground AC.
• Regional jets and larger business aircraft that carry AC systems alongside a DC bus.
• Hangar and maintenance operations where long periods of powered-up systems checks would otherwise burn APU hours.
• Airline and ground handling fleets that want to keep the APU off during turnarounds to cut fuel burn and noise.

What to Watch With AC

• Size in kVA, and leave headroom for the heaviest aircraft you expect to serve.
• Frequency stability and voltage regulation affect how smoothly sensitive equipment behaves.
• Cable length and connector condition still matter. A worn connector is a common source of intermittent faults.

For a closer look at units built for AC work, see our aircraft ground power unit range, or the JTL90D3 90 KVA engine-driven GPU as an example of a 400Hz unit with an optional 28.5V DC rectifier.

28V DC vs. 400Hz AC at a Glance

Factors That Influence Aircraft Ground Power Unit Selection

Once you know which kind of power you need, the rest of the decision is about fit. These are the questions we’d walk through with any operator, and they line up with the points in our guide on how to choose a new ground power unit for your aviation operations.

1. Your Aircraft Mix

Start with the aircraft maintenance manual for every type you serve. It states the voltage, frequency, connector and load the aircraft expects. Don’t rely on a rule of thumb about aircraft size. Work from the documents.

2. Load, Including Peaks

For AC, add up what the aircraft draws and keep a margin for your heaviest visitor. For DC, don’t stop at the continuous rating. Engine starts demand a short, intense surge that a marginal unit can’t deliver.

3. Electrically Powered or Engine-Driven

Plug-in units suit hangars and fixed gates where mains power is available, and they’re quieter and cleaner. Engine-driven units carry their own power and go wherever the aircraft is parked. Read how electric ground power units are changing the aviation industry and how engine driven GPUs support aircraft during ground operations, then browse our electrically powered GPUs and engine driven GPUs.

4. Mobility and Ramp Layout

Think about how far the unit travels in a typical shift. A fixed installation at a gate is a different animal from a trailer-mounted unit that moves between stands. Smaller airfields should look at our notes on portable GPU solutions for remote or regional airports.

5. Climate and Duty Cycle

A unit that runs a few hours a week has different needs from one that works around the clock. Cold-climate sites benefit from options such as cold weather kits and battery blankets, and diesel engines built to current emissions tiers help with compliance.

6. Power Quality and Protection

Clean output and built-in protections guard both the aircraft and your crew. Look for monitoring of voltage, frequency and current, plus fault shutdowns. Our article on the modern standard for ground operations beyond the hangar explains how real-time monitoring helps here.

7. Support and Total Cost

Purchase price is only one line on the ledger. Fuel, maintenance, parts availability and distributor support decide what a GPU really costs over its service life. The checklist in criteria to consider when selecting aircraft ground power units is a useful companion, and airlines should also read what airlines should look for when partnering with a GPU provider.

Do You Need Both DC and AC?

Often, yes. A busy FBO might see a Citation in the morning, a turboprop at noon and a larger jet in the evening, each wanting something different. There are a few ways to cover that:

• A dual-output engine-driven GPU. Some 400Hz units add a 28.5V DC rectifier, such as the optional output on the JTL60D3.
• A transformer-rectifier kit that makes 28.5V DC from an existing 400Hz source.
• Two dedicated units if your volumes justify keeping each type on its own cart.
• A fixed frequency converter at the gate for AC, with a portable DC cart for the rest.

The right mix depends on how often each type of aircraft actually shows up. Count visits for a few months before you commit.

Common Mistakes When Choosing a Ground Power Unit

• Buying on kVA alone and ignoring peak starting current for DC aircraft.
• Skipping the aircraft manual and assuming one connector fits all.
• Forgetting cable length, which affects voltage at the aircraft.
• Choosing the cheapest unit without checking parts and service support.
• Ignoring noise and emissions rules at hangars, gates or night operations.
• Planning only for today’s fleet instead of the aircraft you’ll serve in five years.

Choosing Aircraft Ground Power Units With JETALL GPU

JETALL GPU has built engine-driven and plug-in ground power units since 2003, in 28.5VDC configurations and AC models from 38KVA to 140KVA, and has supplied FBO ramps, military bases, airlines and ground handlers around the world. You can learn more about our company, browse the full product range, or find a local contact through our distributors page.

Frequently Asked Questions About 28V DC and 400Hz AC Ground Power

What is the difference between 28V DC and 400Hz AC ground power?

28V DC (set at 28.5V on the GPU) is direct current that matches the DC bus of smaller aircraft and supplies the heavy current needed to start turbine engines. 400Hz AC, usually 115/200V three-phase, matches the generator output of larger aircraft and runs their AC systems such as avionics, galley, lighting and environmental control while parked.

Why do aircraft use 400Hz instead of 50 or 60Hz?

A higher frequency lets designers use much smaller and lighter transformers and motors for the same power. Weight is expensive in an aircraft, so the industry settled on 400Hz for onboard AC systems.

Why is ground power set to 28.5V rather than 28V?

The aircraft’s nominal system voltage is 28V, but a GPU has to overcome voltage drop in the output cable and keep the aircraft’s battery bus properly supplied. Setting the output slightly above nominal (28.5V) covers that, and many units add line-drop compensation for long cables.

Can one ground power unit supply both DC and AC?

Yes. Some engine-driven units generate 400Hz AC and carry an optional 28.5V DC rectifier, so one machine can serve both types of aircraft. Whether this makes sense depends on how much of each load you actually see on your ramp.

How do I size a GPU for my aircraft?

Start with the aircraft manufacturer’s maintenance manual for the voltage, frequency, connector and load. For AC, size in kVA with headroom for the heaviest aircraft you service. For DC, check both continuous amps and the peak current needed for engine starts.

Is an electric GPU better than an engine-driven one?

Neither is better in every case. Electrically powered units are quieter and cleaner where mains power is available, such as hangars and gates. Engine-driven units are self-contained and suit remote stands, regional airfields and operations without convenient mains supply.