Ground power: two steps, not one
External power is 115V AC. The 787 has three external power receptacles — two forward and one aft — each with its own distinct purpose.
The two forward receptacles are located on the left forward fuselage. Connecting a ground power unit to either one — before anyone has touched a switch in the flight deck — automatically activates Ground Handling mode. The airplane comes partially to life: cargo compartment lighting, cargo door actuation, cargo handling systems, equipment cooling fans, battery chargers, and APU start capability all become available. The ground crew can begin loading and the batteries begin charging without waiting for any flight crew input whatsoever.
Selecting the forward external power switch ON on the flight deck is a separate, deliberate act. This activates External Power On mode, energizing the full electrical system: flight deck equipment, cabin systems, IFE, hydraulics in limited operation, and PECS.
The two-step nature of forward external power is worth understanding precisely. Connecting the GPU triggers Ground Handling mode automatically — the AVAIL light illuminates. Selecting ON triggers External Power On mode — the ON light illuminates. The ground crew's work begins at connection. The flight crew's work begins at selection. These are not the same event.
Two forward external power sources connected and both selected ON maximizes available loads and minimizes load shedding. As established in the opening chapter, this is a capacity problem: each GPU supplies 90 kW and the fully powered airplane can demand more than that. One GPU serves the most critical loads and sheds the rest. Two GPUs provide 180 kW combined — sufficient for the full load. The voltage is correct either way. What changes is the total power capacity available.
Why one GPU cannot power the whole airplane
Imagine a city that needs 180,000 workers to function. Police officers, doctors, teachers, bakers, clerks, engineers — every one of them essential to keep the city running. The city opens at 6am. There is one highway into town, and it can carry 90,000 commuters. At 6am the gates open and workers flood in — but the highway is already full. Half the workforce cannot get through. The city has to make a choice: it lets the most critical workers in first — police officers, doctors, firefighters — and turns the rest away. Some services open. Others stay dark.
The next day, a second identical highway opens alongside the first. Same capacity, same quality of road — just a second one running in parallel. At 6am, both highways open together. 180,000 workers flow into the city. Every post is filled. Every service opens on time.
Nothing changed about the first highway. It was never broken. It was always doing exactly what it was designed to do. The city simply needed two of them running at the same time to meet its full demand.
That is the 787 on a single GPU. One ground power unit delivers 90 kW — exactly what it is designed to deliver. But the fully loaded 787 needs more than that. Connect a second GPU and you have 180 kW. Full capacity. Every system powered. No load shedding.
Notice what did not change between one GPU and two. The voltage was 115V AC in both cases — the same highway, the same surface, the same quality. What changed was the total power capacity — the total number of lanes. This is a capacity problem, not a voltage problem. One source at 90 kW is insufficient not because its voltage is wrong but because its total power capacity cannot meet the total demand. Two sources at 90 kW each solve the problem by doubling the available capacity.
Every load on the airplane draws a certain amount of power, measured in watts. Add up everything running at once — galleys, lighting, avionics, cabin systems, cooling, equipment — and you get the total power the airplane is demanding at that moment. The source supplying it has a power capacity, and the rule is simple: demand must never exceed capacity.
But what happens when demand would exceed capacity and there is no more capacity to add — a generator has failed, or only one source remains? The system does not simply overload and collapse. Instead it sheds load. The BPCU drops the non-essential loads, one group at a time, until the total demand falls back below what the remaining source can actually provide. It is the same arithmetic from the other direction: if you cannot raise the capacity to meet the demand, you lower the demand to meet the capacity. Either way, the inequality must hold — demand below capacity, always.
Volts is the quality of the supply. Capacity is the quantity — how much power the source can deliver. The airplane needs a certain quality — 115V AC for external power — and it needs a certain capacity — more than 90 kW to be fully powered. The distinction matters every time you think about why a limit exists in an electrical system.
If forward external power is selected ON with no APU or engines running, the 115V AC system is energized directly by external power. It then energizes the 235V AC buses through power conversion devices — a reversal of the normal flow direction, where 235V feeds down to 115V. External power flows upward through the system. Once the APU or engines start, the generators energize the 235V buses directly and the normal flow direction is restored.
Why isn't one ground power unit enough to run the whole airplane?
It is a capacity problem, not a voltage one. One unit supplies 90 kW, and a fully powered 787 demands more, so on a single unit the airplane sheds its non-essential loads. Two units give 180 kW — enough to run everything with nothing shed. The voltage was correct all along; there simply was not enough of it.