I have seen several parts that use 28 Volts DC, is this a standard for jet airliners (e.g., Airbuses and Boeings).
You'll see all kinds of power on large aircraft - 28 VDC, 115 VAC 60 Hz (and likely 230 VAC 50 Hz), 115 VAC 400 Hz, 5 VDC (USB power in cockpic/pax seats), 230 VAC, and 270 VDC.
Here's a general idea of what the Boeing 787 Power Distribution System consists of:
Airbus use 28V DC, largely for things like sensors etc., so the overall power usage on the DC system is quite low: As an example, the A350 has 4 Transformer Rectifier Units (TRUs) which convert AC to DC. 2 of them are designated as 'normal' and 2 as 'emergency'. Each one has a nominal power of about 300W. Total DC system usage is about 500W. . Unfortunately I don't know about Boeing.
EDIT: Was curious - looks like Boeing use 28V DC as well - which makes sense, I expect Airbus used 28V DC on the A300 as that was the standard at the time, and just kept using it until today.