I have seen several parts that use 28 Volts DC, is this a standard for jet airliners (e.g., Airbuses and Boeings).
4$\begingroup$ Additionally you may want to know what do the generators/batteries initially deliver, before conversion. $\endgroup$– minsJan 16, 2018 at 0:03
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.
$\begingroup$ 28VDC was in use in aerospace applications already before the A300's introduction. For example, Apollo used 28VDC (see PDF pages 4, 6) and Apollo Spacecraft Familiarization training material for course APC-118 (see PDF page 152). At which point one would have to ask: Why not use what obviously works? Absent good reasons to change, one might just go with what's already used for similar purposes elsewhere in the industry. $\endgroup$– userJan 16, 2018 at 14:27