In the US, it's generally allowed under the National Electrical Code, provided all conductors are insulated for the highest voltage. But the NEC is a safety code, and not a design guide. It sounds like the project is not in the US however.

Hi @dpc,

Thanks for the prompt response. Indeed the project is not in the US but some NEC codes still apply here within the electrical regime.
We are mostly IEC governed. It just does not seem to be a common practice to mix two voltage level conductors in the same cable bundle. In this case, it will be an L1+L2(460VAC) "entering" the transformer and L+N+GND(220VAC) "leaving" the transformer in the same multi-conductor cable. That is why I was wondering if this is acceptable within norms and codes such as IEC/ANSI/IEEE, etc.

Dear Mr. sparkview (Electrical)(OP)4 Apr 22 19:05
"... the project is not in the US but some NEC codes still apply... We are mostly IEC governed. It just does not seem to be a common practice to mix two voltage level conductors in the same cable bundle..... it will be an L1+L2(460VAC) "entering" the transformer and L+N+GND(220VAC) "leaving" the transformer in the same multi-conductor cable. ... I was wondering if this is acceptable within norms and codes such as IEC/ANSI/IEEE, etc..."
1. Cable insulation voltage consideration: I look at it that the (incoming L1-L2 460VAC), the (out going L+N+GND(220VAC). They are within the LOW voltage rating i.e. <1000V per IEC/ANSI/IEEE/NEC, etc... The cable rated good for 460VAC, will be also good for (220VAC). Electrical insulation-wise OK.
2. Safety-wise, (460VAC) and (220VAC) are widely used; technicians generally are aware and considered it of the same danger level.
3. Core marking-wise: IEC L1(Brn),L2(Blk),L3(Gry),N(Blu), E(YwGn) ; ANSI/NEC/IEEE with different colours.
3.1 In your case, perhaps you should use all four 1,2,3 and 4 cores with BLACK colour, with E remain (YwGn). Sleeve core 1 and 2 "460VAC", core 3 (220VAC) L[Brn], core 4 (220VAC) N[Blu]; to avoid confusion and complied with BS7671 or IEC. But NOT to US practice.
Che Kuan Yau (Singapore)

It isn't commonly done because it rarely makes sense. The wire size of the reduced voltage will be twice the size of the primary needs. This means you'll be buying runs of much larger conductor than needed for the primary.

Keith Cress
kcress - http://www.flaminsystems.com