I have 1 by 4 inches pipes can I put 2x16mm2 sq. wires but they are in different circuits?
Is there any in NEC that mention the we can only put 1 circuit per conduit?

Yes, more than one circuit is allowed in a raceway.

If the quantity of current carrying conductors exceeds 4, derating must be done as per NEC Table 310.15 (B) (2) (a).

What article exactly that say I can put two circuits in 1 conduit in NEC.I have here 4 core 16sq. mm. that will be connected in different panel but I have only one 4 inches pipe in my manhole.

Since it is allowed, there is no section that states that this practice is allowed.

You can find references that limit what can be in one raceway, such as NEC 2008 Section 200.3 (D), that explains that grounded conductors of different systems need to be colered coded differently, for example.

I question why your using the NEC, but using the metric sizes of wire?

Are you located in the USA?

Im using NEC because it is stated all the works here as per NEC . All the works here are distribution /utility..
Thanks for the help.
Im working at AFCEE Air force here afghanistan..
I am very well in metric still catching up to convert awg..

They are asking me now what article in NEC that I can put 2 circuits in one raceways/conduit?
As you told its allowed what article can I prove it is allowed. Thanks

Cable tray and a conduits are raceway ? Or only cable tray is not a raceway or maybe its only a support? Or conduits are only the race way?

Not sure who "they" are, but show them Tables in Annex Three of the NEC. Lists quantity of specific conductors in all size conduits.

Why are you having such a difficult time getting "someone" to understand that more than one circuit can be in a conduit?

For example, if I recall correctly, you can fit 9 #12 THHN in a 1/2 EMT.

Also, See Section 110.36.

Cable tray is a raceway, has special requirements and limitations, see Article 392.