I just want to be sure in my computation. The national electrical code provides that, "for each single receptacle shall be considered at no less than 180 watts rating". For me its mean that each convenience outlet is considered to have a maximum load of not less than 180 watts per plug or gang.

If I have 6 convenience receptacles, considering that the there are two plug per outlet, the total number of plug will be: 6 outlets x 2 plug = 12 pieces

The total estimated Load: 12 x 180 watts per outlet = 2,160 watts

The maximum Expected Current Load: 2,160 watts / 230 watts = 9.39 amperes.

In my computation I am right or not ,if not could give some explanation.

This is what is wrong with lay folks trying to read the ever confusing NEC. There is usually way more than meets the eye.
It is also a problem with books that reference the NEC. Many times important information is either misinterpreted or left out all together.

First off:

220.14(I) Receptacle Outlets Except as covered in 220.14(J) and (K), receptacle outlets shall be calculated at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke. A single piece of equipment consisting of a multiple receptacle comprised of four or more receptacles shall be calculated at not less than 90 volt-amperes per receptacle. This provision shall not be applicable to the receptacle outlets specified in 210.11(C)(1) and (C)(2).


See the each yoke comment above. That means a duplex is counted the same as a simplex receptacle. A duplex is NOT two for purposes of this calculation.





Second, and even more important, none of this applies to dwelling units or residences. So you can pretty much forget 220.14(I) all together for residential work:

220.14(J) Dwelling Occupancies In one-family, two-family, and multifamily dwellings and in guest rooms or guest suites of hotels and motels, the outlets specified in (J)(1), (J)(2), and (J)(3) are included in the general lighting load calculations of 220.12. No additional load calculations shall be required for such outlets.
(1) All general-use receptacle outlets of 20-ampere rating or less, including receptacles connected to the circuits in 210.11(C)(3)
(2) The receptacle outlets specified in 210.52(E) and (G)
(3) The lighting outlets specified in 210.70(A) and (B)