Refer to NEC 2005 Article 220 for a complete guide.
Use Australian Standard AS3000 Annex C.
Howver, The sixteenth edition of the IEE's Guide to BS7671 states in paragraph 311 that the maximum demand shall be assessed. If you have more info on this please tell me.
It depends on if the building uses electric heat. Typical demands I have seen for commerical office buildings are (not suitable for design purposes)-
Electric heat - 15 W/sq foot
No electric heat - 10-12 W/sq foot