There really isnt a minimum value of voltage specified by the NEC. However, for proper operation of equipment, there should not be more than around 3% to 5% to drop from utility supply to the farthest device from the service.
If you reading 100-volts at the service, you definetly have problems. 110.3(B) of the 2002 NEC states that equipment shall be used in acordance with any instructions included in the listing or labeling.
If the utility supply is 120-volts nominal, the lowest acceptable would be around 115-volts.
Is this reading on each leg to ground, or just one phase?