are you referring to DOT#?
If you act strictly as a mobile carpet cleaning unit a "
big truck' is not required to have DOT number there are 4 specific provisions for DOT certs
1 A Motor carrier DOT cert is required for commercial transport of property with any vhicle with a GVWR of 10,000lbs or more
2 Or any vehicle of any weight class carrying Federal scheduled hazardous materials...such as explosives, radio active material
3 Any vehicle transporting 5 or more people or has a GVWR or 8,500lbs or greater
4 Any comical vehicle with a GVWR of 26,001 or greater is defined as a commercial carrier
in most states ..as long as you do not transport customers property or goods for sale to customers you do not meet the definition of a "motor carrier"
you can check here
http://www.fmcsa.dot.gov/registration-l ... iption.htm