Manholes:
The maximum spacing between manholes is a function of cleaning equipment capability, and is typically specified by local governments . Maximum manhole spacing ranges from 100 to 150 meters (300 to 500 ft) .
Minimum manhole depth is a function of headroom needed for maintenance workers .
Standard interior manhole diameter is 1 200 mm (4 .0 ft) .
Pipe lengths are measured to the inside of manhole walls .
Manholes are usually custom fabricated of precast concrete or heavy corrugated metal, but may also be laid up with unit masonry in the field .