In congested areas and where required for safety and service continuity, primary
and secondary distribution circuits should be placed underground in duct banks
or direct-buried. Ductbanks should be encased in concrete and further
reinforced if a road crossing is anticipated. A good design practice on new
installations is to allow 25 percent spare ducts in a duct bank for future use.
Cable pulling tension and sidewall pressure should be calculated when the
potential for damage exists.
In general, direct buried cable should be avoided. In certain circumstances, it is
economical to design and install direct buried cable depending on the importance
of the intended service and the security required for these circuits. The
installation depth of the cable and other safety aspects should meet the
requirements of Section 35 of the National Safety Code C2. The following
considerations are related to this type of installation:
The back-fill material should be sand- or rock-free, screened back fill,
which is free from rotting wood or organic matter that might attract
A 3-inch thick (minimum) protective concrete tile or other protective means
should be placed 6 to 8 inches above the cable to warn of buried cable
Underground cables and/or duct systems are provided with physical
protection and identification (e.g., the top surface of a concrete duct bank
may be pigmented with red concrete dye).
Electrical energy metering is recommended at each substation rated 500 kVA or
larger capacity. Kilowatt hour meters with demand indicators should be
considered in the overall facility load management system.
If open wire distribution systems are being used, lightning protection should be
considered. On multi-grounded neutral systems, arresters installed on all three
phases at prescribed intervals may provide adequate protection. If the