standards and guidelines for communication sites - Radio And ...

SPECIAL GROUNDING (EARTHING) SITUATIONS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
• Installation of down conductors to a lower area where there is usable soil for the installation of
vertical ground rods. The down conductors would be more effective if buried below ground like a
radial grounding conductor, wherever possible. The down conductors shall be sized according to
length as shown below in Table 4-8 (based on MIL-HDBK-419A). See Figure 4-69 on page 4-96.
TABLE 4-8 GROUNDING CONDUCTOR SIZING
Conductor length in
linear m (ft)
Conductor size in
mm 2 csa (AWG / MCM)
Less than 10 (Less than 33) 33.62 (2)
10.36 – 12.5 (34 – 41) 42.4 (1)
12.8 – 16 (42 – 53) 52 (1/0)
16.5 – 20 (54 – 66) 67.4 (2/0)
20.4 – 25.6 (67 – 84) 85 (3/0)
25.9 – 32 (85 – 105) 107 (4/0)
32.3 – 38.1 (106 – 125) 126.70 (250 MCM)
38.4 – 45.7 (126 – 150) 152 (300 MCM)
46 – 53.34 (151 – 175) 177 (350 MCM)
53 – 76.1 (176 – 250) 253.4 (500 MCM)
76.4 – 91.39 (251 – 300) 300 (600 MCM)
Greater than 91.39 (300) 380 (750 MCM)
4-94 68P81089E50-B 9/1/05