Ground Resistance Values - AEMC 4620 User Manual

Ground Rod
Resistance-Ohms
R
100
90
80
70
60
50
40
30
20
15
10
9
8
7
6
5
4
3
2
1
1. Select required resistance on R scale.
2. Select apparent resistivity on P scale.
3. Lay straightedge on R and P scale, and allow to intersect with K
scale.
4. Mark K scale point.
5. Lay straightedge on K scale point and DIA scale, and allow to intersect
with D scale.
6. Point on D scale will be rod depth required for resistance on R scale.
4.2

Ground Resistance Values

NEC 2008 article 250.56 regarding the resistance of rod, pipe and plate
®
electrodes states that if the rod, pipe, or plate does not have a resistance
of 25Ω or less to ground shall be augmented by one additional electrode of
any of the types specified by 250.52 (A)(4) through (A)(8). Where multiple
rod, pipe or plate electrodes are installed to meet the requirements of the
section, they shall not be less than 6 feet apart.
Digital Ground Resistance Tester Model 4620 and 4630
Grounding Nomograph
Soil Resistivity
(Ohm-centimeters)
P
100000
50000
40000
30000
20000
15000
10000
5000
4000
3000
2000
1000
500
Figure 7
Rod Depth
Feet
D
K
100
90
80
70
60
50
40
30
20
15
10
5
4
3
2
1
Rod Diameter
Inches
DIA
8
7
6
5
4
3
2
1.5
1
3/4
5/8
1/2
1/4
21