AEMC 4500 - Ground Resistance Values

22 Digital Ground Resistance Tester Model 4500
Grounding Nomograph
Ground Rod
Resistance-Ohms
Soil Resistivity
(Ohm-centimeters)
Rod Depth
Feet
Rod Diameter
Inches
R
P
D
DIA
20
30
40
90
50
60
70
80
100
15
10
1
5
2
4
3
5/8
1/4
1/2
3/4
1
1.5
8
7
6
5
4
3
2
10000
15000
100000
50000
40000
30000
20000
3000
500
1000
2000
4000
5000
20
30
100
90
80
70
60
50
40
15
1
2
3
4
5
6
7
8
9
10
K
Figure 8
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specied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.