In a homogenous soil, the grounding resistance of a single 3 m (10 ft) rod is roughly equivalent to 6 m (20 ft) of a horizontal ground electrode buried approximately 0.6 m (0.2 ft) underground.
A 6-m (20 ft) diameter loop and three 6-m (20-ft) electrodes in a horizontal star is equivalent to three 3 m (10 ft) rods driven in 7.5 m (25 ft) spacing. In practice, the connection wires of the rods are also buried forming a smaller star by themselves so that they lower the resistance and the impedance.
Buried wire lengths can be extended to several tens of meters (several dozens of feet) with almost linear relation to grounding resistance value. In a very poor soil, a maximum length of nearly 50 m (150 ft) per wire may be needed, but it is more cost-efficient to treat the soil chemically.
| Minimum values | Ground rod | Horizontal ground electrode |
|---|---|---|
| Length | Typically 3 m (10 ft) | 10 … 30 m (33 … 100 ft), soil type dependent |
| Loop diameter | – | 5 … 6 m (16 … 20 ft) |
| Spacing | Min. 2 × rod length | – |