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Calcimator

Grounding System Calculator

Design grounding electrode systems: calculate single rod resistance, parallel rod resistance, touch and step voltages.

Inputs

Ω·m

Clay: 40 Ω·m; loam: 100; sand: 1,000; gravel: 2,000; rock: 5,000+ per IEEE Std 80

ft

NEC 250.52(A)(5) minimum: 2.4 m (8 ft); 3 m (10 ft) preferred for lower resistance

mm

NEC 250.52 minimum: 16 mm (5/8") steel; 13 mm (1/2") copper-clad steel

rods

NEC 250.53: if single rod resistance >25 Ω, install supplemental electrode; 2 rods typical

ft

NEC 250.53(B) minimum: 1.8 m (6 ft); IEEE Std 80 optimal: spacing ≥ rod length (3 m)

Results

System Resistance

17.63 Ω

Meets 25Ω Limit?

1 (1=yes, 0=no)

Single Rod Resistance33.49 Ω
Touch Voltage (est.)12,339.41 V
Step Voltage (est.)5,288.32 V
How to Use This Calculator
  1. Enter the Soil Resistivity in Ω·m — measure with a four-point (Wenner) test or estimate from soil type: clay ≈ 40, loam ≈ 100, sand ≈ 1,000.
  2. Enter the Rod Length in meters — NEC minimum is 2.4 m (8 ft); 3 m (10 ft) rods are standard.
  3. Enter the Rod Diameter in mm — standard sizes are 16 mm (5/8") or 19 mm (3/4").
  4. Enter the Number of Rods and the Rod Spacing in meters — rods should be spaced at least 1× their length apart for effective parallel operation.
  5. Read the System Resistance in Ω — NEC recommends below 25 Ω; utility and sensitive equipment installations often require below 5 Ω.
  6. Review estimated Touch Voltage and Step Voltage to identify safety concerns and whether additional rods or a ground ring is needed.

How the result changes with Number of Rods

Number of RodsSystem ResistanceMeets 25Ω Limit?
2.912.16 Ω1 (1=yes, 0=no)
7.654.61 Ω1 (1=yes, 0=no)
132.71 Ω1 (1=yes, 0=no)
181.96 Ω1 (1=yes, 0=no)

What each input means

Soil Resistivity
Soil resistivity in ohm-meters. IEEE Std 80 and NFPA 70 (NEC) Article 250 require adequate grounding system resistance. Clay ≈ 40 Ω·m, loam ≈ 100, sand ≈ 1000, rock ≈ 5000.
Rod Length
Length of each ground rod. NEC 250.52(A)(5) requires grounding electrodes to be at least 2.4 m (8 ft) long; 3 m (10 ft) rods are more common and provide better resistance.
Rod Diameter
Diameter of the ground rod. NEC 250.52(A)(5) requires minimum 15.9 mm (5/8 in) for steel/stainless, 13 mm (1/2 in) for copper-clad steel. Standard: 16 mm (5/8 in) or 19 mm (3/4 in).
Number of Rods
Total number of ground rods to be installed. NEC 250.53(A)(2) requires a supplemental electrode if one rod does not achieve ≤25 Ω; IEEE Std 80 recommends parallel rods for sensitive facilities.
Rod Spacing
Distance between ground rods. NEC 250.53(B) requires supplemental electrodes to be at least 1.8 m (6 ft) apart; IEEE Std 80 recommends spacing at least equal to rod length for maximum effectiveness.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Soil Resistivity = 100, Rod Length = 3, Rod Diameter = 16, Number of Rods = 2 = 5 input(s) provided
  2. Calculate System Resistance
    System Resistance
    17.63 = 17.63
  3. Calculate Meets 25Ω Limit?
    Meets 25Ω Limit?
    1 = 1
  4. Calculate Single Rod Resistance
    Single Rod Resistance
    33.49 = 33.49
  5. Calculate Touch Voltage
    Touch Voltage
    12339.41 = 12339.41

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