Instantly Calculate Ohms by Length for Grounding & Bonding Conductors

This guide explains calculating resistance (ohms) from length for grounding and bonding conductors precisely accurately.

Engineers and electricians require fast formulas, tables, and examples for compliant design and inspection work.

Grounding/Bonding Conductor Resistance Calculator (Ohms by Length)

Conductor length (L) (m)

Length unit Meters (m) Feet (ft)

Conductor material Copper (Cu) Aluminum (Al)

Conductor size (preset, AWG/kcmil) Select size 14 AWG (2.08 mm²) 12 AWG (3.31 mm²) 10 AWG (5.26 mm²) 8 AWG (8.37 mm²) 6 AWG (13.30 mm²) 4 AWG (21.15 mm²) 2 AWG (33.62 mm²) 1 AWG (42.41 mm²) 1/0 AWG (53.48 mm²) 2/0 AWG (67.43 mm²) 3/0 AWG (85.01 mm²) 4/0 AWG (107.2 mm²) 250 kcmil (126.7 mm²) 350 kcmil (177.3 mm²) 500 kcmil (253.4 mm²) 600 kcmil (304.0 mm²) Custom cross-sectional area

Advanced options

Custom cross-sectional area (A) (mm²)

Number of parallel conductors

Conductor operating temperature (°C)

Loop length factor (round-trip) One-way conductor only Round-trip (2 × physical length)

Temperature coefficient α (1/°C)

Resistivity at 20 °C ρ20 (Ω·m)

System type (for documentation) Not specified DC grounding/bonding AC 50 Hz AC 60 Hz

Optionally upload an equipment nameplate or grounding diagram photo to suggest conductor size and length values.

📷 Fill inputs from photo (AI)

⚡ More electrical calculators Reiniciar Copiar Compartir

Enter conductor length and size to obtain the grounding/bonding resistance in ohms.

🧠 Explain my result 🔍 Review my inputs

Formulas used

Base resistance of a uniform conductor:
R = ρ · L / A where:

• R = resistance of the conductor (Ω)
• ρ = resistivity at the operating temperature (Ω·m)
• L = effective conductor length (m), one-way or round-trip as selected
• A = metallic cross-sectional area (m²)

Temperature correction of resistivity (relative to 20 °C):
ρ = ρ20 · [1 + α · (T − 20)] where:

• ρ20 = resistivity at 20 °C (Ω·m)
• α = temperature coefficient of resistivity (1/°C)
• T = conductor temperature (°C)

Multiple parallel conductors of identical size and length:
R_total = R_single / n where:

• R_total = equivalent resistance of the parallel conductors (Ω)
• R_single = resistance of one individual conductor (Ω)
• n = number of conductors in parallel

Reference quick values (copper, 20 °C, one-way length)

Size	Area (mm²)	Resistance per 100 m (Ω)	Resistance per 100 ft (Ω)
6 AWG	13.30	≈ 0.13	≈ 0.041
4 AWG	21.15	≈ 0.081	≈ 0.025
2 AWG	33.62	≈ 0.051	≈ 0.016
1/0 AWG	53.48	≈ 0.032	≈ 0.0098
4/0 AWG	107.2	≈ 0.016	≈ 0.0049

Technical FAQ – Grounding/Bonding Resistance Calculation

Does the calculator give one-way or loop (round-trip) resistance?

By default, the calculator uses the physical one-way conductor length. If you select "Round-trip (2 × physical length)" in the loop length factor, the effective length is doubled to approximate the fault-loop path (out and back).

How accurate is the resistance at different temperatures?

The calculator applies a linear temperature coefficient model based on copper or aluminum data. For typical operating ranges (0–90 °C), this is sufficiently accurate for grounding and bonding studies. For extreme temperatures, detailed manufacturer data may be.

What if my conductor size is not listed in the presets?

Choose "Custom cross-sectional area" in the conductor size list and enter the metallic area in mm² under Advanced options. The calculator will then ignore the preset and use your custom area.

Does this tool consider AC skin effect for large conductors?

No. The resistance is calculated as DC resistance with temperature correction. For typical grounding and bonding conductors at 50/60 Hz, this is usually adequate. For very large conductors or high-frequency applications, dedicated AC resistance and impedance calculations are recommended.

Fundamental relationship for conductor resistance

Grounding and bonding conductor resistance is governed by the classical resistivity relation: R = ρ × L / A Where:

• R = resistance (ohms, Ω)
• ρ = electrical resistivity of conductor material (ohm·m)
• L = length of the conductor (m)
• A = cross-sectional area of the conductor (m²)

This formula produces DC resistance at a reference temperature (commonly 20 °C). For a ready-per-length form used in field calculations: R_per_length = ρ / A So resistance for any length is: R = (ρ / A) × L Typical resistivity values at 20 °C:

• Copper: ρ ≈ 1.724 × 10^-8 Ω·m
• Aluminum (pure): ρ ≈ 2.826 × 10^-8 Ω·m

Temperature correction (practical accuracy)

Conductor resistance varies with temperature. Use a linear approximation: R(T) = R_ref × [1 + α × (T - T_ref)] Where:

• R(T) = resistance at temperature T (°C)
• R_ref = resistance at reference temperature T_ref (commonly 20 °C)
• α = temperature coefficient of resistivity (per °C)

Typical α values:

• Copper: α ≈ 0.00393 /°C (at 20 °C)
• Aluminum: α ≈ 0.0039–0.00403 /°C (commonly 0.0039 /°C used)

Example of application: If R_ref = 0.020 Ω at 20 °C and T = 75 °C, for copper: R(75) = 0.020 × [1 + 0.00393 × (75 − 20)] = 0.020 × [1 + 0.21615] = 0.02432 Ω.

Units, geometry, and conversion essentials

Common field units are feet and AWG (American Wire Gauge) or meters and mm². Conversion links are essential for instant calculation. Cross-sectional area for a circular conductor: A = π × d² / 4 Where:

• d = conductor diameter (m)
• A = area (m²)

To convert AWG to area, use tabulated AWG→mm² values; then convert mm² to m² by multiplying by 1e-6. Length conversions:

• 1 ft = 0.3048 m
• 1 m = 3.28084 ft

For quick field arithmetic, calculate R_per_foot or R_per_meter from ρ and A once, then multiply by the required length.

Extensive standard resistance tables (common conductor sizes)

Notes on table data:

• Copper resistivity = 1.724×10^-8 Ω·m; Aluminum resistivity = 2.826×10^-8 Ω·m at 20 °C.
• Ω/1000 ft values are useful for US practice; Ω/m values provide SI convenience.
• Values shown rounded to typical commercial presentation precision.

Metric conductor table (common mm² sizes)

These metric table values are calculated from the same ρ constants and rounded to useful precision for engineering use.

Instant calculation formulas and field shortcuts

For practical instant calculations, precompute resistances per unit length: R_per_m = ρ / A R_per_ft = R_per_m / 0.3048 Then: R = R_per_m × L_m or R = R_per_ft × L_ft If you maintain a small lookup for R_per_ft or R_per_m for the conductor sizes you commonly use, then calculating resistance for any length is multiplication only. Quick field algorithm:

1. Identify material (Cu or Al) and conductor size (AWG or mm²).
2. Fetch R_per_length from table (Ω/ft or Ω/m).
3. Multiply R_per_length by run length.
4. Apply temperature correction if ambient or conductor temperature deviates from 20 °C.

Example of algebraic implementable formula

Given AWG size with area A (mm²), convert to m²: A_m2 = A_mm2 × 1e-6. Then: R (Ω) = (ρ × L_m) / A_m2 Expressed explicitly: R (Ω) = (1.724e-8 × L_m) / (A_mm2 × 1e-6) for copper.

Grounding and bonding conductor specifics and regulatory context

Grounding electrode conductors, equipment grounding conductors, and bonding jumpers each have different sizing and performance expectations in codes and standards. Key regulatory and normative references:

• NFPA 70: National Electrical Code (NEC) — sections relevant: 250.4, 250.66, 250.122, 250.56.
• IEEE Std 142 (Green Book) — grounding system design and conductor selection guidance.
• IEEE Std 80 — substation grounding design and resistance computations.
• IEC 60228 — conductors of insulated cables; cross-section standardization.
• NIST material property data for resistivity and temperature coefficients.

Useful external authority links:

• NFPA (NEC) information: https://www.nfpa.org
• IEEE Xplore (standards summaries): https://ieeexplore.ieee.org
• NIST material data: https://www.nist.gov
• IEC standards information: https://www.iec.ch

Key code notes:

• The NEC specifies conductor sizing for equipment grounding conductors based on overcurrent protective devices (NEC 250.122) rather than purely resistance limits.
• Grounding electrode conductor size (NEC 250.66) depends on the largest ungrounded service-entrance conductor and electrode type.
• For safety, many jurisdictions and industry practices aim for low system grounding resistance values (for example, ≤25 Ω to earth for a single electrode where possible), but code compliance and system design require comprehensive evaluation beyond a single resistance value.

Worked examples — complete development and detailed solutions

Example 1 — Copper grounding conductor resistance for a short bond

Problem statement: Calculate the DC resistance of a 50 ft copper grounding conductor sized 6 AWG at 20 °C. Then compute the resistance at 75 °C. Given:

• Conductor: Copper, 6 AWG
• Length L = 50 ft
• From table: Copper R_per_1000ft for 6 AWG = 0.3951 Ω/1000 ft
• Reference temperature T_ref = 20 °C
• Target temperature T = 75 °C
• α_copper = 0.00393 /°C

Step-by-step: 1. Convert R_per_1000ft to R_per_ft: R_per_ft = 0.3951 Ω / 1000 ft = 0.0003951 Ω/ft 2. Resistance for 50 ft at 20 °C: R_20 = R_per_ft × L = 0.0003951 × 50 = 0.019755 Ω 3. Temperature correction to 75 °C: ΔT = 75 − 20 = 55 °C R_75 = R_20 × [1 + α × ΔT] = 0.019755 × [1 + 0.00393 × 55] Compute α×ΔT = 0.00393 × 55 = 0.21615 R_75 = 0.019755 × 1.21615 = 0.024023 Ω (rounded to 0.02402 Ω) Solution summary:

• DC resistance at 20 °C: R = 0.019755 Ω (≈0.01976 Ω)
• DC resistance at 75 °C: R = 0.024023 Ω (≈0.02402 Ω)

Interpretation: This low resistance illustrates why short copper grounding conductors are essentially negligible for many fault calculations, but even small resistances can affect touch and step potentials in critical installations. Always confirm conductor temperature when computing actual fault impedance.

Example 2 — Long aluminum grounding conductor for a remote grounding electrode

Problem statement: A grounding electrode conductor is to be installed with an 2/0 AWG aluminum conductor, run length 150 ft (one-way). Determine DC resistance at 20 °C and the adjusted resistance at −10 °C (cold climate). Use 2/0 Al area = 67.43 mm² and aluminum resistivity ρ = 2.826×10^-8 Ω·m. Use α_al = 0.0039 /°C. Given:

• Conductor: Aluminum, 2/0 AWG
• Area A = 67.43 mm² = 67.43 × 10^-6 m²
• Length L = 150 ft = 45.72 m
• ρ = 2.826 × 10^-8 Ω·m
• T_ref = 20 °C, target T = −10 °C
• α = 0.0039 /°C

Step-by-step (direct calculation using resistivity formula): 1. Compute R_per_m: R_per_m = ρ / A_m2 = 2.826e-8 / (67.43e-6) = 0.0004192 Ω/m (rounded) 2. DC resistance for L = 45.72 m at 20 °C: R_20 = R_per_m × L = 0.0004192 × 45.72 = 0.01916 Ω 3. Temperature correction to −10 °C: ΔT = −10 − 20 = −30 °C R_−10 = R_20 × [1 + α × ΔT] = 0.01916 × [1 + 0.0039 × (−30)] Compute α×ΔT = 0.0039 × (−30) = −0.117 R_−10 = 0.01916 × 0.883 = 0.01691 Ω Solution summary:

• DC resistance at 20 °C: R = 0.01916 Ω (one-way)
• DC resistance at −10 °C: R = 0.01691 Ω (one-way)

Notes:

• For two-way fault loops, sum the two conductor resistances in the loop.
• Aluminum is significantly higher resistivity than copper; equivalent ampacity or resistance requires larger aluminum cross-section.

Practical considerations for grounding and bonding design

Designers must consider:

• Loop resistance vs. electrode resistance: grounding electrode system resistance to earth and conductor interconnections both affect overall performance.
• Fault current magnitude and duration: thermal and mechanical effects on bonding conductors require compliance with ampacity and conductor temperature-rise rules.
• Corrosion and mechanical protection: aluminum often requires antioxidant compounds and proper terminations to mitigate galvanic corrosion at dissimilar-metal joints.
• Use of parallel conductors: parallel conductors halve or divide resistance only if equal and placed to share current; code-approved methods and connectors must be used.

Sizing vs. resistance — what code expects

The NEC primarily prescribes grounding and bonding conductor sizes by function and ampacity relationships, not a single numeric maximum resistance. Typical practice:

• Use NEC tables for equipment grounding conductor sizing (NEC 250.122).
• Size grounding electrode conductors per NEC 250.66 (based on largest ungrounded service conductor and reference tables).
• Evaluate grounding system resistance goals (e.g., ≤25 Ω for single electrode) but follow comprehensive site-specific design for safety.

Accuracy, measurement, and verification

When validating calculated resistances with measurements, remember:

• Clamp-on DC resistance meters give high convenience for large conductors but must be validated for accuracy.
• Four-terminal (Kelvin) measurements eliminate contact resistance and are the most accurate for short conductors.
• Megger or earth resistance testers (fall-of-potential) are used for grounding electrode system resistance to earth, which is different than conductor DC resistance.

Reference standards and authoritative sources

For normative procedures and deeper design rules consult:

• NFPA 70, National Electrical Code — available from NFPA: https://www.nfpa.org
• IEEE Std 142 (Green Book) — grounding of industrial and commercial power systems: https://ieeexplore.ieee.org
• IEEE Std 80 — guide for safety in substation grounding: https://ieeexplore.ieee.org
• IEC 60228 — conductors of insulated cables (cross-section classification): https://www.iec.ch
• NIST material properties — reference data for electrical resistivity and temperature coefficients: https://www.nist.gov

Best practices checklist for instant field calculations

• Keep a laminated quick-reference table for the conductor sizes you install frequently (Ω/ft and Ω/m).
• Always confirm conductor material and temperature before final risk assessment.
• Apply temperature correction when conductor runs may operate significantly above or below 20 °C.
• Document calculations and measurement results for inspection and maintenance records.
• When in doubt, oversize for lower resistance and improved safety margins, and verify per code requirements.

Quick mnemonic for rapid mental math

If you remember a conductor’s Ω/1000 ft value, multiply by length in feet and divide by 1000: R ≈ (Ω_per_1000ft × length_ft) / 1000 Or with meters: R ≈ (Ω_per_km × length_m) / 1000 This reduces most field calculations to two arithmetic operations.

Final technical notes

Accurate grounding and bonding conductor resistance calculations depend on correct material properties, precise cross-sectional area, and temperature. For safety-critical systems, complement calculation with measurements, and follow applicable code requirements (NEC, IEEE, IEC). Use the tables above or compute on-demand using R = ρ × L / A and remember to perform temperature adjustments when conditions differ from the reference. References:

• NFPA 70: National Electrical Code — authoritative code for electrical installations in the United States.
• IEEE Std 142 — The Green Book: grounding of industrial and commercial power systems.
• IEEE Std 80 — Guide for Safety in AC Substation Grounding.
• IEC 60228 — Standard on conductor cross-section classes.
• NIST — material properties database for precise resistivity and temperature coefficients.

For further reading and downloadable conductor tables from manufacturers, consult major cable manufacturers’ technical publications and the referenced standards documents for the most up-to-date official tables and practice guidance.