Conductor Resistance Calculator

A conductor resistance calculator determines the DC resistance in ohms of a uniform conductor from its material resistivity, physical length, and cross-sectional area. That resistance is used in voltage-drop review, conductor loss calculations, motor and control-circuit analysis, and verification of a proposed conductor path before selecting or confirming an AWG or kcmil conductor.

The calculator produces two electrical values:

• Resistance — opposition to current flow, expressed in ohms.
• Conductance — the reciprocal of resistance, expressed in siemens (S).

For a known conductor material and geometry, resistance increases directly with conductor length and decreases as cross-sectional area increases. A longer run has more resistance; a larger conductor has less resistance.

Resistivity and Conductor Geometry

The calculation uses the following inputs:

Input	Meaning	Unit
Resistivity	Material property entered at the stated reference condition	ohm-mm2/m
Conductor length	Physical length of the conductor path	m
Cross-sectional area	Metal cross-sectional area of the conductor	mm2

Resistivity is not the same as resistance. Resistivity is a property of the conductor material at a specified condition, while resistance is the result for a specific length and cross-sectional area.

For example, a value entered as 0.0175 ohm-mm2/m represents the resistivity used by the calculation. The calculator does not identify conductor material, convert AWG or kcmil sizes, apply insulation temperature ratings, or substitute manufacturer-published resistance values.

Resistance Formula

The conductor resistance formula is:

\(\displaystyle R = \frac{\rho L}{A}\)

Where:

• (R) = conductor resistance in ohms
• \(\rho\) = Resistivity in ohm-mm2/m
• (L) = Conductor length in m
• (A) = Cross-sectional area in mm2

Conductance is calculated from the resulting resistance:

\(\displaystyle G = \frac{1}{R}\)

Where:

• (G) = conductance in siemens (S)
• (R) = resistance in ohms

The units in the calculator must remain consistent. With resistivity in ohm-mm2/m, use Conductor length in meters and Cross-sectional area in square millimeters.

Calculation Example

Enter the following conductor values:

Input	Value
Resistivity	0.0175 ohm-mm2/m
Conductor length	100 m
Cross-sectional area	10 mm2

Resistance is:

\(\displaystyle R = \frac{0.0175 \times 100}{10}\)

\(\displaystyle R = 0.175\ \text{ohm}\)

The calculator returns:

Result	Value
Resistance	0.175 ohm
Conductance	5.7143 S
Resistivity used	0.0175 ohm-mm2/m
Length used	100 m
Area used	10 mm2

A 100 m conductor with these entered properties has a calculated resistance of 0.175 ohm. If the conductor length doubles while resistivity and area remain unchanged, resistance also doubles to 0.350 ohm. If the cross-sectional area doubles to 20 mm2 at the same length, resistance is reduced by half to 0.0875 ohm.

Voltage-Drop and Circuit Use

Resistance is commonly carried into a voltage-drop calculation:

\(\displaystyle V_D = I R\)

Where \(V_D\) is voltage drop and (I) is current. In a complete two-wire circuit, the resistance calculation must reflect the actual current path. For a supply and return conductor of equal length and size, the loop length is typically the outgoing conductor length plus the return conductor length.

For example, a 100 m one-way run may require 200 m of total circuit conductor length for a two-conductor DC or single-phase loop-resistance calculation. The correct length depends on the actual circuit arrangement, including any grounded conductor, equipment grounding conductor, parallel paths, or three-phase configuration.

Resistance also supports calculation of conductor heating loss:

\(\displaystyle P = I^2R\)

This is useful when comparing proposed conductor sizes, estimating losses in a long branch circuit or feeder, and evaluating why a smaller conductor can produce greater voltage drop and resistive heating at the same load current.

Field Verification

Use the entered Cross-sectional area only when the actual conductor metal area is known. For installed building wire, AWG and kcmil designations are normally converted from a recognized conductor-size reference or verified against manufacturer data. Do not assume a nominal outside diameter, insulation diameter, or raceway-fill dimension is the conductor’s metal cross-sectional area.

The calculation is physics-based and does not replace:

• AWG or kcmil conductor resistance tables.
• Manufacturer DC resistance data for a specific conductor construction.
• AC impedance data where inductive reactance, skin effect, proximity effect, conductor spacing, raceway material, or phase arrangement affect circuit performance.
• Ampacity selection, which requires conductor insulation temperature rating, terminal rating, ambient-temperature correction factor, adjustment factor, and the number of current-carrying conductors.
• Raceway fill, conduit layout, bending limitations, pull tension, or minimum-bending-radius requirements.
• Voltage-drop design review using the installed circuit configuration and load current.
• Applicable NEC requirements, project specifications, utility requirements, and AHJ interpretation.

For branch circuits and feeders, use the calculated resistance as one input in the engineering or installation decision. Confirm the final conductor size using the applicable ampacity rules, terminal limitations, equipment listing instructions, voltage-drop criteria, and actual field routing.

Related workflows: AWG Wire Resistance Calculator and Conductor Resistance Temperature Calculator.