Conductor Resistance Temperature Calculator

Related tools: AWG Wire Resistance Calculator, Voltage Drop Calculator, and Wire Size Calculator.

A conductor’s resistance changes as its temperature changes. The Conductor Resistance Temperature Calculator estimates the resistance at an operating temperature from a known resistance value at a base temperature.

The primary result is Adjusted resistance, expressed in ohms. It is commonly used as an input to voltage-drop calculations, conductor loss review, feeder and branch-circuit analysis, motor-circuit studies, and DC or low-frequency circuit calculations where conductor temperature differs from the published or measured resistance reference temperature.

Published conductor resistance is often referenced to a specific temperature, while installed conductors operate at a temperature influenced by load, ambient conditions, insulation temperature rating, terminations, raceway conditions, and heat from adjacent current-carrying conductors. This calculation converts the known resistance basis to the temperature being evaluated.

The calculator does not select AWG or kcmil, determine conductor material, calculate ampacity, apply an adjustment factor or correction factor, evaluate raceway fill, or establish NEC compliance.

Input Values

Field	Electrical use
Base resistance (ohm)	Known conductor resistance at the stated base temperature. This may come from a manufacturer data sheet, a conductor-resistance table, a test value, or a project calculation.
Base temperature (deg C)	Temperature associated with the entered base resistance.
Operating temperature (deg C)	Temperature at which the conductor resistance is to be estimated.
Temperature coefficient (1/deg C)	Linear resistance-temperature coefficient applicable to the conductor material and temperature range being evaluated.

The entered Base resistance (ohm) must represent the same conductor path under review. For example, voltage-drop work requires resistance for the actual one-way or loop length used by the applicable circuit calculation. A resistance value for 1,000 feet cannot be entered as though it were the resistance of the installed feeder length.

The Temperature coefficient (1/deg C) is an input assumption, not a material-selection result. The calculator does not infer whether the conductor is copper, aluminum, copper-clad aluminum, a particular alloy, or another material.

Resistance-Temperature Formula

The calculator applies a linear temperature-resistance relationship:

\(\displaystyle R_2 = R_1 \left[1 + \alpha(T_2 - T_1)\right]\)

Where:

• R_2 = Adjusted resistance
• R_1 = Base resistance
• \alpha = Temperature coefficient
• T_2 = Operating temperature
• T_1 = Base temperature

The temperature difference is:

\(\displaystyle \Delta T = T_2 - T_1\)

The resistance change is:

\(\displaystyle \text{Resistance change} = \left(\frac{R_2 - R_1}{R_1}\right) \times 100\%\)

For conductors with a positive temperature coefficient, resistance increases as operating temperature rises above the base temperature. If the operating temperature is below the base temperature, the calculated resistance decreases.

Calculation Example

Use the following entered values:

Input	Value
Base resistance (ohm)	1.588
Base temperature (deg C)	20
Operating temperature (deg C)	70
Temperature coefficient (1/deg C)	0.00393

First, calculate the temperature change:

\(\displaystyle \Delta T = 70 - 20 = 50\text{ deg C}\)

Then calculate adjusted resistance:

\(\displaystyle R_2 = 1.588 \left[1 + 0.00393(50)\right]\)

\(\displaystyle R_2 = 1.588(1.1965) \approx 1.9\ \text{ohm}\)

The calculator returns:

Result	Value
Adjusted resistance	1.9 ohm
Resistance change	19.65%
Temperature change	50 deg C

The conductor resistance increases by 19.65% between 20 deg C and 70 deg C using the entered linear coefficient.

Voltage-Drop and Circuit Review

For a branch circuit or feeder, the adjusted resistance can be substituted into the resistance component of a voltage-drop calculation when conductor operating temperature is known or intentionally assumed.

A basic resistive voltage-drop relationship is:

\(\displaystyle V_D = I \times R\)

Where I is circuit current and R is the relevant conductor-path resistance. A higher operating temperature produces higher conductor resistance, which can increase calculated voltage drop and conductor power loss.

For DC circuits and many low-frequency calculations, resistance is often the dominant conductor-loss input. In AC systems, conductor impedance can also include reactance and effects associated with conductor arrangement, raceway type, spacing, parallel conductors, and system characteristics. The adjusted resistance result should therefore be used only within the larger voltage-drop method appropriate to the installation.

For example, a long feeder may have adequate ampacity after applying required ambient-temperature correction factors and adjustment factors for current-carrying conductors, yet still require a larger AWG or kcmil conductor to control voltage drop at the expected operating temperature. Ampacity and voltage drop are separate design reviews.

Field and Code Limits

The calculation is an estimate based only on the entered Base resistance, Base temperature, Operating temperature, and Temperature coefficient. It does not account for:

• Conductor material or alloy verification
• Actual AWG or kcmil selection
• AC resistance, inductive reactance, skin effect, or proximity effect
• Raceway configuration, conductor spacing, or parallel-conductor arrangement
• Installation temperature, ambient temperature, or conductor heat balance
• Insulation temperature rating or terminal rating
• Ampacity, correction factors, adjustment factors, or the number of current-carrying conductors
• Termination resistance, splice resistance, connection condition, or measured contact heating
• Manufacturer resistance tolerances or field test uncertainty
• NEC or local-code compliance, engineering approval, or AHJ acceptance

Use the actual conductor specifications, installed length, circuit configuration, terminal limitations, and applicable code requirements when making a conductor-sizing or installation decision.