Metal conductor resistance generally increases as conductor temperature rises. A wire resistance value listed at 20°C can understate the resistance present when a branch circuit or feeder operates under load at a higher conductor temperature.
The Conductor Resistance Temperature Calculator produces an adjusted resistance value for the selected operating temperature. That adjusted value can then be used in voltage-drop calculations, conductor comparisons, load reviews, and other design checks where the resistance at actual operating conditions is more useful than the published room-temperature value.
For a given conductor size, material, and length, higher temperature means higher resistance. If circuit current stays the same, the higher resistance also increases voltage drop and conductor power loss.
Resistance Temperature Adjustment
The calculation starts with a known resistance at a reference temperature and applies the material’s temperature coefficient:
\(\displaystyle R_T = R_{ref} \times \(1 + \alpha(T - T_{ref})\)\)
Where:
- \(R_T\) = adjusted conductor resistance at the selected operating temperature
- \(R_{ref}\) = conductor resistance at the reference temperature
- \(\alpha\) = temperature coefficient of resistance for the conductor material
- (T) = selected operating temperature
- \(T_{ref}\) = reference temperature associated with the starting resistance value
The calculator uses the reference resistance, reference temperature, material temperature coefficient, and selected operating temperature to return the temperature-adjusted resistance.
A positive temperature difference increases the result for typical metal conductors. A negative temperature difference reduces the calculated resistance relative to the reference value.
Calculation Example
Assume a conductor has a resistance of 1.000 ohm at 20°C. Using a copper temperature coefficient of 0.00393 per °C, calculate its resistance at 75°C.
\(\displaystyle R_{75} = 1.000 \times [1 + 0.00393(75 - 20)]\)
\(\displaystyle R_{75} = 1.000 \times 1.21615\)
\(\displaystyle R_{75} = 1.216\ \Omega\)
The conductor resistance increases from 1.000 ohm at 20°C to approximately 1.216 ohms at 75°C—an increase of about 21.6%.
| Conductor condition | Resistance | Change from 20°C |
|---|---|---|
| Reference condition: 20°C | 1.000 Ω | — |
| Operating condition: 75°C | 1.216 Ω | +21.6% |
The conductor AWG or kcmil size has not changed. The resistance changed because the conductor temperature changed.
Temperature and Voltage Drop
Voltage drop across a conductor is proportional to current and resistance:
\(\displaystyle V_D = I \times R\)
If current remains constant, a 21.6% increase in resistance produces a 21.6% increase in the resistive portion of voltage drop.
For example, if a circuit has 10.0 V of voltage drop using a 20°C resistance value, the same circuit resistance adjusted to 75°C would produce approximately:
\(\displaystyle 10.0 \times 1.216 = 12.16\text{ V}\)
On a 120 V circuit, that changes the voltage-drop percentage from:
\(\displaystyle \frac{10.0}{120} \times 100 = 8.33\%\)
to:
\(\displaystyle \frac{12.16}{120} \times 100 = 10.13\%\)
This relationship is useful when comparing conductor options for long branch circuits, feeders, motor circuits, and other runs where voltage drop affects equipment performance.
Conductor Sizing Applications
Temperature-adjusted resistance is most useful after the basic conductor selection is established. It does not replace ampacity selection, terminal rating review, insulation temperature rating review, or any applicable correction and adjustment factors.
Use the adjusted resistance result when evaluating:
- Voltage drop on long branch circuits and feeders
- Motor starting conditions where conductor voltage loss may affect terminal voltage
- Comparisons between copper and aluminum conductors at a common operating temperature
- Resistance-based power loss using \(I^2R\)
- Circuit performance where published resistance data is stated at a temperature different from expected operation
- Resistance inputs for custom electrical calculations, spreadsheets, or load-review models
For a two-conductor AC or DC circuit, account for the complete current path. A voltage-drop calculation normally needs the resistance of both the outgoing and return conductor path unless the calculation method already includes the full circuit length.
Calculator Inputs and Result
Enter the starting conductor resistance at its stated reference temperature, then apply the temperature coefficient and selected operating temperature. The resulting adjusted resistance should be paired with the actual conductor length and circuit configuration used in the voltage-drop calculation.
A practical sequence is:
1. Determine the conductor resistance at the stated reference temperature.
2. Set the reference temperature that applies to that resistance value.
3. Apply the material temperature coefficient.
4. Select the operating temperature to be evaluated.
5. Use the adjusted resistance in the voltage-drop or power-loss calculation.
Resistance values may be expressed per unit length, such as ohms per 1,000 ft, or as total resistance for a specific conductor run. Keep the units consistent throughout the calculation. If the resistance is stated per 1,000 ft, convert it using the actual circuit length before calculating voltage drop.
Field Verification
The calculation adjusts resistance for temperature only. For critical work, use manufacturer resistance data when available and confirm the temperature basis of that data before applying it.
Actual conductor temperature depends on loading, ambient temperature, conductor installation, raceway arrangement, number of current-carrying conductors, insulation temperature rating, terminations, and heat dissipation conditions. Ampacity, raceway fill, conductor adjustment factors, terminal limitations, and AHJ requirements remain separate field and code-review decisions.
Use the Calculator
Use these Elecatrix tools for the resistance input, temperature adjustment, and voltage-drop review: