Copper and aluminum conductors with the same nominal AWG size do not have the same electrical resistance. When AWG, conductor length, and temperature remain constant, aluminum produces a higher resistance value than copper. That higher resistance increases voltage drop for the same current and circuit length.
The result to compare is usually resistance in ohms per 1000 ft and the calculated total conductor or circuit resistance in ohms. Those values feed directly into voltage-drop review, conductor sizing, feeder and branch-circuit planning, motor-load calculations, and evaluation of available voltage at the load.
A resistance comparison must identify the conductor material. An AWG number alone does not establish resistance unless the calculation also specifies copper or aluminum.
Resistance Per 1000 Feet
Resistance is the opposition a conductor presents to current flow. For a fixed AWG size, resistance changes with conductor material, conductor length, and temperature.
For a material comparison, hold these inputs constant:
- AWG size
- One-way conductor length
- Conductor temperature
- Circuit arrangement
- Load current, when reviewing voltage drop
Then change only the conductor material from copper to aluminum.
The calculator’s resistance-per-1000-ft result provides a normalized value for comparing conductors before applying the actual installed length. At the same AWG and temperature, aluminum’s resistance per 1000 ft is higher than copper’s.
That difference becomes more consequential as conductor length and load current increase. A short branch circuit serving a light load may show little practical voltage-drop difference, while a long feeder, a high-current branch circuit, or a motor circuit can lose more voltage with aluminum conductors of the same AWG.
Total Conductor Resistance
The total resistance calculation applies the conductor’s resistance per 1000 ft to the installed length.
\(\displaystyle R_{\text{total}} = R_{\text{per 1000 ft}} \times \frac{L}{1000}\)
Where:
- \(R_{\text{total}}\) is conductor resistance in ohms
- \(R_{\text{per 1000 ft}}\) is the selected copper or aluminum resistance value
- (L) is conductor length in feet
For a two-conductor circuit, voltage-drop review generally requires the resistance of the complete current path. The outgoing conductor and return conductor both add resistance.
\(\displaystyle R_{\text{circuit}} = 2 \times R_{\text{one-way conductor}}\)
Use the actual circuit path. A 150 ft one-way run has 300 ft of conductor path for a typical two-wire line-to-line or line-to-neutral resistance calculation.
The resistance result is electrical arithmetic. The final conductor selection still requires a separate review of ampacity, insulation temperature rating, terminal rating, overcurrent protection, equipment listing, termination compatibility, installation method, and applicable AHJ requirements.
Copper and Aluminum Comparison
The following comparison holds AWG size, length, temperature, and load current constant.
| Condition | Copper Conductor | Aluminum Conductor |
|---|---|---|
| Nominal AWG size | Same | Same |
| Resistance per 1000 ft | Lower | Higher |
| Total resistance at the same length | Lower | Higher |
| Voltage drop at the same current | Lower | Higher |
| Voltage available at the load | Higher | Lower |
| Conductor size needed for equal resistance | Smaller size may work | Larger conductor may be required |
Aluminum is often selected in larger feeder and service conductors where conductor cost, weight, and installation economics are part of the design. It cannot be treated as a direct resistance equivalent to copper at the same AWG. If the design target is a particular voltage-drop limit, the aluminum conductor may need a larger AWG or kcmil size to provide comparable circuit resistance.
Voltage-Drop Calculation Example
Run the same scenario through the AWG Wire Resistance Calculator twice:
| Input | First Calculation | Second Calculation |
|---|---|---|
| Conductor material | Copper | Aluminum |
| AWG size | Same AWG | Same AWG |
| One-way length | Same length | Same length |
| Temperature | Same temperature | Same temperature |
| Load current | Same current | Same current |
Start with the copper result and record:
- Resistance per 1000 ft
- Total conductor resistance
- Total circuit resistance, where applicable
Then change only the material selection to aluminum and record the same values.
The aluminum calculation will return a higher resistance value. Enter the same current, voltage, conductor length, and conductor material into the Voltage Drop Calculator to compare the resulting voltage drop.
For a resistance-based voltage-drop review:
\(\displaystyle V_{\text{drop}} = I \times R_{\text{circuit}}\)
Where:
- \(V_{\text{drop}}\) is voltage drop in volts
- (I) is circuit current in amperes
- \(R_{\text{circuit}}\) is total circuit resistance in ohms
If current remains unchanged, the conductor with higher total circuit resistance produces higher voltage drop.
For example, if an aluminum conductor’s calculated circuit resistance is greater than the equivalent copper conductor’s resistance, the aluminum circuit will have less voltage-drop margin at the same load current. Increasing conductor size reduces resistance and can restore voltage-drop margin.
Conductor Sizing Workflow
Resistance calculations are commonly used after an initial ampacity selection. Ampacity and voltage drop answer different electrical questions.
- Ampacity evaluates whether a conductor can carry the calculated load under the applicable installation conditions.
- Voltage drop evaluates the voltage lost through conductor impedance at a given current and distance.
- Resistance supplies the conductor-loss portion of the voltage-drop calculation.
- Conductor material changes the resistance used in that calculation.
A practical workflow is:
1. Establish the load and select a preliminary conductor based on required ampacity.
2. Confirm whether the conductor is copper or aluminum.
3. Calculate resistance using the actual AWG or kcmil size, conductor length, and temperature.
4. Calculate voltage drop using the circuit voltage and design current.
5. Increase conductor size if the voltage-drop result does not meet the project requirement.
- Verify the final conductor, terminals, lugs, and equipment are suitable for the selected material and conductor size.
For long feeders, large loads, and motor circuits, the conductor material selection can change the final size even when both copper and aluminum options satisfy the initial ampacity requirement.
Temperature and Field Conditions
Conductor resistance rises as conductor temperature rises. A copper-versus-aluminum comparison is valid only when both calculations use the same temperature basis.
Do not compare a copper value at one temperature with an aluminum value at another temperature and attribute the entire difference to conductor material. Keep temperature constant when evaluating the material effect, then use the appropriate temperature assumptions for the final design review.
Field installation also extends beyond resistance arithmetic. Aluminum conductor use requires review of the equipment’s conductor-material compatibility, termination requirements, torque requirements, conductor preparation, and the selected terminal rating. Raceway routing, bends, pull conditions, raceway fill, conductor quantity, and adjustment or correction factors may also affect the final installation decision even though they do not change the basic resistance formula.
Use the Calculators
Use the following Elecatrix tools to calculate and review the resistance and voltage-drop values: