Wire resistance is the conductor’s opposition to current flow, expressed in ohms. For a branch circuit, feeder, control circuit, or low-voltage run, the resistance result becomes an input for voltage-drop review:
\(\displaystyle V_{\text{drop}} = I \times R\)
The calculation starts with the selected AWG conductor’s resistance per reference length—commonly ohms per 1,000 ft—and scales that value to the actual conductor path. The key output is the resistance of one conductor or the complete outgoing-and-returning circuit path.
For one conductor:
\(\displaystyle R = R_{1000} \times \frac{L}{1000}\)
Where:
- (R) = conductor resistance in ohms
- \(R_{1000}\) = resistance of the selected wire size in ohms per 1,000 ft
- (L) = one-way conductor length in feet
For a two-conductor circuit, use the total circuit path. If the load is 100 ft from the source, current travels 100 ft on the ungrounded conductor and returns 100 ft on the grounded conductor or other return conductor. The electrical path is therefore 200 ft.
\(\displaystyle R_{\text{loop}} = R_{1000} \times \frac{2L}{1000}\)
Use the AWG Wire Resistance Calculator to identify the resistance associated with a selected wire size. The Conductor Resistance Calculator can then scale resistance by conductor length.
Resistance per 1,000 Feet
AWG describes conductor size, not the total resistance of an installed circuit. A smaller AWG number identifies a larger conductor, and a larger conductor generally has lower resistance per 1,000 ft. For example, 12 AWG has less resistance than 20 AWG, while 20 AWG has less resistance than 22 AWG.
The resistance-per-1,000-ft value is the starting point because it allows the calculation to be scaled to any installed length. A 75 ft conductor has 7.5% of the resistance listed per 1,000 ft; a 250 ft conductor has 25%.
For a one-way 100 ft conductor:
\(\displaystyle R_{100\text{ ft}} = R_{1000} \times \frac{100}{1000}\)
\(\displaystyle R_{100\text{ ft}} = 0.10 \times R_{1000}\)
The one-conductor resistance at 100 ft is therefore one-tenth of the reference resistance per 1,000 ft.
Two-Conductor Loop Resistance
Voltage-drop calculations require the resistance of the full current path, not only the one-way distance shown on a plan.
For a typical single-phase two-wire circuit:
\(\displaystyle L_{\text{loop}} = 2 \times L_{\text{one-way}}\)
For a load located 100 ft from the source:
| Circuit measurement | Length used |
|---|---|
| One conductor from source to load | 100 ft |
| Outgoing and return conductor path | 200 ft |
| Resistance calculation for voltage drop | 200 ft total path |
The loop-resistance formula becomes:
\(\displaystyle R_{\text{loop}} = R_{1000} \times \frac{200}{1000}\)
\(\displaystyle R_{\text{loop}} = 0.20 \times R_{1000}\)
Doubling the path doubles the resistance. Using only the one-way distance in a two-conductor voltage-drop calculation understates the circuit resistance and voltage drop.
Calculation Example
Assume a selected AWG conductor has a reference resistance of \(R_{1000}\) ohms per 1,000 ft and the one-way circuit distance is 100 ft.
One-conductor resistance
\(\displaystyle R_{\text{one conductor}} = R_{1000} \times \frac{100}{1000}\)
\(\displaystyle R_{\text{one conductor}} = 0.10R_{1000}\)
Two-conductor loop resistance
\(\displaystyle R_{\text{loop}} = R_{1000} \times \frac{100 + 100}{1000}\)
\(\displaystyle R_{\text{loop}} = 0.20R_{1000}\)
If the circuit load current is (I), calculate voltage drop from the complete loop resistance:
\(\displaystyle V_{\text{drop}} = I \times R_{\text{loop}}\)
For a 120 V circuit, voltage-drop percentage is:
\(\displaystyle \%V_{\text{drop}} = \frac{V_{\text{drop}}}{120} \times 100\)
Enter the conductor size, circuit length, and load current in the Voltage Drop Calculator to review the resulting voltage drop.
Applying Resistance to Conductor Sizing
Resistance calculations support conductor-size decisions when voltage drop may affect equipment operation or load performance. Longer branch circuits and feeders have more conductor resistance, even when the conductor ampacity is adequate for the calculated load.
Common uses include:
- Reviewing voltage drop on long branch circuits serving receptacles, lighting, HVAC equipment, or remote panels
- Comparing AWG sizes when a larger conductor may reduce voltage drop
- Estimating the resistance of control wiring, instrumentation wiring, and low-voltage circuits
- Reviewing feeder conductor length between service equipment, distribution equipment, and downstream panelboards
- Checking conductor path assumptions when routing through raceways, cable tray, underground runs, or building spaces
- Estimating voltage loss that may affect motor starting, contactor operation, electronic equipment, or other load behavior
Conductor resistance does not replace ampacity selection. Ampacity depends on the applicable conductor, insulation temperature rating, terminal rating, ambient temperature, adjustment factor, correction factor, and the number of current-carrying conductors. Resistance is used separately to evaluate the voltage lost across the installed conductor length.
Field Length and Circuit Path
Use installed conductor length, not a simplified straight-line building measurement. Raceway routing, vertical risers, offsets, pull-box routing, panel location, equipment terminations, and slack can all increase the actual conductor path.
For a practical calculation:
1. Identify the AWG or kcmil conductor size.
2. Obtain the resistance per 1,000 ft for that conductor.
3. Measure or estimate the one-way installed conductor route.
4. Use one-way length for one-conductor resistance.
5. Use the full outgoing-and-return path for a two-conductor loop.
6. Multiply loop resistance by circuit current to calculate voltage drop.
For multi-conductor or three-phase systems, the current path and voltage-drop method depend on the circuit configuration. Do not automatically double the one-way length without first identifying the actual current return path used by the calculation method.
Field Verification
Resistance varies with conductor material and temperature. Copper and aluminum conductors of the same AWG do not have the same resistance, and conductor resistance rises as temperature rises. Splices, terminals, terminations, damaged conductors, and poor connections can also add resistance beyond the calculated straight-conductor value.
Use verified manufacturer data when conductor resistance affects a design, acceptance test, equipment performance decision, or compliance determination. Confirm conductor material, insulation temperature rating, terminal limitations, actual routing, circuit configuration, and applicable project or AHJ requirements separately from the calculator arithmetic.