AC Wire by Feet Calculator
Use this advanced calculator to estimate the recommended wire size for an AC circuit based on current, run length in feet, voltage, conductor material, and allowable voltage drop. It is designed for fast planning and educational sizing comparisons for branch circuits, feeders, shop equipment, HVAC runs, and long-distance AC wiring.
Wire Size Calculator
Enter the electrical load and distance details below. The calculator recommends the smallest conductor size that satisfies both ampacity and voltage drop targets using common resistance values.
Expert Guide to Using an AC Wire by Feet Calculator
An AC wire by feet calculator is a practical planning tool that helps estimate the correct conductor size for an alternating current circuit when run length matters. In the real world, wire length is not just a convenience detail. As conductors get longer, resistance increases, voltage drop rises, and equipment may receive less than the intended supply voltage. That can reduce motor torque, increase heat, shorten equipment life, and lead to nuisance failures. A proper calculator helps bridge the gap between basic amp ratings and actual installation performance.
Most people start with amperage. That is necessary, but it is not enough. A conductor may be able to carry the current thermally, yet still produce too much voltage drop over a long distance. This is why electricians, engineers, and serious DIY users frequently combine three factors: load current, one-way run length in feet, and conductor material. Once voltage is also considered, you can estimate how much loss occurs and whether a larger conductor should be selected. The calculator above does exactly that by checking wire sizes against both ampacity and voltage drop criteria.
Why wire length in feet changes the answer
Every conductor has resistance. Resistance is measured in ohms, and the value rises with conductor length. In simple terms, a longer wire acts like a longer road with more friction. Electrical energy is still delivered, but more of it is lost along the way. With AC branch circuits and feeders, this reduction shows up as voltage drop. If the voltage drop becomes excessive, lights may dim, motors may start hard, compressors may overheat, and electronics may operate unpredictably.
For single-phase circuits, the electrical path includes both the outgoing and returning conductors. That means the total path length is roughly twice the one-way run. For three-phase circuits, the formula differs slightly, but distance still has a direct impact. This is why a 20 amp load at 20 feet may work perfectly on one wire size, while the same 20 amp load at 180 feet might require a significantly larger conductor to stay within a 3% target.
Key idea: Wire sizing is usually limited by either heat capacity or voltage drop. Short runs are often governed by ampacity. Long runs are often governed by voltage drop. The farther the equipment is from the panel, the more likely it is that a larger wire will be required.
What this calculator considers
This AC wire by feet calculator evaluates common wire sizes and compares them against two practical screening rules:
- Ampacity check: The wire must be capable of carrying the current without exceeding a conservative ampacity value for the selected material.
- Voltage drop check: The estimated voltage drop must stay at or below the allowable percentage you selected, such as 2%, 3%, or 5%.
- Material adjustment: Copper and aluminum are not interchangeable in performance. Aluminum has higher resistance, so it usually requires a larger size for the same load and distance.
- Phase adjustment: Single-phase and three-phase circuits use different voltage drop relationships.
The result is a planning recommendation, not a permit-ready code decision. Final conductor selection must still reflect the latest electrical code, insulation type, termination temperature rating, ambient temperature, conduit fill, continuous load rules, and local jurisdiction requirements.
Understanding copper vs aluminum in long AC runs
Copper is the benchmark conductor material in most residential and light commercial work because it offers high conductivity, strong mechanical properties, and familiar termination practices. Aluminum is lighter and usually less expensive per amp delivered, which is why it is widely used on larger feeders and service conductors. However, aluminum has higher electrical resistance than copper, so a larger conductor is typically needed to achieve the same voltage drop performance.
| Conductor Material | Electrical Resistivity at 20°C | Approximate Conductivity | Relative Weight | Common Planning Implication |
|---|---|---|---|---|
| Copper | 1.68 × 10-8 Ω·m | 100% IACS | Heavier | Smaller size often meets voltage drop targets more easily |
| Aluminum | 2.82 × 10-8 Ω·m | About 61% IACS | Lighter | Usually requires upsizing for equal performance |
The resistivity values above help explain why a copper conductor and an aluminum conductor with the same nominal size do not behave the same way over distance. When the run gets longer, those differences become more important. For example, a branch circuit for a detached garage, workshop compressor, or mini-split condenser may need an upsized aluminum feeder sooner than many people expect.
Sample voltage drop comparison at 20 amps over 100 feet
The following examples illustrate why wire gauge changes when run length and voltage matter. These values are based on common conductor resistance data and a single-phase 120 V circuit with a 100-foot one-way run, which means about 200 feet of conductor path for the round trip.
| Wire Size | Copper Resistance (Ω per 1000 ft) | Estimated Voltage Drop | Percent Drop at 120 V | Practical Meaning |
|---|---|---|---|---|
| 12 AWG | 1.588 | 6.35 V | 5.29% | Often too high for a 3% target at this load and distance |
| 10 AWG | 0.999 | 4.00 V | 3.33% | Close, but still above a strict 3% branch-circuit target |
| 8 AWG | 0.6282 | 2.51 V | 2.09% | Comfortably meets a 3% target in this example |
| 6 AWG | 0.3951 | 1.58 V | 1.32% | Excellent voltage stability with added future margin |
These are exactly the kinds of relationships an AC wire by feet calculator makes easy to visualize. It is not enough to ask, “What wire for 20 amps?” The better question is, “What wire for 20 amps over 100 feet at my voltage and my voltage-drop limit?”
How to use the calculator correctly
- Enter the load current. Use the expected running current in amps. For continuous loads, remember that code requirements may require additional sizing considerations.
- Enter the one-way distance. This is the physical distance from source to load, not the round-trip distance. The calculator handles the path relationship internally.
- Select system voltage. Higher voltages usually reduce percentage voltage drop for the same conductor and load.
- Choose single-phase or three-phase. This affects the drop formula.
- Choose copper or aluminum. Material changes conductor resistance and ampacity assumptions.
- Select allowable voltage drop. Many designers use about 3% for branch circuits and keep total feeder-plus-branch drop around 5% as a planning benchmark.
- Review the recommendation and chart. The output shows the selected wire size, estimated drop, and several comparison points so you can see if upsizing makes sense.
When upsizing is smart even if the calculator says a smaller wire works
A calculator recommendation is usually the minimum size that meets the chosen criteria. In practice, upsizing can still be a smart move. Long-term operating costs, future expansion, voltage-sensitive equipment, and motor starting behavior may justify one more size increase. If you are feeding HVAC equipment, well pumps, air compressors, EV charging equipment, or detached buildings, extra margin often pays off in performance stability.
- Motors and compressors benefit from better voltage at startup.
- Future load growth is easier to accommodate if the conductor is not already at its limit.
- Voltage-sensitive electronics may behave more reliably with lower drop.
- Energy losses decrease when conductor resistance decreases.
Common mistakes people make with wire-by-feet calculations
The most common mistake is using ampacity alone. A 30 amp circuit does not always use the same conductor when the load is 15 feet away versus 175 feet away. Another mistake is confusing one-way distance with total conductor length. Many charts and formulas expect one-way distance, but the circuit physics still involve the full electrical path. A third error is forgetting that aluminum and copper do not share the same resistance values. There is also a tendency to ignore supply voltage. A conductor that produces a certain voltage loss at 120 V has the same absolute voltage drop at 240 V, but the percentage drop is cut in half because the nominal voltage is higher.
Another practical issue is temperature. Resistance rises as conductor temperature rises. This calculator uses standard resistance values for planning, but actual installed conditions may differ. Conduit fill, rooftop heat, ambient temperature, bundled conductors, and termination limitations can all affect the final design. For permit work or critical equipment, always verify with the governing code and manufacturer instructions.
Residential examples where this calculator is useful
In residential work, an AC wire by feet calculator is especially useful for detached garages, gate openers, subpanels, spas, workshops, shed circuits, landscape transformer feeds, and long outdoor receptacle runs. It is also useful for dedicated appliance circuits where the run is longer than normal, such as a far-end air handler, electric heater, or pump. Homeowners often assume that if a breaker size is known, the wire size is fixed. In reality, distance can easily change the answer.
For example, a 20 amp circuit serving a workshop 140 feet from the panel might technically be protected by a 20 amp breaker with 12 AWG under some thermal assumptions, yet the voltage drop at the tool could be poor. Upgrading to 10 AWG or even 8 AWG may be the better practical choice, especially for motorized equipment. The calculator helps reveal that difference before material is purchased.
Commercial and light industrial use cases
In commercial environments, long branch circuits to rooftop units, corridor equipment, warehouse receptacles, and remote mechanical rooms are common. Feeder runs to distribution panels or process loads can also be substantial. For these applications, planning with both voltage drop and ampacity is not optional. Small voltage losses may seem harmless on paper, but they compound across startup events and heavily loaded periods. Over time, undervoltage can increase motor heating and reduce operational efficiency.
Three-phase systems also deserve attention. Although three-phase can be more efficient in power delivery, long runs still produce measurable drop. This is why the calculator includes a three-phase option and why charts remain useful even for experienced installers. Seeing the drop trend across wire sizes helps with cost-performance decisions.
Best practices after using an AC wire by feet calculator
- Verify the selected conductor against the latest electrical code table applicable to your insulation and termination temperature ratings.
- Check whether the load is continuous and whether derating or the 125% rule affects the final conductor or overcurrent device.
- Confirm local amendments, especially for aluminum conductors, outdoor conditions, or underground raceways.
- Review equipment manufacturer instructions for minimum circuit ampacity and maximum overcurrent protection.
- If the run is especially long, compare one size larger to see whether future performance gains justify the added cost.
Authoritative references for further review
For safety and energy context, review guidance from OSHA electrical safety, broad efficiency resources from the U.S. Department of Energy Energy Saver program, and engineering measurement resources from the National Institute of Standards and Technology.
Final takeaway
An AC wire by feet calculator is most valuable when it is used as a decision support tool rather than a shortcut. It helps you see the real effect of distance, current, voltage, and material choice before installation begins. That reduces guesswork, improves equipment performance, and helps prevent under-sized conductors on long runs. If you treat the result as a strong starting point and then confirm the final design against code and equipment requirements, you will make better electrical choices with fewer surprises in the field.