Choose a wire size by amps, power, distance and voltage drop
HomDera’s updated wire size calculator works with both common metric cable sizes and North American AWG or kcmil designations. It can start from a known current or derive current from load power, then compare the conductor required by voltage drop with an optional reference ampacity requirement.
Instead of hiding the reason behind one answer, the results show three separate decisions: the size required by voltage drop, the size required by the selected ampacity reference, and the final standard conductor size that satisfies every enabled check.
The result is a planning estimate, not an approved wiring design. It does not certify compliance with the NEC, CEC, BS 7671, AS/NZS 3000, IEC-based national rules or any local inspection requirement. A qualified electrical professional must verify the actual cable, protection, terminations and installation conditions.
Two unit systems without converting AWG into a fake decimal gauge
Available sizing modes
| Mode | Distance input | Standard conductor series | Displayed recommendation |
|---|---|---|---|
| Metric | metres | 0.5 to 400 mm² | the next standard metric cross-sectional area |
| US customary | feet | 14 AWG through 4/0 AWG, then 250 to 600 kcmil | the smallest listed AWG or kcmil size that passes the enabled checks |
AWG is a discrete gauge system: a smaller gauge number means a larger conductor, and sizes above 4/0 are commonly expressed in kcmil. The calculator therefore keeps the actual designation, such as 10 AWG or 250 kcmil, while also showing its approximate area in square millimetres.
What the calculator now includes
- current-based and power-based input modes;
- separate DC, single-phase AC and three-phase AC calculations;
- copper and aluminum/aluminium conductors;
- one-way run length in metres or feet;
- a selectable voltage-drop limit and current sizing margin;
- optional peak or starting current;
- power factor and efficiency when current is derived from power;
- an optional NFPA 70 Table 310.16 reference ampacity check;
- ambient-temperature and conductor-grouping corrections for that reference check;
- a detailed table comparing nearby conductor sizes.
Start with current or calculate current from kilowatts
Use the current mode when the equipment nameplate, manufacturer documentation or a suitable measurement already provides amperes. Use the power mode when you know active load power but still need to estimate line current.
DC: I = P / (V × efficiency)
Single-phase AC: I = P / (V × power factor × efficiency)
Three-phase AC: I = P / (√3 × V × power factor × efficiency)The current sizing margin is then applied to the normal load current. When a larger peak or starting current is entered, the calculator uses that higher value as the design current. This is useful for preliminary checks involving motors, pumps, compressors and inverters, but it is not a complete motor-circuit calculation.
Voltage-drop sizing for DC, single-phase and three-phase circuits
The entered length is always the one-way distance from source to load. A DC or single-phase calculation includes the outgoing and return path automatically. A balanced three-phase calculation uses the √3 relationship.
DC or single-phase AC:
Area = 2 × current × one-way length × resistivity / allowed voltage loss
Balanced three-phase AC:
Area = √3 × current × one-way length × resistivity / allowed voltage lossCopper and aluminum use different resistivity values. The resistance estimate is adjusted to the selected conductor-temperature column, which produces a more conservative voltage-drop result than treating every conductor as if it remained at 20°C. AC reactance is not included, so long feeders and large conductors may require a fuller impedance calculation.
What the NFPA ampacity reference does — and does not do
When enabled, the ampacity check starts with values from NFPA 70 Table 310.16 for insulated conductors in raceway, cable or earth under the table’s base conditions. The selected 60°C, 75°C or 90°C column is adjusted for ambient temperature and for more than three current-carrying conductors.
General small-conductor overcurrent limits are also applied so that a higher temperature column does not allow the calculator to present 14 AWG, 12 AWG or 10 AWG as if their common protection limits did not exist.
The reference ampacity is not a code-compliance decision. Terminal ratings, conductor insulation, cable type, wet or dry location, rooftop exposure, thermal insulation, bundling, flexible-cord rules, equipment listings, continuous-load rules and local amendments can all change the permitted conductor size.
Using the NEC reference while displaying a metric size
The NFPA table is organised by AWG and kcmil, not by the standard metric series. In metric mode, the calculator uses the nearest AWG reference conductor whose area does not exceed the metric conductor area, then rounds the final recommendation up to a standard metric size. This is intentionally conservative, but it is still not a substitute for IEC, BS, CEC or AS/NZS cable tables.
How to read the result dashboard
Key outputs
| Output | Meaning |
|---|---|
| Recommended standard conductor size | the final AWG, kcmil or mm² size that passes every enabled check |
| Calculated normal load current | the entered current or the current derived from power, power factor and efficiency |
| Design current | the larger of current with margin and the entered peak current |
| Minimum by voltage drop | the first standard size that stays within the selected percentage |
| Minimum by reference ampacity | the first standard size whose corrected reference ampacity reaches the design current |
| Actual voltage drop | the volts and percentage lost at the recommended size |
| Estimated load voltage | source voltage minus the calculated conductor loss |
| Maximum one-way length | the longest one-way run for the recommended size at the selected drop limit |
Open the detailed calculation to compare nearby sizes. Each row shows conductor area, corrected reference ampacity when enabled, voltage drop and a plain-language pass or fail result. This makes it clear when a larger conductor is required by distance rather than by current.
Why 12 V and 24 V systems often need unexpectedly large wire
A 0.5 V loss is more than 4% of a 12 V supply but only about 0.2% of a 230 V supply. Battery cables, inverter leads, vehicle circuits, LED lighting and other low-voltage runs therefore become voltage-drop limited very quickly. Shortening the run can be as important as increasing the wire size.
Why can the same 20 A load need a much larger conductor at 12 V than at 120 V or 230 V?
Answer: At a 3% limit, a 12 V circuit can lose only 0.36 V. A 120 V circuit can lose 3.6 V and a 230 V circuit can lose 6.9 V. The lower-voltage circuit therefore needs much lower conductor resistance over the same distance.
Explanation: Allowed voltage loss in volts equals supply voltage multiplied by the selected percentage.
Copper versus aluminum or aluminium
Aluminum has higher resistivity than copper, so it generally needs a larger cross-sectional area for the same current, distance and voltage-drop target. It also requires terminals, compounds and installation methods specifically approved for aluminum conductors. The calculator changes both the voltage-drop calculation and the available ampacity-reference rows when the material is changed.
Common mistakes that the calculator cannot prevent
- entering round-trip length when the field asks for one-way distance;
- confusing wire diameter with cross-sectional area;
- using normal current while ignoring a documented starting current;
- choosing a 90°C ampacity column when the terminals are limited to a lower temperature;
- assuming an online AWG recommendation is also the correct breaker size;
- using building-wire ampacity values for flexible cords, battery cable or another separately regulated wiring type;
- ignoring cable grouping, thermal insulation, roof temperature, soil conditions or enclosure heating;
- replacing copper with the same-size aluminum conductor;
- treating voltage drop as the only electrical safety requirement.
When voltage-drop-only mode is appropriate
Voltage-drop-only mode is useful when the correct ampacity comes from a separate product-specific table, such as manufacturer data for flexible battery cable, automotive wire, marine cable or another specialised conductor. In that case, HomDera determines the resistance-based size but deliberately labels ampacity as not checked.
Skipping the built-in ampacity reference does not mean ampacity is unimportant. It means that you are responsible for checking it against the correct table for the exact cable and installation.
Questions about AWG and cable sizing
Does a lower AWG number mean a smaller wire?
No. AWG runs in the opposite direction: 10 AWG is larger than 12 AWG, 4 AWG is larger than 6 AWG, and 1/0 is larger than 1 AWG. After 4/0, large conductor sizes are commonly stated in kcmil.
Can the recommended AWG be used as the circuit-breaker rating?
No. A conductor size and an overcurrent-device rating are related but not interchangeable. The protective device must be coordinated with the load, conductor ampacity, equipment requirements, fault current, starting characteristics and the rules that apply to the installation.
What voltage-drop percentage should I choose?
Three percent is a common preliminary target, but it is not universal. Sensitive electronics, low-voltage equipment or a feeder-plus-branch-circuit combination may need a different limit. Follow the equipment manufacturer and the electrical rules that apply to the complete installation.
Why can the ampacity result be larger than the voltage-drop result?
A short run may have very little voltage loss even with a relatively small conductor, but that conductor may still be unable to carry the design current under the selected temperature and grouping conditions. The final recommendation therefore uses the larger requirement.
Why can voltage drop require a larger conductor than ampacity?
This is common on long runs and low-voltage circuits. A conductor may carry the current thermally but still lose too much voltage before the load. Increasing conductor area reduces resistance and keeps more voltage available at the equipment.
Important limits before installation
The calculator does not model conductor reactance, harmonic current, neutral loading, fault-loop impedance, short-circuit withstand, voltage unbalance, motor protection, earth-fault protection, conductor fill, mechanical protection or product listing. It also cannot inspect the actual cable route, terminals, enclosure, insulation or environmental exposure.
Use the result to compare options and prepare questions for an electrician—not as permission to energise a circuit. Incorrect conductor or protection selection can cause overheating, equipment damage, electric shock or fire.
Summary
The new calculator legitimately supports the search intent behind “wire size calculator”, “AWG calculator”, “wire size by amps” and “voltage drop calculator”. It works in feet or metres, outputs AWG/kcmil or mm², accepts amps or kilowatts, and explains whether distance or reference ampacity controlled the answer. The final installation still requires the exact cable data, correct protective device and a jurisdiction-specific professional check.