AWG wire size equation
The calculator checks each AWG size from small to large and selects the first conductor that passes both ampacity and voltage-drop limits.
Select first AWG where Iamp ≥ Ireq and Vd% ≤ Vlimitwhere:
- Ireq
- Required conductor ampacity after continuous-load and bundling adjustment[A]
- Iload
- Load current[A]
- Fc
- Continuous-load factor, 1.25 when selected
- Kbundle
- Bundled current-carrying conductor derating factor
- Vd
- Voltage drop[V]
- Rft
- AWG conductor resistance per foot after material adjustment[ohm/ft]
- L
- One-way conductor length[ft]
- Iamp
- Reference ampacity for the AWG conductor[A]
- Vlimit
- Maximum allowed voltage drop[%]
The internal ampacity table is simplified for planning; final conductor sizing must follow the applicable NEC, IEC, BS, or local standard.
How to calculate AWG wire size
AWG wire sizing must satisfy both current capacity and voltage drop. The calculator applies a continuous-load factor when selected, adjusts reference ampacity for the number of current-carrying conductors, and checks each listed gauge against the chosen voltage-drop limit.
- Select DC, single-phase, or balanced three-phase.
- Enter current, voltage, and one-way length.
- Select conductor material and continuous-load treatment.
- Enter the number of current-carrying conductors for bundling adjustment.
- Choose the first gauge that passes both ampacity and voltage-drop checks.
AWG wire size example
For a 20 A, 120 V single-phase circuit with a 100 ft one-way run, copper conductors, a 3% voltage-drop limit, and three current-carrying conductors, 8 AWG is the smallest size in the calculator's reference table that passes both checks.
| Input or result | Value |
|---|---|
| Circuit | 120 V single-phase |
| Load current | 20 A |
| One-way length | 100 ft |
| Conductor | Copper |
| Maximum voltage drop | 3% |
| Preliminary result | 8 AWG |
Ampacity vs. voltage drop when choosing wire gauge
Short circuits often select wire size by ampacity, while long or low-voltage circuits may require a much larger conductor because of voltage drop. The governing gauge is the larger size required by either condition.
| Check | What controls the result |
|---|---|
| Ampacity | Load current, continuous duty, conductor material, bundling, terminals, insulation, and installation method |
| Voltage drop | Current, voltage, one-way distance, conductor resistance, material, and circuit type |
| Final design | The larger compliant conductor after all applicable checks |
How current-carrying conductors affect wire size
Multiple loaded conductors in the same raceway or cable can retain heat and reduce allowable ampacity. The calculator uses simplified adjustment bands as a planning aid; final derating must follow the adopted code and the actual conductor temperature rating.
Complete the wire sizing check
After selecting an AWG size, verify terminal ratings, insulation type, ambient temperature, raceway fill, equipment grounding conductor, overcurrent protection, and short-circuit performance.
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Assumptions
- Reference ampacity table is simplified for planning
- Bundling derating follows common 4-6, 7-9, and 10-20 conductor adjustment bands
- Voltage drop uses copper resistance with material factor
Important Warnings
- Final conductor sizing must follow the locally adopted NEC/IEC/BS standard, insulation rating, terminal temperature, ambient correction, and installation method.
- Small-signal conductors may have product-specific limits not captured by building-wire ampacity tables.