Wire Gauge Calculator
Minimum AWG for a run from current, one-way distance, voltage and allowable drop, with the actual voltage drop, conductor resistance and a full AWG table.
Wire Gauge Calculator: with the default inputs, minimum wire gauge is 12 AWG.
The continuous current the run will carry, not the breaker size.
Source to load. The calculator doubles it for the return conductor.
3% to the load is the usual design target; 5% total including feeders.
The thinnest AWG that keeps the drop within your limit.
- Actual voltage drop (V)
- 2.897
- Actual drop
- 2.41%
- Voltage at the load (V)
- 117.1
- Resistance of the run (Ω)
- 0.1932
- Power lost as heat in the wire (W)
- 43.46
- Minimum conductor area (mm²)
- 2.663
- Area of the chosen gauge (mm²)
- 3.309
- Total conductor length (there and back)
- 100 ft
Assumptions
- Two-conductor DC or single-phase AC; the current path is twice the one-way distance.
- Solid conductor geometry from the ASTM B258 AWG definition; stranded cable is a few percent worse.
- Resistivity is corrected linearly for temperature from the 20 °C value; the default 75 °C matches NEC Chapter 9 Table 8.
- Voltage drop only — ampacity, derating, conduit fill and code compliance are not evaluated.
- No allowance for connector, splice or termination resistance.
| Gauge | Area (mm²) | Ω per 1,000 ft | Drop (V) | Drop | Within budget? |
|---|---|---|---|---|---|
| 4/0 AWG | 107.219 | 0.0596 | 0.089 | 0.07% | yes |
| 3/0 AWG | 85.029 | 0.0752 | 0.113 | 0.09% | yes |
| 2/0 AWG | 67.431 | 0.0948 | 0.142 | 0.12% | yes |
| 1/0 AWG | 53.475 | 0.1195 | 0.179 | 0.15% | yes |
| 1 AWG | 42.408 | 0.1507 | 0.226 | 0.19% | yes |
| 2 AWG | 33.631 | 0.19 | 0.285 | 0.24% | yes |
| 3 AWG | 26.67 | 0.2396 | 0.359 | 0.3% | yes |
| 4 AWG | 21.151 | 0.3022 | 0.453 | 0.38% | yes |
| 6 AWG | 13.302 | 0.4805 | 0.721 | 0.6% | yes |
| 8 AWG | 8.366 | 0.764 | 1.146 | 0.95% | yes |
| 10 AWG | 5.261 | 1.2147 | 1.822 | 1.52% | yes |
| 12 AWG | 3.309 | 1.9315 | 2.897 | 2.41% | yes |
| 14 AWG | 2.081 | 3.0712 | 4.607 | 3.84% | no |
| 16 AWG | 1.309 | 4.8834 | 7.325 | 6.1% | no |
| 18 AWG | 0.823 | 7.765 | 11.647 | 9.71% | no |
| 20 AWG | 0.518 | 12.3468 | 18.52 | 15.43% | no |
| 22 AWG | 0.326 | 19.6322 | 29.448 | 24.54% | no |
Ω per 1,000 ft is for a single conductor and can be checked against NEC Chapter 9 Table 8 at 75 °C. The drop columns already count both conductors.
How this is worked out
The formula
Allowed drop Vdrop = Vsystem × drop% ÷ 100 Loop resistance Rmax = Vdrop ÷ I Conductor length L = 2 × one-way distance (out and back) Minimum area A = ρ × L ÷ Rmax Resistivity ρ(T) = ρ₂₀ × (1 + α(T − 20 °C)) AWG diameter d = 0.127 mm × 92^((36 − n)/39) ρ₂₀ = 1.7241e-8 Ω·m (copper), 2.8264e-8 Ω·m (aluminium) α = 0.00393 /°C (copper), 0.00403 /°C (aluminium)
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Load current
- The continuous current the run will carry, not the breaker size.0 or more · defaults to 15
- One-way distance
- Source to load. The calculator doubles it for the return conductor.in ft, m, yd, km, mi · 0 or more · defaults to 50
- System voltage
- A number.0 or more · defaults to 120
- Acceptable voltage drop
- 3% to the load is the usual design target; 5% total including feeders.a percentage · from 0 to 100 · defaults to 3
- Conductor material
- Choose one of 2 options.Copper · Aluminium
- Conductor temperature(under More options)
- 75 °C is the standard design assumption for a loaded conductor and matches NEC Chapter 9 Table 8. Use 20 °C for a bench measurement.in °C · from -40 to 200 · defaults to 75
What you get back
- Minimum wire gaugemain answer
- The thinnest AWG that keeps the drop within your limit.
- Actual voltage drop (V)
- Actual drop
- Voltage at the load (V)
- Resistance of the run (Ω)
- Power lost as heat in the wire (W)
- Minimum conductor area (mm²)
- Area of the chosen gauge (mm²)
- Total conductor length (there and back)
What this assumes
- Two-conductor DC or single-phase AC; the current path is twice the one-way distance.
- Solid conductor geometry from the ASTM B258 AWG definition; stranded cable is a few percent worse.
- Resistivity is corrected linearly for temperature from the 20 °C value; the default 75 °C matches NEC Chapter 9 Table 8.
- Voltage drop only — ampacity, derating, conduit fill and code compliance are not evaluated.
- No allowance for connector, splice or termination resistance.
About this calculator
Wire has resistance, resistance times current is voltage, and that voltage never reaches your load — it turns into heat somewhere in the wall. This sizes the conductor so the loss stays inside a budget you choose. Enter the current, the one-way distance, the system voltage and the drop you can tolerate, and you get the thinnest AWG that works, the drop you would actually see, and the resistance and heat of the run.
The two separate questions
Sizing a conductor answers two independent questions, and they have different answers:
- Ampacity — how much current the wire can carry without its insulation cooking. That is a code table (NEC 310.16, IEC 60364-5-52), and it depends on insulation rating, how many conductors share the conduit, and ambient temperature.
- Voltage drop — whether enough voltage survives the trip. That is physics, and it is what this calculator does.
The required size is the larger of the two. On short runs ampacity almost always wins; past 30 m or so, or on any 12/24 V system, voltage drop takes over and demands wire far thicker than ampacity alone. This calculator is a design aid for question 2 — it is not a substitute for code compliance, and it does not know your conduit, insulation or ambient temperature.
Why low-voltage runs eat copper
The drop budget is a percentage of system voltage. At 120 V, 3% is 3.6 V; at 12 V it is 0.36 V — ten times less voltage for the same current. Because the required area scales as I·L/Vdrop, halving the system voltage quadruples the copper needed for the same delivered power. A 20 A load 30 m away needs 12 AWG at 120 V and 2/0 at 12 V. That is the whole argument for 48 V solar and DC distribution.
Temperature, and why the numbers may not match the book
Copper gets about 0.4% more resistive per degree. A conductor running at its rated 75 °C has roughly 22% more resistance than the same wire at 20 °C on your bench. Under More options you can set the conductor temperature; 75 °C is the default because it is the standard design assumption, and at that setting the per-1,000-ft column reproduces NEC Chapter 9 Table 8. If you compare against a calculator that quietly uses 20 °C you will see about a 20% difference — neither is wrong, they are answering different questions.
Caveats
- Single-phase or DC, two conductors. Three-phase uses √3 × one-way length instead of 2×, so this over-estimates the drop by about 15% for a balanced three-phase run.
- Stranded cable has slightly more resistance than the solid-wire geometry for the same nominal gauge, and terminations, splices and connectors add their own.
- AC skin effect is negligible below about 1/0 at 60 Hz but is not modelled.
- Motors and inverters draw large inrush currents; size for the steady state here, then check the sag on start.
Frequently asked questions
▸What gauge wire do I need for a 20 amp circuit?
12 AWG copper for ampacity, but voltage drop can force something thicker: at 120 V, 20 A over 100 ft one way drops about 6%, so you would go to 10 AWG. Always take the larger of the two requirements.
▸Why do you double the distance?
Current goes out on one conductor and back on the other, so it passes through twice the one-way length of copper. Enter the distance from source to load and the calculator handles the return.
▸How much voltage drop is acceptable?
The common design target is 3% to the load and 5% total including feeders — informative recommendations in NEC 210.19(A) and 215.2(A), not enforceable rules. Sensitive electronics and long LED runs often want tighter.
▸Is aluminium wire acceptable?
Yes for feeders and services, and it is cheaper per amp, but it needs roughly 1.64 times the copper area, AL/CU-rated terminations and antioxidant compound. Aluminium branch-circuit wiring in older homes is a known fire hazard.
▸Does this replace an electrician or the code book?
No. It answers the voltage-drop question only. Ampacity, breaker sizing, conduit fill, derating for bundling and ambient temperature, and permitted conductor types all come from the code, and the work usually needs an inspection.
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