AmpSizerHow We Calculate
Every calculator on this site runs the same three steps in the same order an electrician does them: work out the current, look up what the conductor can carry, then check whether the run is too long for it. Whichever of the last two asks for more copper is the answer.
The formulas
- Voltage drop:
k × amps × R × one-way feet ÷ 1,000, wherekis 2 for single-phase and DC and 1.732 for balanced three-phase, andRis ohms per 1,000 ft. - Percent drop: volts lost ÷ nominal system voltage × 100.
- Longest run at a target:
target% ÷ 100 × volts × 1,000 ÷ (k × amps × R). - Continuous load: amps × 1.25, per 210.19(A)(1) and 215.2(A)(1). Lighting, EV charging and process heat are continuous; a general receptacle circuit is not.
- Usable ampacity: table ampacity × ambient factor × bundling factor.
- Conduit fill: total conductor area ÷ internal raceway area. The ceiling is 53% for one conductor, 31% for two, 40% for more than two, and 60% in a nipple no longer than 24 in.
- Box fill: a volume allowance for every conductor, twice the largest conductor for each device yoke, one allowance for all internal clamps together, one for all grounding conductors together.
- Motors: conductors at 125% of the table full-load current (430.22), inverse-time breaker up to 250% (430.52), overload relay at 115–125% of the nameplate current (430.32).
- Transformers:
kVA × 1,000 ÷ (k × volts)on each side, then 125% of that current for the protective device.
The tables we read, and where they are
| What | NEC table | What we use it for |
|---|---|---|
| Allowable ampacity | Table 310.16 | 60, 75 and 90 °C columns, copper and aluminium |
| Ambient correction | Table 310.15(B)(1) | anything hotter or colder than 86 °F (30 °C) |
| Bundling adjustment | Table 310.15(C)(1) | more than three current-carrying conductors together |
| Small-conductor ceiling | 240.4(D) | 14 AWG at 15 A, 12 at 20, 10 at 30 |
| Standard device ratings | 240.6(A) | rounding a load up to a breaker that exists |
| Equipment grounding conductor | Table 250.122 | the ground wire, and 250.122(B) when the circuit is upsized |
| Conductor area and resistance | Chapter 9, Table 8 | kcmil, mm², diameter, ohms per 1,000 ft |
| Insulated conductor area | Chapter 9, Table 5 | THHN/THWN-2 and XHHW-2 cross-sections |
| Raceway internal area | Chapter 9, Table 4 | EMT, PVC 40 and 80, RMC, IMC, ENT, FMC |
| Fill percentages | Chapter 9, Table 1 | 53 / 31 / 40 %, and the nipple note |
| Box volume allowances | 314.16(B) and Table 314.16(A) | cubic inches needed, and the boxes that are sold |
| Flexible cord ampacity | Table 400.5(A)(1) | extension cords, which 310.16 does not cover |
| Motor full-load current | Tables 430.248 and 430.250 | single-phase and three-phase motors |
The full conductor table
This is the file every calculator on the site reads. The reference pages draw the same rows, so a chart here and an answer in a widget cannot disagree.
| Size | kcmil | mm² | Cu 60 °C | Cu 75 °C | Cu 90 °C | Al 75 °C | Cu Ω/kft | Al Ω/kft |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 4.11 | 2.08 | 15 A | 20 A | 25 A | — | 3.14 | — |
| 12 AWG | 6.53 | 3.31 | 20 A | 25 A | 30 A | 20 A | 1.98 | 3.18 |
| 10 AWG | 10.38 | 5.261 | 30 A | 35 A | 40 A | 30 A | 1.24 | 2 |
| 8 AWG | 16.51 | 8.367 | 40 A | 50 A | 55 A | 40 A | 0.778 | 1.26 |
| 6 AWG | 26.24 | 13.3 | 55 A | 65 A | 75 A | 50 A | 0.491 | 0.808 |
| 4 AWG | 41.74 | 21.15 | 70 A | 85 A | 95 A | 65 A | 0.308 | 0.508 |
| 3 AWG | 52.62 | 26.67 | 85 A | 100 A | 115 A | 75 A | 0.245 | 0.403 |
| 2 AWG | 66.36 | 33.62 | 95 A | 115 A | 130 A | 90 A | 0.194 | 0.319 |
| 1 AWG | 83.69 | 42.41 | 110 A | 130 A | 145 A | 100 A | 0.154 | 0.253 |
| 1/0 AWG | 105.6 | 53.49 | 125 A | 150 A | 170 A | 120 A | 0.122 | 0.201 |
| 2/0 AWG | 133.1 | 67.43 | 145 A | 175 A | 195 A | 135 A | 0.0967 | 0.159 |
| 3/0 AWG | 167.8 | 85.01 | 165 A | 200 A | 225 A | 155 A | 0.0766 | 0.126 |
| 4/0 AWG | 211.6 | 107.2 | 195 A | 230 A | 260 A | 180 A | 0.0608 | 0.1 |
| 250 kcmil | 250 | 127 | 215 A | 255 A | 290 A | 205 A | 0.0515 | 0.0847 |
| 300 kcmil | 300 | 152 | 240 A | 285 A | 320 A | 230 A | 0.0429 | 0.0707 |
| 350 kcmil | 350 | 177 | 260 A | 310 A | 350 A | 250 A | 0.0367 | 0.0605 |
| 400 kcmil | 400 | 203 | 280 A | 335 A | 380 A | 270 A | 0.0321 | 0.0529 |
| 500 kcmil | 500 | 253 | 320 A | 380 A | 430 A | 310 A | 0.0258 | 0.0424 |
| 600 kcmil | 600 | 304 | 350 A | 420 A | 475 A | 340 A | 0.0214 | 0.0353 |
| 700 kcmil | 700 | 355 | 385 A | 460 A | 520 A | 375 A | 0.0184 | 0.0303 |
| 750 kcmil | 750 | 380 | 400 A | 475 A | 535 A | 385 A | 0.0171 | 0.0282 |
Where the figures come from
Ampacity, resistance, conductor area, raceway area, fill percentages and motor currents are all published in NFPA 70, the National Electrical Code; NFPA provides free read-only access to the current edition, and each table is named above so you can check any number against it. Extension cord ampacity is Table 400.5(A)(1), which is a different table from 310.16 for a reason: a cord is not a raceway. Nothing on this site is an estimate, an average of several sources, or a figure remembered from a job.
Units: US first, metric one tap away
Feet, Fahrenheit, AWG and horsepower are the units the NEC tables are written in, so they are the defaults. None of them means anything to most of the world, so every physical quantity on this site has a switch beside the field, and the choice travels in the link.
| Quantity | Default | Alternative | Exactly |
|---|---|---|---|
| Conductor size | AWG, kcmil | mm² | 1 kcmil = 0.50670748 mm² |
| Run length | feet | metres | 1 ft = 0.3048 m |
| Ambient temperature | °F | °C | °C = (°F − 32) × 5/9 |
| Motor rating | horsepower | kilowatts | 1 mechanical HP = 745.7 W |
| Box volume | cubic inches | cm³ | 1 cu in = 16.387064 cm³ |
| Tray width, conductor diameter | inches | millimetres | 1 in = 25.4 mm |
Volts, amps, watts, ohms, VA and kVA get no switch, because they are SI units already.
Horsepower is two units, not one. Mechanical horsepower is 745.7 W and is what a US motor nameplate means. Metric horsepower — PS, CV — is 735.5 W. The motor calculator prints both so a nameplate can be checked against the right one.
The arithmetic never changes with the units. Inputs are converted back to the units the code tables use before they touch a table, and the results are converted on the way out. Calculations follow the US NEC; outside the US, follow your local electrical code — the metric columns are a conversion for reading and for buying cable, not a claim of compliance with IEC 60364 or any other national standard, whose ampacity tables are organised differently and give different figures for the same cross-section.
What this is not
It is not a design and it is not a permit. Three things routinely make the right table give the wrong answer on a real job. Your local code is not the NEC — states adopt different editions and add amendments, and some cities write their own. Your equipment terminals carry their own temperature rating, which is often lower than the wire’s, and the lower of the two governs. And the conditions of use — sun on a rooftop conduit, a bundle through insulation, a run under a slab — change the derating. A licensed electrician resolves all three; a calculator cannot.
Nothing you type is sent anywhere. Every calculation happens in your own browser, no account is needed for any tool, and the site keeps no record of what you worked out.