Wire Size Calculator
Determine the correct AWG wire size for any single-phase or three-phase circuit. Based on NEC Table 310.15(B)(16-17) ampacity ratings, conductor resistance, and voltage drop limits.
Source: NFPA 70, National Electrical Code, Table 310.15(B)(16-17)
📥 Input Parameters
Source voltage at the start of the circuit (V)
Maximum current flowing through the circuit (A)
One-way length of the wire run (ft)
Material of the wire's conductive core
📤 Calculation Results
🧵 Actual Size — Cross-Section View
Scale: ~5 px = 1 mm- ◆8 AWG (3.26 mm bare) is smaller than a standard wooden pencil.
- Selected conductor: Copper 8 AWG.
- Insulation rating: 75°C — THWN, XHHW, USE.
- Installation method: Raceway (conduit / cable tray).
- Ampacity reference: NEC Table 310.15(B)(16).
ℹ️ How this calculator works
This tool finds the smallest AWG conductor that satisfies bothNEC ampacity (Table 310.15(B)(16) for raceway/cable/buried, Table 310.15(B)(17) for free air) and your selected voltage drop limit. Voltage drop is calculated as Vd = k × L × R × I / 1000, where k = 2 for single-phase and k = √3 for three-phase, L is one-way length in feet, R is conductor resistance per 1000 ft at 75°C.
⚠️ For planning only. Always verify with the latest NEC edition and local amendments, and consult a licensed electrician before installation. Motor loads should use 125% of nameplate FLA.
📊 Quick Wire Gauge ChartNEC 310.15 · Vd ≤ 3%
Recommended AWG by amperage (rows) and one-way distance (columns) — values update live with your calculator parameters above.
| Amps ↓ \ Distance → | 25 ft | 50 ft | 100 ft | 150 ft | 200 ft | 300 ft |
|---|---|---|---|---|---|---|
| 10 A | 14 AWG | 14 AWG | 14 AWG | 12 AWG | 10 AWG | 8 AWG |
| 30 Ayour load | 10 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG | 4 AWG |
| 50 A | 8 AWG | 8 AWG | 6 AWG | 4 AWG | 4 AWG | 2 AWG |
| 70 A | 4 AWG | 4 AWG | 4 AWG | 4 AWG | 3 AWG | 1 AWG |
| 90 A | 3 AWG | 3 AWG | 3 AWG | 3 AWG | 2 AWG | 1/0 AWG |
| 110 A | 2 AWG | 2 AWG | 2 AWG | 2 AWG | 1 AWG | 2/0 AWG |
📋 How to Use This Wire Size Calculator
Eight steps from nameplate data to a code-safe conductor. The whole flow takes less than a minute once you know your circuit's voltage, current, and run length.
- 1
Enter the source voltage
Type the voltage at the start of the circuit — 120 or 240 V for most homes, 208/240/480 V for three-phase equipment, or 12/24/48 V for battery systems.
- 2
Enter the amperes
Use the maximum current the circuit will carry. For motors, multiply nameplate FLA by 1.25; for EV chargers and other continuous loads, use the rated amps — the code already treats them at 125%.
- 3
Enter the one-way distance
Measure from the panel to the load in feet. The calculator doubles this internally for single-phase and DC return paths, so never enter the round-trip length yourself.
- 4
Pick the conductor material
Copper is the default. Choose aluminum only for appropriately rated terminations — aluminum needs to be about two sizes larger and never smaller than 8 AWG.
- 5
Open Advanced Parameters
Set the phases, insulation temperature rating, and installation method (raceway, cable, buried, or free air) if they differ from the defaults.
- 6
Choose the voltage drop limit
3% is the sensible default for branch circuits. Tighten to 1–2% for sensitive electronics, long pump runs, or low-voltage DC systems.
- 7
Read the recommended size
The result is the smallest AWG that satisfies both the NEC ampacity table and your voltage drop target, plus the actual voltage the load will see.
- 8
Cross-check with the chart
The dynamic lookup chart below the results shows the whole amp-by-distance matrix for your current settings — handy for spotting how close you are to the next size up.
Plan the circuit before you pull wirePhoto: UnsplashTip: gather three numbers before you start — the load's rated amps (or the breaker size protecting it), the supply voltage, and the measured or estimated run length in feet.
🎚️ Every Input Explained
What each field means, where the number comes from, and why it changes the answer. Read this once and the calculator stops being a black box.
⚡ Source Voltage
The nominal supply at the circuit origin, not the voltage you hope to see at the load. Single-phase residential circuits are typically 120 V or 240 V; three-phase systems run 208 V, 240 V, or 480 V. Battery-based DC systems are usually 12 V, 24 V, or 48 V. If you are unsure, measure at the panel or read the breaker feeding the circuit.
🔢 Amperes
The maximum steady current the circuit must carry. Take it from the equipment nameplate, the appliance specification, or the branch-circuit breaker. Motors follow a special rule — multiply full-load amps (FLA) by 1.25 before entering the value. EV chargers, water heaters, and anything running three or more hours straight count as continuous loads and are sized at 125% as well.
📏 Distance
The one-way length of the run in feet. Resistance — and therefore voltage drop — scales directly with length, which is why distance decides the wire size on long feeds long before ampacity does. A 30 A load survives 14 AWG ampacity-wise, but at 150 ft the same load needs 6 AWG copper just to hold a 3% drop.
🔀 Phases
Single-phase circuits (two hots, or one hot plus a neutral) return current through a second conductor, so the drop formula doubles the one-way resistance. Three-phase circuits share the return across three conductors and use a √3 factor instead — roughly a 13% smaller conductor for identical conditions. Residential work is almost always single-phase; commercial and agricultural buildings frequently use three-phase.
🌡️ Insulation Rating
The maximum continuous temperature the insulation tolerates: 60°C (TW, UF), 75°C (THWN, XHHW, USE), or 90°C (THHN, THWN-2, XHHW-2). Higher ratings permit more current in the same copper, but your ampacity must match the weakest termination in the circuit — most breaker lugs are listed at 75°C, so 90°C wire is usually assigned the 75°C value.
🧵 Conductor Material
Copper conducts about 60% better than aluminum, so aluminum must be roughly two AWG sizes larger for the same job. Aluminum is still the economical choice for large services and feeders, provided every termination is aluminum-rated (AL/CU marked), connections are torqued properly, and antioxidant compound is applied. NEC 310.106(B) forbids aluminum smaller than 8 AWG in building wiring.
🔧 Installation Method
Heat escapes differently depending on how the wire is run. Conductors in free air cool best and use the higher Table 310.15(B)(17) values. Raceway (conduit or cable tray), factory cables (NM, URD, SE), and direct burial all use Table 310.15(B)(16). Direct-buried cable in warm soil can dissipate heat well, but compacted dry soil insulates — hence conservative values.
📉 Voltage Drop Limit
The percentage of source voltage you are willing to lose as heat in the copper. The NEC's informational notes suggest 3% for a branch circuit and 5% total across feeder plus branch. Sensitive loads — inverters, pumps with long runs, low-voltage DC — deserve 2% or less. Tighter limits mean bigger wire; looser limits save copper at the cost of performance.
Conductor Sizing — Why Wire Size Matters
The National Electrical Code requires conductors to be sized so they do not overheat under continuous load. Ampacity — the maximum current a wire can carry safely — depends on conductor material (copper or aluminum), insulation temperature rating, ambient temperature, and the number of current-carrying conductors bundled together. The cross-sectional area of a conductor determines its resistance: a larger wire (lower AWG number) has lower resistance, lower I²R heating, and higher allowable ampacity.
Voltage drop is the second constraint. On long runs, the wire's resistance causes the voltage at the load to fall below the source voltage. The NEC recommends a maximum voltage drop of 5% for combined feeder and branch circuits, with 3% as a sensible practical target. Excess voltage drop causes motors to overheat, lights to dim, and electronics to fail. This calculator picks the smallest wire that satisfies both ampacity and your selected voltage drop limit.
AWG vs kcmil
American Wire Gauge (AWG) numbers run from 14 AWG (smallest common power conductor) down through 4/0 AWG. Above 4/0, sizes are expressed in thousands of circular mils (kcmil): 250 kcmil, 350 kcmil, 500 kcmil and so on. Each step larger reduces resistance and increases ampacity. For very large services, parallel runs of smaller conductors are often more practical than a single huge cable — consult an engineer when loads exceed 600 A.
Single-Phase vs Three-Phase
Single-phase circuits use two wires (line and neutral, plus a ground) — current flows out on one and returns on the other, so the voltage drop formula doubles the one-way resistance. Three-phase circuits use three current-carrying conductors, and the geometric return path means voltage drop is proportional to √3 instead of 2. Common single-phase voltages are 120 V and 240 V; common three-phase voltages are 208 V, 240 V, 480 V.
Insulation Temperature Ratings
The 60°C column covers older TW and UF insulation. 75°C covers THWN, XHHW, and USE — the most common modern residential and commercial wire types. 90°C covers THHN, THHW, and XHHW-2, used in higher-temperature environments. Always select the column that matches the lowest-rated termination in the circuit — a 90°C wire terminated to a 75°C breaker must use the 75°C ampacity.
🧮 The Math Behind the Calculator
Two physical laws and one unit convention produce every wire size on this page. Here is the full derivation, in the same order the calculator applies it.
1. From Ohm's law to wire area
Ohm's law (V = I × R) tells us how much voltage a current loses across a resistance. Pouillet's law (R = ρ × L / A) says that resistance equals the material's resistivity ρ times length L, divided by cross-sectional area A. Substitute one into the other and solve for A, and the minimum conductor area for a single-phase (or two-wire DC) circuit falls out:
where the factor of 2 accounts for the outbound and return conductors, I is the load current, L is the one-way length, and Vdrop is the absolute voltage you are willing to lose (source voltage × drop limit). For three-phase, the return path is shared across three lines, so the 2 becomes √3:
In metric units ρ is measured in Ω·m and A comes out in m² (multiply by 1,000,000 for mm²). At 20°C, ρ is 1.68 × 10⁻⁸ Ω·m for copper and 2.82 × 10⁻⁸ Ω·m for aluminum.
2. The circular-mils shortcut
North American electricians rarely touch metric area. Instead the trade uses circular mils (cmil): the area of a circle one mil across, numerically equal to the square of the wire's diameter in mils. Working in feet, amps, and volts, the same equation becomes:
K is the resistivity constant in Ω·cmil/ft. At 75°C operating temperature — the basis of this calculator's resistance table — K is about 12.9 for copper and 21.2 for aluminum. At room temperature the copper figure drops closer to 10.4, which is why handbooks that assume cooler conductors quote slightly smaller K values.
3. Temperature correction of resistivity
Copper and aluminum both get more resistive as they warm. If you need the resistivity at a temperature other than the reference, scale it linearly:
with α ≈ 0.004 per °C for copper and 0.0043 per °C for aluminum. Example: copper at 50°C is 1.68 × 10⁻⁸ × (1 + 0.004 × 30) ≈ 1.88 × 10⁻⁸ Ω·m — about 12% more resistance than at room temperature, which is exactly why voltage drop calculators reference 75°C values rather than optimistic 20°C ones.
4. How AWG numbers relate to diameter
AWG is a logarithmic scale pinned at 36 AWG = 0.005 in and 4/0 AWG = 0.46 in of bare-conductor diameter. The gauge for any diameter d (inches) is:
Useful mental math from that scale: every 3-gauge jump halves the area and doubles resistance; every 6-gauge jump quarters the area; and −39/39 makes 4/0-to-36 a 92-step ladder. That is why upsizing three sizes roughly doubles your copper — and why the calculator always rounds up to the next standard size.
✏️ Worked Examples
Three real circuits, solved end to end — a household appliance run, a multi-motor feeder, and a low-voltage DC battery circuit. Every number below is reproducible in the calculator above.
240 V dryer circuit
Single-phase · copper · 75°C · raceway · 3% target
- Circuit: 30 A clothes dryer, 100 ft from panel.
- Ampacity check: 10 AWG copper is rated 35 A at 75°C and just clears the NEC 240.4(D) 30 A cap — ampacity alone would say 10 AWG.
- Drop check: Vd = 2 × 100 × 1.21 × 30 / 1000 = 7.26 V, which is 3.03% of 240 V — a hair over the 3% limit.
- Upsize: 8 AWG (R = 0.764 Ω/1000 ft) drops only 4.58 V = 1.91%.
Pump station feeder
Three-phase 460 V · NEC Table 430.250 FLA
- Load: three pumps — 25 HP (34 A), 7.5 HP (11 A), 2 HP (3.4 A).
- Feeder rule (Art. 430): 125% of the largest motor plus the full load of the rest: (34 × 1.25) + 11 + 3.4 = 56.9 → 57 A.
- Enter 57 A at 460 V, 200 ft, three-phase, 3% target.
- Ampacity: 6 AWG copper is rated 65 A at 75°C — passes.
- Drop: Vd = 1.732 × 200 × 0.491 × 57 / 1000 = 9.7 V = 2.11% — passes.
12 V RV lighting circuit
DC battery system · 3% target
- Load: 10 A of LED lighting, 15 ft from the fuse panel.
- Budget: 3% of 12 V is only 0.36 V — a tiny absolute number.
- 10 AWG test: Vd = 2 × 15 × 1.21 × 10 / 1000 = 0.363 V → 3.02% — just misses.
- 8 AWG test: Vd = 0.229 V → 1.91% — comfortable margin.
- Contrast: the same 10 A on a 120 V circuit would pass easily in 14 AWG. Low voltage, not amps, is what inflates DC wire.
⚙️ Sizing Wire for Electric Motors
Motors are the most common reason a "correct-looking" wire turns out undersized. NEC Article 430 adds three rules on top of ordinary ampacity.
125% of full-load amps
Branch-circuit conductors feeding a single motor must be sized at no less than 125% of the motor's full-load current (NEC 430.22). Take the FLA from the nameplate, or from NEC Tables 430.248 (single-phase) / 430.250 (three-phase) when the nameplate is unreadable.
Inrush current
At startup a motor can draw five to seven times its FLA for a fraction of a second. The conductor must tolerate that surge, and the breaker must not trip on it — which is why motor breakers are often time-delayed (inverse-time) rather than instant-trip.
Multi-motor feeders
For a feeder serving several motors, size for 125% of the largest motor's FLA plus the full load of every other motor on the feeder (NEC 430.24). Enter that combined figure in the calculator's Amperes field — see the pump-station example above.
Practical workflow
- Read the motor nameplate: voltage, FLA, service factor, and whether it is single- or three-phase.
- Multiply FLA by 1.25 (or apply the multi-motor rule) and enter the result as Amperes.
- Add distance and set a 3% drop limit — motor loads are notoriously intolerant of sag at the terminals.
- Check the result against the overload and short-circuit protection required by Article 430 Part IV.
- For HVAC compressors, use the data-plate MCA (minimum circuit ampacity) instead of doing the 125% math yourself — the manufacturer has already done it.
🌡️ Derating Factors — When the Table Value Is Too Optimistic
Published ampacities assume a cool, uncrowded raceway at 30°C. Real attics, engine rooms, and conduit banks are harsher, and the NEC cuts the allowable current accordingly.
Ambient temperature correction
Multiplier applied to the 75°C ampacity column (NEC Table 310.15(B)(1)):
| Ambient temperature | Factor |
|---|---|
| 31–35°C (87–95°F) | 0.94 |
| 36–40°C (96–104°F) | 0.88 |
| 41–45°C (105–113°F) | 0.82 |
| 46–50°C (114–122°F) | 0.75 |
| 51–55°C (123–131°F) | 0.71 |
Bundled current-carrying conductors
Adjustment when more than three current-carrying conductors share a raceway or cable (NEC Table 310.15(B)(3)(a)):
| Conductors in raceway | Ampacity retained |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
Grounding and bonding conductors are not counted; neutrals on balanced 3-phase or split 1-phase circuits are not either.
Worked derating example
An 8 AWG copper THWN conductor is rated 50 A at 75°C. It runs through a hot attic at 51–55°C (×0.71) bundled with five other current-carrying conductors (×0.80). Effective capacity: 50 × 0.71 × 0.80 = 28.4 A — the wire is now smaller than the 30 A breaker above it, and one size up is mandatory. This is precisely the class of situation the calculator's disclaimer warns about, so apply derating before finalizing any installation in a hostile environment.
Continuous loads (the invisible derate)
Any load running three hours or more — water heaters, EV chargers, HVAC, refrigeration — must have its conductor and overcurrent device sized at 125% of the continuous current (NEC 210.19(A)(1), 215.2(A)(1)). A 24 A continuous load therefore needs a 30 A circuit and a conductor good for at least 30 A at 75°C, before ambient or bundling derates.
🛡️ Pairing Wire with Breakers & Fuses
The breaker does not protect the appliance — it protects the wire. Its one job is to interrupt current before the conductor exceeds its temperature rating.
Rule 1 — Breaker never exceeds the wire
The overcurrent device must be rated at or below the conductor's effective ampacity (NEC 240.4). Pick a 50 A breaker and you owe the circuit at least 50 A of wire at 75°C after every derate — 8 AWG copper or 6 AWG aluminum, whichever table value you are using. Our Reverse Ampacity tab automates exactly this: enter the existing wire and run length, and it returns the largest standard breaker the conductor can legally carry.
Rule 2 — Small conductors are capped
NEC 240.4(D) overrides the ampacity tables for small wire: 14 AWG copper is limited to 15 A protection, 12 AWG copper to 20 A, and 10 AWG copper to 30 A — regardless of the higher values in Table 310.15(B)(16). This is the small-conductor rule flagged in the calculator's results panel.
Rule 3 — Use standard sizes
Breakers and fuses come in standard ampere ratings per NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, and upward. A conductor whose precise safe limit is 42 A gets a 40 A device — the calculator rounds down to the next standard size for you.
Why fuses are sized from the wire, not the load: if a fan motor shorts internally, the fault current flows through everything upstream of the short. The fuse must open before the cable insulation cooks — so the fuse rating follows the conductor's ampacity, while the load's needs only set a floor. This is also why adding loads to an existing circuit requires re-checking the wire, not just swapping the breaker.
🔋 12V / 24V / 48V DC Wire Sizing
Solar arrays, RVs, vans, boats, golf carts, and off-grid cabins all live or die by low-voltage DC wiring. The same physics applies — but the margins are brutally thin.
Why low voltage is unforgiving
Voltage drop is an absolute number of volts. Half a volt lost in the copper is a meaningless 0.4% on a 120 V circuit — but it is a noticeable 4.2% on a 12 V battery system, where an inverter may cut out below 11.5 V. Multiply by the currents involved (a 2,000 W inverter draws roughly 200 A from a 12 V bank) and it is obvious why battery cables are the fattest wires in any RV.
Rules of thumb for DC runs
- Keep drop at or under 3% for critical circuits; ABYC marine guidance allows up to 10% for non-critical loads.
- The two-wire round trip means the effective length is double the physical distance — the calculator's single-phase factor of 2 already handles this.
- Raising system voltage is the cheapest upgrade there is: going 12 V → 24 V halves the current, and 48 V cuts it to a quarter, shrinking the required copper dramatically.
- Motive loads — trolling motors, winches, starters — want even tighter drop because motor torque collapses with voltage.
Off-grid DC systems live on thin voltage marginsPhoto: UnsplashA note on standards: this calculator implements NEC ampacity tables, which are written for AC building wiring at 60/75/90°C ratings. Marine and mobile installations are usually built to ABYC E-11 with 105°C tinned marine-grade copper and their own bundling rules — the numbers differ, but the workflow (drop limit, then ampacity, then fuse matching) is identical. When a marine rating conflicts with the NEC value shown, follow the stricter of the two for your application.
Wire Size Calculators for DC, Solar, Marine & Automotive Systems
The main calculator above follows NEC building-wire tables for AC premise wiring. Low-voltage systems — RVs, boats, off-grid solar, cars, and landscape lighting — run under different standards (ABYC E-11, NEC 690.8, SAE J1128) with their own fuse rules, temperature columns, and drop budgets. These five purpose-built tools apply the right rule set to each job.
🏠 Common Applications — Quick Answers
Starting points for the circuits people actually build. Treat these as initial guesses, then confirm with the calculator using your true voltage, amps, and distance.
🏠 100 A residential service
Using the NEC dwelling allowance (83% of service rating). A full 100 A continuous demand needs 3 AWG Cu / 1 AWG Al. Upsize for long underground laterals.
🏘️ 200 A residential service
The classic service-entrance pair at 75°C. Distance is rarely an issue at service length, but conduit fill and ambient matter.
🛠️ Detached garage / shed subpanel
Ampacity is easy; the trench length is what decides. A 60 A feed at 300 ft can end up at 4 AWG copper once a 3% drop is enforced.
🚗 Level 2 EV charger
Charging is continuous, so the breaker is 125% of the charger rating: 50 A for a 40 A unit, 60 A for 48 A. Hardwired or EV-rated receptacle per Article 625.
🚰 Well pump / irrigation
A 1 HP 230 V pump draws only ~8 A — 14 AWG would carry it. But pumps at the bottom of a well or out in a field routinely need 10 or 8 AWG purely for voltage drop, and sag makes pumps stall.
❄️ AC condenser / heat pump
Manufacturers print the Minimum Circuit Ampacity and Max Overcurrent Protection directly on the data plate (e.g., MCA 35 A → 10 AWG Cu, 45 A max breaker). Never substitute your own math.
🏭 Welder circuit
Welders are sized by duty cycle under Article 630 — a 40%-duty machine may legally use a smaller conductor than its input amps suggest. Check the welder's supply-connection data.
☀️ Solar & battery banks
See the DC section above: keep drop ≤3%, size for round-trip length, and fuse to the wire. Inverter battery cables frequently reach 2/0–4/0 AWG.
📏 AWG Quick Reference Chart
Physical properties of common building-wire sizes. Ampacity is copper in raceway at 75°C — for aluminum, move two sizes larger. The dynamic chart near the calculator shows drop-limited sizes for your exact inputs.
| Wire size | Diameter | Area (kcmil) | Area (mm²) | Cu Ω/1000 ft @75°C | Cu ampacity 75°C | Typical use |
|---|---|---|---|---|---|---|
| 14 AWG | 0.064" · 1.63 mm | 4.11 | 2.08 | 3.07 | 20 A | 15 A lighting & receptacle circuits |
| 12 AWG | 0.081" · 2.05 mm | 6.53 | 3.31 | 1.93 | 25 A | 20 A kitchen / bath receptacles |
| 10 AWG | 0.102" · 2.59 mm | 10.4 | 5.26 | 1.21 | 35 A | Water heaters, 30 A appliance circuits |
| 8 AWG | 0.128" · 3.26 mm | 16.5 | 8.37 | 0.764 | 50 A | Ranges, EV chargers on 50 A circuits |
| 6 AWG | 0.162" · 4.12 mm | 26.2 | 13.3 | 0.491 | 65 A | 60 A subpanels, 48 A EV charging |
| 4 AWG | 0.204" · 5.19 mm | 41.7 | 21.2 | 0.308 | 85 A | 100 A feeders & dwelling services |
| 3 AWG | 0.229" · 5.83 mm | 52.6 | 26.7 | 0.245 | 100 A | 100 A continuous feeders |
| 2 AWG | 0.258" · 6.54 mm | 66.4 | 33.6 | 0.194 | 115 A | Large residential feeders |
| 1 AWG | 0.289" · 7.35 mm | 83.7 | 42.4 | 0.154 | 130 A | 125–150 A distribution |
| 1/0 AWG | 0.325" · 8.25 mm | 106 | 53.5 | 0.122 | 150 A | 150 A services, long feeders |
| 2/0 AWG | 0.365" · 9.27 mm | 133 | 67.4 | 0.0967 | 175 A | 175 A distribution |
| 3/0 AWG | 0.410" · 10.4 mm | 168 | 85.0 | 0.0766 | 200 A | 200 A service entrance (copper) |
| 4/0 AWG | 0.460" · 11.7 mm | 212 | 107 | 0.0610 | 230 A | 200 A services (aluminum), large feeders |
Ampacity per NFPA 70 (NEC) Table 310.15(B)(16), 30°C ambient, ≤3 current-carrying conductors. Free-air ratings are higher (see the calculator's Installation option).
Metric mm² ↔ AWG conversion
| Metric (IEC) | Nearest AWG |
|---|---|
| 1.5 mm² | 16 AWG |
| 2.5 mm² | 14 AWG |
| 4 mm² | 12 AWG |
| 6 mm² | 10 AWG |
| 10 mm² | 8 AWG |
| 16 mm² | 6 AWG |
| 25 mm² | 4 AWG |
| 35 mm² | 2 AWG |
| 50 mm² | 1/0 AWG |
| 70 mm² | 2/0 AWG |
| 95 mm² | 3/0 AWG |
| 120 mm² | 4/0 AWG |
Approximate ampacity equivalents — metric and AWG series do not align exactly.
Wire Size Calculator — Frequently Asked Questions
How do I calculate wire size for a 100 amp service?▾
For a 100 A single-phase 120/240 V residential service, the NEC dwelling allowance (sizing at 83% of the service rating, formerly 310.15(B)(7)) generally permits 4 AWG copper or 2 AWG aluminum at 75°C. If the load is a true 100 A continuous demand, you need 3 AWG copper or 1 AWG aluminum in raceway instead. Long underground runs often force upsizing — at 200 ft with a 3% target you will typically need 1/0 AWG copper or 2/0 AWG aluminum. Use the Wire Size Calculator tab with your exact voltage, amps, and distance to confirm.
What's the difference between 60°C, 75°C, and 90°C insulation?▾
The number is the maximum sustained operating temperature of the insulation in a 30°C ambient. Higher temperature ratings allow higher ampacity for the same AWG. 60°C insulation (TW, UF) is older and used for wet locations. 75°C (THWN, XHHW, USE) is the most common modern rating. 90°C (THHN, THHW, XHHW-2) is for high-temperature applications. The ampacity used must match the lowest-rated termination in the circuit — most breakers and lugs are rated 75°C.
Why does voltage drop matter on long runs?▾
Every conductor has resistance. On a long run, that resistance causes the voltage at the load to drop below the source voltage. The NEC recommends a maximum 5% combined voltage drop (3% on branch circuits). Excess voltage drop causes motors to draw more current and run hotter, lights to dim, and resistive loads (heaters, incandescent lamps) to produce less output. Voltage drop is calculated as Vd = k × L × R × I / 1000, where k = 2 (single-phase) or √3 (three-phase).
Can I use aluminum wire instead of copper?▾
Yes — aluminum is significantly cheaper and lighter than copper for the same ampacity, but it requires roughly two wire sizes larger (e.g. 2/0 aluminum replaces 1/0 copper for the same ampacity). Aluminum is widely used for service entrance and large feeder runs. Always use aluminum-rated lugs and antioxidant compound, and follow NEC 110.14(C) for termination requirements. The calculator has a Conductor switch for both materials.
What is the reverse ampacity calculator for?▾
The Reverse Ampacity tab answers: "I have an existing wire in the wall — how many amps can I safely put through it?" Enter the AWG size, length, and source voltage, and the tool returns the maximum safe current. The result is the lower of (a) the conductor's NEC ampacity and (b) the current that would push voltage drop past your target. Useful when sizing a new breaker for an existing circuit.
Does this calculator handle motor circuits?▾
For motors, the NEC requires conductors sized at 125% of the motor's full-load amps (FLA). Enter your motor's nameplate FLA multiplied by 1.25 in the Amperes field. For continuous loads (on for 3+ hours), the NEC also requires the conductor and breaker be sized at 125% of the load. Always consult the motor's nameplate and Article 430 for motor branch-circuit requirements.
Is this calculator code-compliant?▾
This tool uses NEC Table 310.15(B)(16) (raceway/cable/buried) and Table 310.15(B)(17) (free air) ampacity values at 30°C ambient with no more than 3 current-carrying conductors. It does NOT apply derating for higher ambient temperatures, more than 3 conductors in a raceway, or special equipment requirements (motors, welders, EV chargers). Always verify with the latest NEC edition and local amendments, and consult a licensed electrician.
How do I calculate wire size for a three-phase circuit?▾
Three-phase power uses three current-carrying conductors with no return path, so the drop formula uses √3 instead of 2. In circular-mils form: CM = (√3 × K × I × L) / Vd, where K is 12.9 for copper or 21.2 for aluminum, I is the line current, L is the one-way length in feet, and Vd is the allowable drop in volts. A handy shortcut: a three-phase conductor can be about 86.6% (√3/2) of the area a single-phase circuit would need for the same voltage, current, length, and drop limit.
Why does the required wire size grow with distance?▾
Conductor resistance is directly proportional to length, so doubling the run doubles the resistance and doubles the voltage drop for the same load. To hold the drop percentage constant while doubling distance, the cross-sectional area must also double — which means going up roughly three AWG sizes. This is why distance, not ampacity, is usually the deciding factor for pumps, well motors, detached buildings, and any other long feeder.
How is AWG calculated from wire diameter?▾
American Wire Gauge follows a logarithmic scale anchored at two points: 36 AWG is exactly 0.005 inches and 4/0 AWG is 0.46 inches in diameter. The gauge number is n = −39 × log₉₂(d / 0.005) + 36, where d is the bare conductor diameter in inches. Practical takeaway: every 3-gauge step roughly doubles the metal area, and every 6-gauge step quadruples it — which is why 10 AWG carries about twice the current of 16 AWG.
What is a circular mil, and what does kcmil mean?▾
A circular mil (cmil) is the area of a circle one mil (1/1000 inch) in diameter — about 0.000507 mm². It makes wire math easy because the area in circular mils is simply the square of the diameter in mils: a wire 100 mils across is 10,000 cmil. One kcmil (thousand circular mils, historically written MCM) is 1,000 cmil. AWG ends at 4/0 (211,600 cmil); anything larger — 250, 350, 500 kcmil and beyond — is expressed in kcmil. 250 kcmil is roughly 127 mm².
Does ambient temperature change the wire size I need?▾
Yes. Ampacity tables assume a 30°C (86°F) ambient. In hotter surroundings the wire cannot shed heat as well, so NEC Table 310.15(B)(1) applies correction factors — for a 75°C conductor: 0.88 at 36–40°C, 0.82 at 41–45°C, and 0.75 at 46–50°C. A 50 A conductor in a 50°C attic is only good for about 37 A. Hot attics, rooftop equipment, engine rooms, and dense conduit banks are the usual culprits that force a size bump.
What's the difference between THHN, THWN, and XHHW wire?▾
These are single-conductor building wire insulation types. THHN is rated 90°C in dry locations only; THWN is 75°C and works in wet locations; the -2 suffix (THWN-2, XHHW-2) means 90°C dry and 75°C wet, which is why modern THHN/THWN-2 dual-rated wire is the shop standard. XHHW uses cross-linked polyethylene that handles heat and moisture better than PVC-based THHN. Remember that in practice you almost always assign ampacity from the 75°C column, because breaker lugs are typically rated 75°C.
How do I convert metric mm² sizes to AWG?▾
Common approximate equivalents by ampacity: 1.5 mm² ≈ 16 AWG, 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 6 mm² ≈ 10 AWG, 10 mm² ≈ 8 AWG, 16 mm² ≈ 6 AWG, 25 mm² ≈ 4 AWG, 35 mm² ≈ 2 AWG, 50 mm² ≈ 1/0 AWG, 70 mm² ≈ 2/0 AWG, 95 mm² ≈ 3/0 AWG, and 120 mm² ≈ 4/0 AWG. The mapping is never exact — IEC metric sizes and AWG steps don't align perfectly — so when a certificate or print specifies one system, stay within that system.
Do the same rules apply to residential and industrial wiring?▾
The physics and the ampacity tables are identical, but the surrounding requirements differ. Residential work is mostly single-phase 120/240 V with romex-style cable and 60–75°C terminations. Industrial facilities run three-phase 208/480 V systems in conduit, apply conductor derating for dense wireways, and follow additional rules for motors (Article 430), welders (Article 630), and continuous process loads. Jurisdictions may also enforce local amendments and stricter engineering review on industrial installations.