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Wire Size Calculator

Ready to calculate
NEC 310.16 Verified.
AC + DC + 3-Phase.
Copper & Aluminum.
100% Free.
Privacy Secure.

How it Works

01Voltage & Current

Enter system voltage (120/240/480) and load in amps. Pick AC 1-ph, 3-ph, or DC.

02One-Way Length

Distance from source to load in feet. Round-trip is doubled automatically.

03Material & Temp

Copper or aluminum. Insulation temp rating 60 / 75 / 90 °C.

04AWG + Voltage Drop

Recommended AWG, ampacity, actual voltage drop — both branch 3% and feeder 5% tested.

What is a Wire Size Calculator?

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A wire size calculator is the universal conductor sizer every electrician, solar installer, and electrical estimator keeps one click away. Unlike preset amp calculators (100A, 200A, 12V), this tool handles the full range: AC or DC, any voltage, any amperage, any run length, copper or aluminum, using NEC Table 310.16 ampacity and the standard voltage-drop formula. It's the calculator that replaces three different reference cards in your toolbox.

The underlying math runs in two steps. First, find the minimum AWG for the load per NEC 310.16 at your selected insulation temperature column (60°C for old-house NM-B, 75°C for most modern terminations, 90°C for free-air derated applications). Second, apply the voltage-drop formula — V = (K × L × I) ÷ CM for AC one-way, or V = (2 × K × L × I) ÷ CM for DC round-trip — where K is the conductor resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), L is distance, I is current, and CM is circular mils. If drop exceeds the target (3% for branch circuits, 5% for combined feeder + branch), the calculator upsizes AWG until drop is within limits.

Inputs: amperage, voltage, one-way run length, conductor material (copper or aluminum), insulation temperature rating, and AC or DC system. Output includes required AWG, actual voltage drop %, ampacity margin, wattage lost to resistance, and whether the final size is ampacity-limited or drop-limited. Also reports typical breaker/fuse size for the circuit.

Typical use cases: branch circuit sizing (15A to 60A residential), feeder sizing (60A to 400A subpanels and services), solar home-run sizing (10 AWG to 4/0 for string PV), RV and marine DC (16 AWG to 4/0 for 12V-48V systems), and commercial 208V/480V three-phase (any AWG). For the specialized applications (100A residential service, 12V DC, 220V appliances), use the dedicated calculators in the same family.

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How the Wire Size Calculator Works

Enter voltage and amps: System voltage (120/240/480) and the load current draw.
Enter one-way length: Distance from source to load in feet. Round-trip is handled automatically.
Pick material + temp: Copper (standard) vs aluminum (78% ampacity). 60°C (TW/UF), 75°C (THW/THWN), or 90°C (THHN/XHHW).
Set max drop: NEC recommends 3% on branch circuits, 5% total feeder + branch.
Get recommended AWG: Smallest wire that satisfies both ampacity and voltage drop limits.

Wire Size Formula

Voltage Drop = (2 × K × L × I) ÷ Circular Mils

  • K = 12.9 Ω·cmil/ft for copper at 75°C
  • K = 21.2 Ω·cmil/ft for aluminum at 75°C
  • L = one-way length in feet (factor of 2 accounts for return path)
  • I = current in amps

Solving for minimum circular mils: cmils = (2 × K × L × I) ÷ max voltage drop

Then pick the smallest AWG whose circular mils ≥ calculated minimum AND whose ampacity ≥ load.

Real-World Example

Example: 20 Amp Circuit, 100 ft Run, 120V

Load: 20A at 120V. Length: 100 ft. Copper. Max 3% drop = 3.6V.

  • Min circular mils = (2 × 12.9 × 100 × 20) ÷ 3.6 = 14,333 cmils
  • 10 AWG = 10,380 cmils → too small (voltage drop)
  • 8 AWG = 16,510 cmils → OK
  • 8 AWG ampacity (75°C) = 50A → massive headroom for 20A
  • Actual drop = (2 × 12.9 × 100 × 20) ÷ 16,510 = 3.12V = 2.6%

Without voltage-drop, 12 AWG would have been enough for 20A (25A ampacity) — but the 100ft run pushes it to 8 AWG.

Who uses a wire size calculator?

1
Electricians pulling branch circuits, feeders, and service entrance conductors.
2
DIY homeowners wiring garages, hot tubs, well pumps, and EV chargers.
3
Solar and RV installers sizing DC runs between panels, charge controllers, and batteries.
4
Low-voltage contractors for landscape lighting, doorbells, and security systems.
5
Marine technicians working to ABYC E-11 standards on boats and yachts.

AWG Reference Table — Copper at 75°C

AWGAmpacityCircular MilsTypical Use
1420A4,11015A branch circuits
1225A6,53020A branch circuits
1035A10,38030A dryer / well pump
850A16,51040-50A range, EV charger
665A26,24060A AC, subpanel feeder
485A41,74070-85A feeder
3100A52,620100A service feeder
2115A66,360100-125A feeder w/ drop
1130A83,690125A feeder
1/0150A105,600150A service
2/0175A133,100150A long run
3/0200A167,800200A service
4/0230A211,600200A long run

Key Takeaways

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A universal wire-size calculator is only as good as the two checks it runs: ampacity (can the wire safely carry the current without overheating?) and voltage drop (does the wire deliver the rated voltage at the load?). Skip either check and you end up with undersized cable that passes inspection but runs hot, or oversized cable that wastes hundreds of dollars in copper with no performance benefit.

Every new circuit or service must also respect the surrounding code: overcurrent protection matches the wire (not the load), conduit fill stays within NEC Chapter 9 Table 1 limits, ground conductors follow NEC 250.66 (service grounds) or 250.122 (equipment grounds), and all terminations are torqued to the manufacturer spec. This calculator handles the conductor math; a licensed electrician and a pulled permit handle the rest. For anything larger than a simple branch circuit, always involve the AHJ and pull the inspection ticket.

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Wire Size Calculator FAQs

What size wire do I need for a 20 amp circuit?
For a standard 20A circuit at 120V: 12 AWG copper handles 20A with 25A ampacity at 75°C. For runs over 75 ft, step up to 10 AWG to keep voltage drop under 3%. For 20A at 240V, 12 AWG copper works up to 150 ft runs.
What size wire for 30 amps?
10 AWG copper handles 30A at 35A ampacity. For runs over 100 ft, step up to 8 AWG. Aluminum needs 8 AWG for 30A (78% ampacity factor). A 30A dryer circuit typically uses 10 AWG.
What size wire for 50 amps?
8 AWG copper handles 50A (50A ampacity at 75°C, 55A at 90°C). For long runs (over 100 ft), step up to 6 AWG for voltage drop. Aluminum needs 6 AWG for 50A. Common 50A uses: electric range, EV charger.
What size wire for 100 amp service?
3 AWG copper or 1 AWG aluminum is the standard 100A service entrance conductor. For very long feeder runs (over 150 ft), step up to 2 AWG copper / 1/0 aluminum for voltage drop.
What size wire for 200 amp service?
3/0 AWG copper or 4/0 AWG aluminum is the standard 200A service. For long runs, step up to 4/0 copper or 250 kcmil aluminum.
How do I calculate voltage drop?
Voltage drop (V) = (2 × K × L × I) ÷ circular mils, where K is 12.9 for copper or 21.2 for aluminum, L is one-way length in feet, and I is current in amps. Divide the voltage drop by the source voltage to get percent drop.
What is the difference between copper and aluminum wire?
Aluminum has about 78% the ampacity of copper, so you need to go up one AWG size for the same amp rating. Aluminum is cheaper and lighter for long service feeders, but requires anti-oxidant compound at terminations and is prohibited on small branch circuits by most local codes.
What is the NEC voltage drop recommendation?
NEC 210.19 recommends a maximum of 3% drop on branch circuits and 5% combined drop on feeders plus branches. These are recommendations, not hard code requirements — but exceeding them wastes energy and may cause motor/appliance issues.
Does wire length matter for wire size?
Yes — significantly. Ampacity sets a minimum based on heat, but voltage drop over long distances may force you to a larger wire. A 20A / 100 ft circuit may need 8 AWG (from the typical 12 AWG for 20A) to keep voltage drop under 3%.
Can I use a smaller wire if I upgrade the breaker?
No — always size the wire for the actual load plus NEC safety factors, not the breaker. The breaker protects the wire, not the other way around. Upsizing the breaker on undersized wire is a fire hazard and code violation.

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Disclaimer

Educational reference. For permit / inspection, always verify with a licensed electrician and local AHJ amendments.