What is AWG wire gauge?

AWG (American Wire Gauge) is the standard system for measuring wire diameter. Smaller AWG numbers = thicker wire = more current capacity. 14 AWG is thinner than 12 AWG, which is thinner than 10 AWG. Below 1 AWG, sizes are expressed as 1/0 (one-ought), 2/0, 3/0, 4/0. One AWG step changes diameter by about 26% and area by about 50%. Wire gauge determines maximum safe amperage (ampacity) for a given insulation temperature rating.

What is the NEC ampacity for common wire sizes?

NEC 310.16 ampacity for copper at 75°C (THWN/THHN): 14 AWG = 20A, 12 AWG = 25A, 10 AWG = 35A, 8 AWG = 50A, 6 AWG = 65A, 4 AWG = 85A, 3 AWG = 100A, 2 AWG = 115A, 1 AWG = 130A, 1/0 = 150A, 2/0 = 175A, 3/0 = 200A, 4/0 = 230A. Circuit breaker sizes are lower: 14 AWG = 15A breaker, 12 AWG = 20A, 10 AWG = 30A per NEC 240.4(D).

How do I calculate voltage drop?

Voltage Drop = (2 × Length × Current × Resistance per ft) for single-phase. VD = (√3 × Length × Current × Resistance per ft) for three-phase. Keep VD under 3% for branch circuits and 5% total (feeder + branch). Example: 12 AWG copper, 100 ft run, 20A at 120V: VD = 2 × 100 × 20 × 0.00193 = 7.72V (6.4%). This exceeds 3% — upsize to 10 AWG: VD = 2 × 100 × 20 × 0.00122 = 4.88V (4.1%), or use 8 AWG for under 3%.

Can I use aluminum wire instead of copper?

Yes, but aluminum requires 1-2 AWG sizes larger for the same ampacity. Aluminum is cheaper per foot but has higher resistance, more thermal expansion, and requires anti-oxidant compound on all connections. Use only AL-rated devices (outlets, breakers, lugs). Aluminum is commonly used for service entrance cables (SE), feeder cables, and large branch circuits (40A+) where cost savings are significant. Never mix aluminum and copper without listed connectors.

What is the 80% rule for wire sizing?

NEC requires that continuous loads (running 3+ hours) not exceed 80% of the wire and breaker rating. A 20A breaker on a continuous load is limited to 16A (20 × 0.80). This means wire sizing for continuous loads should be based on 125% of the actual load. Example: 30A continuous load requires wire rated for 37.5A — use 8 AWG copper minimum. This prevents overheating from sustained current flow.

Wire Size Calculator — Free AWG, Ampacity & Voltage Drop Wire Gauge Selector 2026 | AllInOneTools

🔌 Electrical & Energy

Wire Size Calculator

Select the correct AWG wire gauge based on amperage, voltage, distance, and conductor material. Includes NEC ampacity tables, voltage drop calculation, and breaker matching.

Circuit Requirements

Load Current

Amps

System Voltage

One-Way Distance

feet

Phase

Conductor Material

Max Voltage Drop

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Wire Specifications

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NEC Ampacity Reference

📝 Step-by-Step Selection

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Wire Size Calculator: The Complete Guide to Selecting the Correct AWG Gauge

Selecting the correct wire size is critical for safety, code compliance, and performance. Undersized wire overheats — causing insulation damage, circuit breaker nuisance tripping, and fire risk. Oversized wire wastes money. The right wire size balances ampacity (safe current-carrying capacity) and voltage drop (acceptable power loss over distance) per NEC requirements.

Wire Selection Formula

Step 1 — Ampacity: Select wire with NEC ampacity ≥ load amps
Step 2 — Voltage Drop: VD = (2 × L × I × R/ft) for 1-phase
VD = (√3 × L × I × R/ft) for 3-phase
Step 3 — VD% = (VD ÷ System Voltage) × 100
Step 4 — If VD% > max allowed, upsize wire
Step 5 — Use the LARGER of ampacity-based and VD-based sizes

Worked Example — 20A Kitchen Circuit, 75 ft

Load: 20A, 120V single-phase, copper, 75 ft one-way
Ampacity: 12 AWG (NEC breaker max 20A on 12 AWG) ✓
12 AWG resistance: 1.93 mΩ/ft
VD = 2 × 75 × 20 × 0.00193 = 5.79V (4.8%)
4.8% exceeds 3% limit → upsize to 10 AWG
10 AWG: VD = 2 × 75 × 20 × 0.00122 = 3.66V (3.05%)
Still borderline → use 10 AWG copper (30A rated)

NEC Ampacity Table (75°C Copper)

AWG	Ampacity (Cu)	Ampacity (Al)	Breaker (Cu)	Resistance (mΩ/ft)
14	20A	—	15A	3.07
12	25A	20A	20A	1.93
10	35A	30A	30A	1.21
8	50A	40A	40A	0.764
6	65A	50A	55A	0.491
4	85A	65A	70A	0.308
3	100A	75A	85A	0.245
2	115A	90A	95A	0.194
1	130A	100A	110A	0.154
1/0	150A	120A	125A	0.122
2/0	175A	135A	150A	0.0967
3/0	200A	155A	175A	0.0766
4/0	230A	180A	200A	0.0608

Pro Tip — Always Use the Larger Wire

When ampacity and voltage drop calculations suggest different wire sizes, always use the larger (lower AWG number). For long runs (over 50 ft at 120V), voltage drop usually requires upsizing beyond the minimum ampacity requirement. The extra cost of larger wire is minimal compared to the performance and safety benefit. Also consider future load growth — installing slightly larger wire now is much cheaper than re-wiring later.

Safety — Wire Sizing is Code-Critical

Undersized wire is a leading cause of residential electrical fires. Always follow NEC 310.16 for ampacity and NEC 210.19/215.2 for voltage drop recommendations. The NEC breaker-to-wire relationship in 240.4(D) is absolute: 14 AWG = 15A max breaker, 12 AWG = 20A, 10 AWG = 30A. Never install a larger breaker to "fix" tripping — the breaker protects the wire. If a breaker trips frequently, the circuit is overloaded and needs to be split or the wire upsized.

Frequently Asked Questions

What size wire do I need?▼

Depends on amps, distance, and voltage. Common: 15A = 14 AWG, 20A = 12 AWG, 30A = 10 AWG, 50A = 6 AWG, 100A = 1 AWG (copper). For long runs, upsize for voltage drop. Our calculator checks both ampacity and voltage drop automatically.

What is AWG?▼

American Wire Gauge — standard measuring system. Smaller number = thicker wire = more capacity. 14 AWG is thinner than 12, which is thinner than 10. Below 1 AWG: 1/0, 2/0, 3/0, 4/0. Each AWG step changes area by ~50%.

What is voltage drop and why does it matter?▼

Voltage loss over wire distance due to resistance. NEC recommends max 3% for branch circuits, 5% total. Too much drop causes dim lights, motor overheating, and equipment malfunction. Longer runs need larger wire to keep drop acceptable.

Copper or aluminum wire?▼

Copper: better conductor, smaller size, more expensive. Aluminum: cheaper, requires 1-2 sizes larger, needs anti-oxidant on connections. Aluminum is common for service entrance and large feeders (40A+). Never mix without listed connectors. Use AL-rated devices only.

What is the 80% rule?▼

Continuous loads (3+ hours) can't exceed 80% of wire/breaker rating per NEC. A 20A breaker on continuous load = 16A max. Size wire for 125% of continuous load. This prevents overheating from sustained current.

What size wire for 200 amp service?▼

Copper: 2/0 AWG. Aluminum: 4/0 AWG. Ground: 4 AWG copper. Most residential 200A services use aluminum SE cable (much cheaper). Conduit: 2" minimum for copper, 2.5" for aluminum.

How far can I run 12 gauge wire?▼

At 120V, 20A load, 3% max drop: about 50 feet one-way. At 15A: about 65 feet. At 10A: about 100 feet. Beyond these distances, upsize to 10 AWG. At 240V, distances double because the percentage drop is halved. Use our calculator for exact distances.