Quick answer: Use 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A and 6 AWG for 50 A circuits wired with NM-B cable. On long runs, go up one size or more to keep voltage drop under 3%.
Choosing a wire size comes down to two questions. First, can the wire carry the current without overheating? That is its ampacity. Second, will too much voltage be lost along the way? That is voltage drop. The right wire is the smallest one that passes both tests.
Wire Size by Amps
These are the minimum copper sizes most homeowners will see for common breakers, using the 60°C column of NEC Table 310.16 that applies to NM-B (Romex) cable:
| Breaker | Minimum copper wire | Typical use |
|---|---|---|
| 15 A | 14 AWG | Lighting, general outlets |
| 20 A | 12 AWG | Kitchen, bathroom and garage outlets |
| 30 A | 10 AWG | Electric dryer, water heater |
| 40 A | 8 AWG | Electric range, cooktop |
| 50 A | 6 AWG | Range, EV charger, subpanel |
| 60 A | 4 AWG | Subpanel |
14, 12 and 10 AWG copper are limited to 15, 20 and 30 A breakers by the NEC small-conductor rule, even though the wire itself can carry slightly more.
Full Ampacity Chart
| AWG | Copper 60°C | Copper 75°C | Aluminum 60°C | Aluminum 75°C |
|---|---|---|---|---|
| 8 | 40 | 50 | 35 | 40 |
| 6 | 55 | 65 | 40 | 50 |
| 4 | 70 | 85 | 55 | 65 |
| 3 | 85 | 100 | 65 | 75 |
| 2 | 95 | 115 | 75 | 90 |
| 1 | 110 | 130 | 85 | 100 |
| 1/0 | 125 | 150 | 100 | 120 |
| 2/0 | 145 | 175 | 115 | 135 |
| 3/0 | 165 | 200 | 130 | 155 |
| 4/0 | 195 | 230 | 150 | 180 |
The 75°C column applies to individual THHN/THWN conductors in conduit when every terminal is also rated 75°C. When in doubt, use the 60°C column.
Skip the math: enter amps, voltage and distance and it checks ampacity and voltage drop together. Open the free Wire Size Calculator →
Why Distance Matters: Voltage Drop
Voltage drop = 2 × K × Amps × One-way length (ft) ÷ Circular mils
K is 12.9 for copper and 21.2 for aluminum. The NEC recommends keeping branch circuits under 3% voltage drop. For a 20 A load at 120 V:
| One-way distance | 12 AWG drop | 10 AWG drop | 8 AWG drop |
|---|---|---|---|
| 25 ft | 1.6% | 1.0% | 0.7% |
| 50 ft | 3.3% | 2.1% | 1.3% |
| 75 ft | 4.9% | 3.1% | 2.0% |
| 100 ft | 6.6% | 4.1% | 2.6% |
| 150 ft | 9.9% | 6.2% | 3.9% |
So 12 AWG is fine for a 20 A circuit up to about 40 ft, 10 AWG works to about 70 ft and longer runs need 8 AWG. At 240 V the percentage drop is half as much, so long 240 V runs are easier.
Continuous Loads
Loads that run for three hours or more, such as EV chargers, must be sized at 125% of their current. A 40 A EV charger therefore needs wire and a breaker rated for 50 A.
Safety Comes First
- Never put a larger breaker on a smaller wire.
- Derate wire when more than three current-carrying conductors share a conduit or when it runs through hot attics.
- Most electrical work requires a permit, and many areas require a licensed electrician. Use this guide for planning only.
Wire Size for Common Household Circuits
| Circuit | Typical breaker | Typical copper wire |
|---|---|---|
| Bedroom and living room outlets, lighting | 15 A | 14 AWG |
| Kitchen countertop, bathroom, laundry outlets | 20 A | 12 AWG |
| Dishwasher, disposal, microwave | 15–20 A | 14–12 AWG |
| Electric water heater (4,500 W) | 30 A | 10 AWG |
| Electric dryer | 30 A | 10 AWG |
| Electric range | 40–50 A | 8–6 AWG |
| Level 2 EV charger (40 A) | 50 A | 6 AWG |
| Central AC or heat pump | per nameplate | per nameplate |
Appliance nameplates list the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP). For HVAC equipment always size to those values rather than to a general table.
How to Read a Wire Gauge
The American Wire Gauge (AWG) system runs backwards: the smaller the number, the thicker the wire. Sizes above 1 AWG are written 1/0, 2/0, 3/0 and 4/0 (pronounced one-aught, two-aught and so on). Each step down of three gauge numbers roughly doubles the copper area, which halves the resistance and voltage drop.
Copper vs Aluminum
Aluminum conducts about 61% as well as copper, so it must be larger for the same current. It is cheaper and lighter, which is why it is common for service entrance cables and long feeders to subpanels or detached garages. Use only terminals and breakers marked AL/CU, apply anti-oxidant compound where required, and torque connections to the manufacturer’s specification.
Planning a Subpanel or Detached Garage Feed
For a 60 A subpanel 100 ft away at 240 V, 6 AWG copper (55 A at 60°C) is too small, so 4 AWG copper or 2 AWG aluminum is typical. Check voltage drop as well: 4 AWG copper at 60 A over 100 ft drops about 3.7 V, or 1.5% at 240 V, which is well within the recommendation.
Sources
Frequently Asked Questions
What size wire for a 20 amp breaker?
12 AWG copper is the minimum. For runs longer than about 40 ft at 120 V, 10 AWG keeps voltage drop under 3%.
What size wire for 50 amps?
6 AWG copper NM-B cable, or 8 AWG copper THHN in conduit when all terminals are rated 75°C.
Can I use 14 gauge wire on a 20 amp circuit?
No. 14 AWG copper is limited to a 15 A breaker.
Is aluminum wire safe?
Modern aluminum alloy conductors are safe for feeders and service when installed with rated lugs, but they must be one or two sizes larger than copper.
What size wire for a 30 amp dryer?
10 AWG copper is the standard for a 30 A dryer circuit.
What size wire for an EV charger?
A 40 A Level 2 charger is a continuous load, so it needs a 50 A circuit, typically 6 AWG copper NM-B.
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