Choosing the right wire is one of the most important decisions in any electrical project. Wire that’s too small for the current can overheat, damage insulation and start a fire. This guide explains how wire sizes work in the U.S. and how to match a wire to a circuit.
Safety first: electrical work can be dangerous and often requires a permit. Use this guide to understand and plan, and hire a licensed electrician when your local rules require it or when you’re not sure.
How AWG wire sizes work
In the U.S., wire is sized by the American Wire Gauge (AWG) system. The numbers run backwards: the smaller the number, the thicker the wire. A 10 AWG wire is thicker than a 14 AWG wire and can safely carry more current. Above 1 AWG, sizes continue as 1/0, 2/0, 3/0 and 4/0 (“one aught” to “four aught”).
Match the wire to the breaker
The breaker protects the wire. The wire must be able to carry at least the breaker’s rating. For typical copper wire in homes, the usual pairings are:
| Breaker | Copper wire | Typical uses |
|---|---|---|
| 15 A | 14 AWG | Lighting, bedroom and living room outlets |
| 20 A | 12 AWG | Kitchen countertop, bathroom and laundry outlets |
| 30 A | 10 AWG | Electric dryer, many water heaters |
| 40 A | 8 AWG | Some electric ranges and cooktops |
| 50 A | 6 AWG | Ranges, EV chargers, subpanels |
These are common pairings for copper conductors under standard conditions. The appliance’s installation instructions and your local code have the final say.
Why wire type and temperature rating matter
The same gauge can carry different amounts of current depending on its insulation and how it’s installed. The National Electrical Code (NEC) lists ampacities in temperature columns. NM-B cable, the common sheathed cable often called by the brand name Romex, is generally sized using the 60°C column. Individual conductors such as THHN in conduit may use the 75°C column when every termination is rated for it.
Long runs and voltage drop
Every foot of wire adds resistance, so some voltage is lost along the way. On a short run this is negligible. On a long run, say to a detached garage or a well pump, the drop can be large enough to make motors run hot and lights dim. A common design target is to keep voltage drop to about 3% on a branch circuit. If the run is long, the fix is usually to step up one or more wire sizes.
Our free Wire Size Calculator checks both ampacity and voltage drop, and tells you when a long run needs a larger wire.
Continuous loads need extra room
Equipment that runs at full current for three hours or more, such as an EV charger or a water heater, is a continuous load. Conductors and breakers for these loads are sized at 125% of the load current. For example, a charger drawing 32 amps continuously needs a circuit rated for at least 40 amps.
Copper vs. aluminum
Most branch circuits in homes use copper. Aluminum is sometimes used for large feeders and service entrances, but it needs a larger size for the same current and connectors rated for aluminum. Never mix the two without the right hardware.
Common mistakes to avoid
- Putting a bigger breaker on an existing wire to stop it tripping. The breaker is there to protect the wire.
- Ignoring voltage drop on long runs to outbuildings.
- Mixing 14 AWG and 12 AWG on a 20-amp circuit.
- Skipping the permit and inspection where they’re required.
Key takeaways
- Lower AWG number = thicker wire = more current.
- Typical copper pairings: 15 A/14 AWG, 20 A/12 AWG, 30 A/10 AWG.
- Long runs and continuous loads often need a larger wire.
- When in doubt, ask a licensed electrician.
