Homeowner's Guide
Every breaker in your panel is matched to a specific wire gauge. Get it wrong and the breaker won't trip before the wire overheats — that's how electrical fires start inside walls. This guide walks through the wire sizes for every common home circuit (15A, 20A, 30A, 40A, 50A), the difference between copper and aluminum, and how to know if the wiring behind your outlets is undersized.
These are copper conductor sizes based on the NEC 60°C column for typical residential NM-B (Romex) branch circuits. Ampacity ratings for THHN in conduit and for aluminum feeders differ — see the notes below the chart.
| Breaker | Copper Wire Gauge | Common Cable | Typical Uses |
|---|---|---|---|
| 15A | 14 AWG | 10/2 or 14/2 NM-B | General lighting circuits, most bedroom and living-room outlets |
| 20A | 12 AWG | 12/2 or 12/3 NM-B | Kitchen small-appliance circuits, bathroom outlets, garage, laundry |
| 30A | 10 AWG | 10/2 or 10/3 NM-B | Electric dryers (10/3), water heaters (10/2), window AC units |
| 40A | 8 AWG | 8/2 or 8/3 NM-B / THHN | Electric ranges up to ~10 kW, some Level 2 EV chargers (32A continuous) |
| 50A | 6 AWG | 6/3 NM-B or 6 AWG THHN in conduit | Electric ranges, hot tubs, welders, 40–48A Level 2 EV chargers, sub-panels |
| 60A | 6 AWG (copper) / 4 AWG (aluminum) | 6/3 copper or 4/3 aluminum SER | Small sub-panels, detached garage feeders |
| 100A | 3 AWG copper / 1 AWG aluminum | SER or THHN in conduit | Sub-panels, small home services, large detached shops |
| 200A | 2/0 copper / 4/0 aluminum | SER, USE, or THHN in conduit | Main residential service entrance for most modern homes |
NM-B (Romex) is limited to the 60°C column of NEC Table 310.16 regardless of the insulation rating printed on the jacket. THHN in conduit can use the 75°C column, which sometimes lets you drop one gauge — but only when every terminal in the circuit is rated 75°C.
AWG (American Wire Gauge) is a reverse scale — the smaller the number, the thicker the wire. 14 AWG is thinner than 12 AWG, which is thinner than 10 AWG, and so on. A thicker wire has less resistance, dissipates heat better, and can carry more current without its insulation breaking down.
Cable labeling like 12/2 or 10/3 tells you the gauge and how many current-carrying conductors are inside (plus a ground). "12/2 with ground" is 12 AWG cable with a hot, a neutral, and a bare copper ground — what you'll find behind most 20A kitchen outlets. "10/3 with ground" is 10 AWG with two hots, a neutral, and a ground — that's a 240V dryer circuit.
Copper is the default for everything inside your home — every branch circuit, every outlet, every switch leg. It's more conductive per square inch, doesn't oxidize the way aluminum does, and terminates cleanly under standard screw and back-stab terminals.
Aluminum shows up in two places today: service entrance conductors from the meter to the panel, and sub-panel feeders to detached buildings. It's roughly one-third the cost of copper for the same ampacity, but you have to upsize — a 100A copper feeder is 3 AWG, the aluminum equivalent is 1 AWG. Every aluminum termination needs antioxidant compound (Noalox) and a torque-rated lug tightened to spec.
Old aluminum branch wiring (small-gauge aluminum in 15A and 20A circuits, installed in the late 1960s and 1970s) is a different story. It expands and contracts more than copper, works loose under standard screw terminals, and has caused enough fires that most insurance carriers require remediation. If your outlets have "AL" or "Aluminum" stamped on the yoke, get an inspection.
A breaker is a wire protector, not an appliance protector. Its job is to trip before the wire behind it gets hot enough to damage insulation. When someone replaces a tripping 15A breaker with a 20A "to stop it from tripping," or extends a circuit with the wrong gauge, the breaker no longer protects anything — the wire will carry more current than it can safely dissipate, heat up inside the wall cavity, and eventually ignite framing or insulation.
The most common patterns we find on service calls: 14 AWG spliced onto a 20A circuit for a bathroom addition, 12 AWG extended to a 30A dryer outlet, or an oversized breaker installed after nuisance tripping. If you've ever felt a warm outlet cover, smelled hot plastic near a receptacle, or seen a breaker that's hard to reset, stop using that circuit and call an electrician.
Voltage drop is the main reason to go up a gauge. Long circuit runs — EV chargers in detached garages, well pumps at the back of a property, outbuilding sub-panels — lose voltage over distance. The NEC recommends keeping total voltage drop under 5% for a branch circuit. As a working rule, if a 20A circuit runs more than ~100 feet, we run 10 AWG instead of 12; a 50A circuit past ~75 feet gets 4 AWG instead of 6.
Continuous loads are the other case. Level 2 EV chargers pull their rated current for hours at a time, so the NEC requires the circuit be sized for 125% of the charger's continuous draw. A 40A charger needs a 50A circuit and 6 AWG wire — not the 40A/8 AWG the chart would suggest for a non-continuous load.
A 30 amp breaker requires 10 AWG copper wire. For a 240V dryer circuit you typically run 10/3 NM-B (two hots, a neutral, and a ground). For a 240V water heater with no neutral, 10/2 NM-B is standard. Never use 12 AWG on a 30 amp breaker — it will overheat before the breaker trips.
10/2 NM-B (Romex) is 10-gauge cable with one hot, one neutral, and a bare ground. It's most commonly used for 30 amp, 240V circuits that don't need a neutral at the appliance — electric water heaters, baseboard heater circuits, and some window AC units. It's rated for 30 amps at 60°C.
A 40 amp breaker needs 8 AWG copper. For a range or EV charger you'll usually pull 8/3 NM-B or 8 AWG THHN in conduit. Some older code references allowed 8 AWG on ranges up to 50 amps, but for a dedicated 40 amp circuit today, 8 AWG is the correct size.
A 50 amp circuit uses 6 AWG copper — typically 6/3 NM-B for ranges, hot tubs, and 40–48A Level 2 EV chargers. In conduit, 6 AWG THHN is standard. If you're running the circuit a very long distance, we may upsize to 4 AWG to compensate for voltage drop.
Yes, but aluminum has to be one to two gauges larger for the same amperage — a 100A copper feeder is 3 AWG, the aluminum equivalent is 1 AWG. Aluminum is common on service entrances and sub-panel feeders because it's much cheaper. It's not used for branch circuits inside the home due to expansion, oxidation, and terminations that require antioxidant paste and torque-rated lugs.
This is one of the most common causes of house fires. The breaker only trips when current exceeds its rating, so a 30 amp breaker on 12 AWG wire will let the wire carry 25+ amps continuously — well above what 12 AWG can safely dissipate. The insulation melts, the wire arcs inside the wall, and by the time smoke shows there's already fire in the framing.
Yes. Long runs cause voltage drop, and the fix is to upsize the wire. As a rule of thumb, if your circuit runs more than about 100 feet from the panel, go up one wire gauge. EV chargers, well pumps, and detached-garage sub-panels are the most common places we upsize for length.
In Franklin County and most St. Louis jurisdictions, yes — any new circuit, panel change, or service upgrade requires a permit and an inspection. We pull the permit, coordinate the inspection, and handle Ameren for anything that touches the meter or service drop.
CAS Electric provides licensed electrical safety inspections across Franklin County and the St. Louis metro. We'll check your panel, breaker sizing, and any circuits you're worried about — free written estimate on any repairs.