What Size Wire Do I Need? Ampacity and Voltage Drop

"What size wire?" looks like a one-number question, and it is actually two. The first number is ampacity: how much current the conductor can carry continuously without overheating its insulation - a heat question, answered by a table. The second is voltage drop: how much voltage the run loses to its own resistance - a distance question, answered by arithmetic. A wire can pass the ampacity table with room to spare and still be wrong, because it's feeding a pump at the far end of a 120-foot run that now starts hard and runs hot.

This guide walks both numbers for the circuits homeowners actually plan: branch circuits for tools and appliances, subpanel feeders to garages and sheds, and the service-entrance class of wire that only an electrician touches but that every owner ends up buying. It sits downstream of a different question - how many amps does the whole house need? - which has its own guide in NEC Article 220 Service Load, Explained.

Ampacity: The NEC Table

Wire ampacity is not folklore - it's measured heat rise, tabulated in the NEC's Table 310.16, and organized by conductor temperature rating. The column that matters depends on the insulation: household Romex (NM-B cable) is used at the 60°C column, while THHN pulled in conduit can usually be counted at 75°C where the terminations allow it. The table below is the copper extract homeowners plan around, with the 60°C and 75°C columns side by side.

One override trumps the whole table for small wires: NEC 240.4(D), the small-conductor rule, caps 14 AWG at 15 amps, 12 AWG at 20 and 10 AWG at 30 no matter what the higher-temperature columns say. That's why the "Typical use" column above doesn't exploit 75°C ratings for branch circuits. The rule exists because a breaker protecting a small wire at a higher rating still lets the conductor run hotter than its jacket and terminations tolerate in real walls.

The 80 Percent Rule

A circuit's breaker protects the wire; the wire must be able to carry what the load draws continuously. The NEC's definition of continuous is three hours or more, and the rule is that a continuous load lands at no more than 80% of the breaker's rating: 16 amps on a 20-amp circuit, 24 on a 30, 32 on a 40, 48 on a 60. Anything that heats while it works - a water heater, an EV charger, a baseboard heater, a pool pump in July - is continuous for planning purposes. A garbage disposal or a drill press is not.

This is why an EV charger that draws 48 amps is served by a 60-amp circuit rather than a 50: 48 ÷ 0.80 = 60 exactly. It's also why a 1,500-watt space heater (12.5 amps) and a hair dryer on the same 15-amp bedroom circuit trip the breaker - not a malfunction, just two continuous-ish loads meeting a rule written for exactly this case.

Common Circuits, Already Solved

Most household planning never needs the table - it needs this list, which matches breaker, wire and the appliance that drives the sizing. The wire column assumes copper NM-B cable, the default for interior branch runs; the conduit-and-THHN alternative is usually one size smaller at the 75°C column.

Voltage Drop Over Distance

Resistance lives in every foot of wire, and the losses scale with both current and length: doubling the run doubles the drop. The engineering formula is compact - VD = 2 × K × I × L ÷ CM, where K is the resistivity constant (12.9 for copper), I is current, L is the one-way length, and CM is the circular-mil cross-section of the wire (4,110 for 14 AWG; 6,530 for 12; 10,380 for 10; 16,510 for 8). The 2 accounts for the round trip. The NEC's advisory limits are 3% drop for a branch circuit and 5% for the combined feeder-plus-branch path - 3.6 volts and 6.0 volts at 120.

You don't need the formula to plan; you need its conclusions. For 120-volt circuits at exactly 3% drop, the one-way lengths where each standard combination lands:

Read the table as a budget, not a cliff edge - a 50-foot run of 12 AWG on a 20-amp circuit is at 3.3%, which is a rounding error, not a failure. But a 100-foot run to a detached workshop at 20 amps on 12 AWG loses 7.9 volts, more than double the budget, and the symptoms arrive as motors that start slowly and lights that dim when the compressor kicks in. The fix is always the same: upsize one wire size, which buys you roughly 60% more distance each step. And on 240-volt circuits the budget doubles to 7.2 volts, so dryers and EV circuits tolerate twice the length at the same current.

Copper, Aluminum, and Where Each Belongs

Aluminum conductors carry the same amps at a bigger size - the practical shorthand is two AWG sizes up (10 AWG copper ≈ 8 AWG aluminum at 30 amps). For that weight and cost saving, aluminum is standard on service entrances and large feeders: a 100-amp residential service is typically 2 AWG aluminum, a 200-amp service 4/0 aluminum, both sized under the NEC's service conductor allowance. Those terminations are lugs rated for aluminum, torqued to spec, and coated with antioxidant paste - the details that make the material boring, which is the goal.

Branch-circuit aluminum is the opposite story. The small-gauge aluminum wiring installed in the late 1960s and early 70s expanded, oxidized and loosened at devices, and it remains the one wiring class that insurers ask about directly. If a house has it: don't panic, do get an electrician's evaluation, replace devices with CO/ALR-rated ones where they remain, and consider remediation pigtailing. New aluminum branch wiring is not a thing - the modern choice for interior runs is copper NM-B or copper THHN in conduit, and the aluminum lives on the service side where its terminations are engineered for it.

Grounding and Conduit Fill

The equipment grounding conductor - the bare or green wire - follows its own smaller table (NEC 250.122) and does not need to match the hots: 14 AWG ground for 15- and 20-amp circuits, 10 AWG up through 60 amps, 8 AWG for 100 amps. It carries fault current for the milliseconds it takes a breaker to trip, not continuous load, which is why it can be smaller.

In conduit, the planning question is fill and derating. THHN conductors are color-coded and pulled together, and once a pipe carries more than three current-carrying conductors, their heat has to share the pipe: four to six conductors knocks ampacity to 80% of the table value, seven to nine to 70%. A ground wire doesn't count, and on 240-volt circuits with no neutral (a water heater, an EV circuit), only the two hots count. The classic trap is the "just add one more circuit" pipe that ends up with six conductors - fine for the new 15-amp lighting run, quietly illegal for the 20-amp receptacle run sharing the space.

Worked Example: A Garage Subpanel

Here is the whole guide in one run: a detached garage 90 feet from the house panel, planned loads of a 32-amp EV charger on a 50-amp breaker plus 20 amps of tools and lighting, fed by a 60-amp feeder with a small subpanel at the far end.

Ampacity first: 60 amps needs 6 AWG copper THHN (65 A at 75°C) or 4 AWG aluminum. Voltage drop second: 60 amps one-way 90 feet on 6 AWG: 2 × 12.9 × 60 × 90 ÷ 26,240 = 5.3 volts. This is a 240-volt feeder, where the 5% feeder budget is 12 volts - 5.3 is comfortable, so no upsizing. Then the details: three current-carrying conductors (two hots, one neutral) plus a 10 AWG ground - four wires total, but only three count for derating, so the full table value stands. The run fits in 1-inch EMT at about half the allowed fill. And the close-out: a grounding electrode at the detached building (ground rod), a main breaker or disconnect at the subpanel, and burial depth per code if it goes underground in conduit - details your inspector will confirm before the trench is filled back in.

Swap the numbers and the method is unchanged for a shed at 40 feet on a 20-amp multiwire circuit, or a well pump at 150 feet that wants upsized wire purely for starting voltage. Ampacity from the table, 80% for anything continuous, drop over the distance, then the grounding and fill details.

When This Guide Stops Applying

Wire sizing is planning knowledge; installation is licensed work in most jurisdictions for good reason - the failure modes are fire and shock, not inconvenience. Permits and inspections are part of the process, not an admission fee, and local amendments to the NEC change real details (conduit types, burial depths, even which table applies in your jurisdiction). Use this guide to size the project, sanity-check an electrician's quote, and understand what the inspector is checking - then let the person pulling the wire pull it.

Planning guidance only, based on NEC ampacity and voltage-drop practice. Electrical work requires permits and a licensed electrician in most jurisdictions, and local code amendments govern - always verify against your adopted code edition.

Where to Read Next

• NEC Article 220 service load, explained — the upstream question: how many amps the whole house needs before any of this wire gets bought.
• The electrical service load calculator — both NEC methods in, calculated amps and a suggested service size out; run it before planning any feeder.
• Water heater sizing — the appliance whose 30-amp circuit appears in the table above, sized from the hot-water side.
• The Home Maintenance category — electrical, plumbing, insulation and the seasonal rhythm that catches both.
• All field guides — the full library across every category.

The bottom line: size the wire twice - once for the amps it carries (NEC 310.16, with the 80% rule for anything that runs for hours) and once for the distance it travels (3% drop on branches, 5% on the combined path). Upsizing one gauge is cheap; a burnt termination or a motor starved of voltage is not. And the actual pulling of that wire belongs to a licensed electrician with a permit.