Wire Size Explained: AWG, Ampacity, Breaker Sizing, and Voltage Drop

Wire size is chosen by three separate checks, and a conductor that passes one can fail another. Ampacity decides whether the wire can carry the current without overheating. The breaker decides whether the circuit is protected if something goes wrong. Voltage drop decides whether the load at the far end receives enough voltage to work. This guide explains the American Wire Gauge system, how ampacity is read from the NEC tables with derating, how breaker sizes relate to wire sizes, and how the voltage-drop check changes the answer on long runs. It explains the method and cites the NEC; it does not reproduce the full ampacity tables, which depend on insulation type, temperature rating, and installation conditions and must be read from the current Code and the local rules that adopt it.

AWG: the gauge system

American Wire Gauge numbers the sizes from large to small in reverse: 0000 (4/0) is the largest common size, and the numbers rise as the wire gets smaller — 10, 12, 14, 16, 18 — down to the fine wires used in electronics. Each step of 3 gauges roughly doubles or halves the cross-sectional area, and each step of 6 roughly doubles or halves the resistance. The practical consequence is that going up two sizes — from 12 to 10 AWG — cuts resistance by about 37%, which is why one or two sizes is usually enough to fix a voltage-drop problem.

Cross-sectional area is also quoted in circular mils in the NEC tables, and in square millimetres in metric practice. The conversion is fixed: 12 AWG is about 3.31 mm², 10 AWG about 5.26 mm², 8 AWG about 8.37 mm². The area, not the gauge number, is what carries the current, and the gauge is a convenient label for a standard set of areas. Stranded and solid conductors of the same gauge have the same cross-sectional area of copper and the same ampacity; they differ in flexibility and in how they terminate, not in current-carrying capacity.

Copper and aluminium are not interchangeable at the same gauge. Aluminium has higher resistance, so an aluminium conductor must be larger — typically two gauge sizes larger — to carry the same current as copper, and it needs terminations rated for aluminium, with the preparation the manufacturer specifies. The NEC tables give separate columns for copper and aluminium, and the wrong column is a common source of undersized aluminium circuits.

Try the method on your own figures with the Wire Size Calculator.

Ampacity: Table 310.16 and derating

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. The NEC’s main table, Table 310.16, lists ampacities by size, material, and insulation temperature rating — 60°C, 75°C, and 90°C columns — for conductors in raceway, cable, or direct burial. The 90°C column looks generous, but it can rarely be used at face value: the terminations — breakers, lugs, devices — are usually rated 60°C or 75°C, and the conductor’s ampacity at the termination is limited to the lowest temperature rating in the circuit. The usable ampacity is usually the 60°C or 75°C figure, whatever the insulation allows.

Derating adjusts the table value for the actual conditions. High ambient temperature reduces ampacity, because the conductor starts hotter and has less margin before its limit. Several current-carrying conductors bundled in one raceway or cable reduce it further, because they heat each other. The NEC gives adjustment factors for both, and they multiply: a circuit in a hot attic, bundled with several others, can have an ampacity well below its table value. Continuous loads — loads running three hours or more — add a further requirement: the conductor and the overcurrent device must be sized at 125% of the continuous load, so a 16-amp continuous load needs a conductor and breaker rated for 20 amps.

Because the full tables depend on insulation type, temperature rating, installation method, and the edition of the Code in force locally, this article does not print a simplified ampacity table. A simplified table that omits derating is worse than no table: it invites a choice that looks right and overheats. Read the current NEC Table 310.16, apply the temperature and bundling adjustments, respect the termination limits, and confirm the local amendments. The calculator applies the standard table values for common residential sizes and flags the conditions that need derating, but the Code and the local authority are the final word.

Breakers: protection, not permission

The breaker protects the conductor, not the load. Its rating must not exceed the ampacity of the conductor it protects, with the standard sizes and the exceptions the Code allows for specific loads such as motors and air conditioners. The familiar pairings — 15-amp breaker on 14 AWG, 20-amp on 12 AWG, 30-amp on 10 AWG — hold for general-purpose branch circuits at the usual temperature ratings, and they are the result of the ampacity rules, not a separate rule of their own.

Breaker sizing and wire sizing are therefore decided together, from the load. Find the load current, add the 125% factor for continuous loads, choose the conductor whose derated ampacity meets it, and choose the breaker that protects that conductor. A larger breaker on a smaller wire does not give more capacity; it removes the protection. A smaller breaker on a larger wire is safe but limits the circuit to the breaker’s rating, which is sometimes the right choice for a dedicated load.

Ground-fault and arc-fault protection are separate requirements that sit alongside overcurrent protection. GFCI protection is required in wet and outdoor locations, AFCI protection in most living areas in current editions, and the requirements vary by edition and locality. They do not change the wire size, but they change the device, and a circuit planned without them is incomplete however correct its conductor.

Voltage drop: the check that changes the answer

Voltage drop is the third check, and it is the one that most often changes a wire size that ampacity alone would approve. The drop depends on current, length, and resistance, and it is calculated with the formulas explained in the voltage-drop guide: single-phase, Vd = (2 × L × I × R) ÷ 1,000, with R from NEC Chapter 9, Table 8. The NEC recommends no more than 3% drop on a branch circuit and no more than 5% combined, as informational notes for reasonable efficiency.

Worked example, repeated here because it is the decision the calculator exists for: 120 V, 150 feet one way, 20 A, 12 AWG copper at 1.98 ohms per 1,000 feet. The drop is 11.88 V, or 9.9% — far over the recommendation, even though 12 AWG is the correct size for 20 A on a short run. In 6 AWG, at 0.491 ohms per 1,000 feet, the drop is 2.95 V, or 2.5%, which is inside it. The wire that carries the current is not always the wire that delivers the voltage, and length is what separates the two cases.

The practical order is therefore: size for ampacity, check voltage drop, and upsize if the drop fails, then re-check ampacity and conduit fill for the larger size. Upsizing for drop also reduces energy wasted as heat in the cable and gives the circuit margin for future load growth, which is why electricians commonly upsize long runs by one size even when the calculation only just passes.

Choosing a size, step by step

First, find the load current, including the 125% factor for continuous loads. Second, choose the installation conditions: material, insulation type, ambient temperature, and how many current-carrying conductors share the raceway. Third, read the derated ampacity from Table 310.16 and choose the smallest size that meets the load. Fourth, calculate the voltage drop for the actual length, and upsize until it meets the 3% recommendation. Fifth, choose the breaker that protects the conductor, and check the termination temperature ratings, conduit fill, and local requirements.

Common residential sizes, as a sanity check rather than a table: 14 AWG copper for 15-amp general circuits, 12 AWG for 20-amp kitchen, bathroom, and laundry circuits, 10 AWG for 30-amp appliances such as dryers and water heaters, and 6 AWG for 50- to 60-amp feeders and ranges, subject in every case to the derating and drop checks above. If a calculation gives a size far from these norms, the inputs — usually the length or the continuous-load factor — deserve a second look before the wire is bought.

When in doubt, go one size larger. The cost difference between adjacent sizes on a residential run is small, the larger wire runs cooler, drops less voltage, and leaves room for a future load. The cost of a wire that is too small is paid in heat, in voltage, and sometimes in a re-pull. Electrical work beyond simple replacement should be done or checked by a qualified electrician, and local permits and inspections apply.

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