Conductor ampacity and wire sizing for DIY electrical work

Conductor ampacity is the maximum continuous current, in amperes, that an electrical conductor can carry under specified installation conditions without exceeding the temperature rating of its insulation or the ratings of its terminations. In North American practice, conductor ampacity for building wiring is usually determined from Table 310.16 of NFPA 70, the National Electrical Code (NEC). The table is widely reproduced in trade publications and online references, including a wire ampacity chart that presents the 2023 edition values for copper and aluminum conductors.

This article explains how to read the table, why it has three temperature columns, and how the surrounding code rules modify the tabulated values. It is written for do-it-yourself installers, appropriate-technology practitioners, and students who need to understand the reasoning before they size a circuit. It is not a substitute for the adopted electrical code in a particular jurisdiction or for the judgment of a licensed electrician.

A common misunderstanding is that each wire size has one ampacity. In fact, a conductor has a range of ampacities that depend on four factors:

• the temperature rating of the conductor insulation;
• the temperature rating of the terminals and overcurrent devices it connects to;
• the ambient temperature of the installation;
• the number of current-carrying conductors bundled together.

Table 310.16 addresses the first two factors by providing separate columns for 60 °C, 75 °C, and 90 °C. The remaining factors are handled by correction and adjustment tables in Article 310. A conductor that is adequate in one installation can be inadequate in another even though the wire size and material are identical.

Table 310.16 lists allowable ampacities for insulated conductors rated up to 2,000 volts, installed in raceways, cables, or directly buried in earth.^[1] The values assume an ambient temperature of 30 °C (86 °F) and not more than three current-carrying conductors in the raceway or cable. Equipment grounding conductors are not counted as current-carrying conductors for this purpose.

The following extract shows the common sizes used in residential and light commercial work. The dash indicates that a size is not listed for that material in the table.

The full table continues through 2,000 kcmil. Flexible cords and fixture wires are covered separately by Article 400 and Article 402. The copper wire ampacity by AWG pages present the same data one size at a time, which is useful when comparing the 60 °C, 75 °C, and 90 °C columns for a single conductor.

The temperature rating printed on a conductor describes the maximum temperature its insulation can withstand continuously. Common insulation types include TW (60 °C), THW and THWN (75 °C), and THHN, THWN-2, and XHHW-2 (90 °C). A higher-rated insulation can carry more current through the same copper because it tolerates more heat.

The conductor is only one part of the circuit. The terminals, circuit breakers, receptacles, and equipment it connects to also have temperature ratings. NEC 110.14(C) requires the ampacity to be selected so that it does not exceed the lowest temperature rating of any connected termination, conductor, or device.

For circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, the default assumption is 60 °C terminations unless the equipment is listed and marked for a higher temperature. For circuits over 100 amperes, or marked for conductors larger than 1 AWG, the default is 75 °C terminations. Modern residential breakers are often marked 75 °C, which permits the 75 °C column when the conductor is also rated at least 75 °C. The 90 °C column is generally used as the starting point for derating calculations, not as the final ampacity, unless the terminations are rated for 90 °C.

One important cable rule illustrates the principle. Type NM-B cable, commonly called Romex, has conductors with 90 °C insulation, but NEC 334.80 limits its ampacity to the 60 °C column. A 6/3 NM-B cable therefore provides 55 amperes, not 75, even though the individual conductors are marked with a 90 °C rating. Installers who look only at the insulation print will overestimate the cable.

The table gives an ampacity for continuous operation, but the load itself may be classified as continuous or noncontinuous. A continuous load is one that operates for three hours or more. NEC 210.19(A)(1) requires branch-circuit conductors to have an ampacity of at least 125 percent of the continuous load plus 100 percent of the noncontinuous load. NEC 210.20(A) applies the same 125 percent factor to the overcurrent device.

The factor is applied to the load, not to the conductor's ampacity. A 30-ampere continuous load requires a conductor ampacity of at least 37.5 amperes. An 8 AWG copper conductor at 75 °C provides 50 amperes and satisfies the requirement; a 10 AWG conductor at 75 °C provides 35 amperes and does not. The overcurrent device must be at least 37.5 amperes, and the next standard rating is 40 amperes.

Storage-type water heaters, electric vehicle chargers, and many commercial lighting loads are treated as continuous. A common error is to add up nameplate currents and compare the total directly with a breaker rating, which omits the 125 percent factor and can undersize the conductor.

Two multipliers modify the table ampacity after the temperature column has been selected.

Ambient temperature correction. Table 310.16 assumes 30 °C ambient. Where the ambient is higher, the allowable ampacity is reduced according to Table 310.15(B)(1). For example, a 75 °C conductor in a 40 °C ambient is corrected by 0.88; in a 50 °C ambient, by 0.75. A 90 °C conductor in the same ambients is corrected by 0.91 and 0.82, respectively. The higher-temperature insulation retains more of its ampacity in hot locations, which is why 90 °C conductors are common in attics and boiler rooms.

Conductor count adjustment. Table 310.15(C)(1) reduces the ampacity when more than three current-carrying conductors share a raceway or cable. The factors are 80 percent for four to six conductors, 70 percent for seven to nine, 50 percent for ten to twenty, and progressively lower for larger groups. Equipment grounding conductors are excluded from the count, as are neutral conductors that carry only the unbalanced current of a three-wire or four-wire circuit under the conditions described in NEC 310.15(E).

The two multipliers are applied multiplicatively to the table ampacity, and the result is then compared with the termination limit. For example, a 6 AWG copper conductor with 90 °C insulation, installed with four current-carrying conductors in a 45 °C ambient and terminated on 75 °C lugs, starts at 75 amperes in the 90 °C column. Applying 0.87 for ambient and 0.80 for conductor count gives 52.2 amperes. The 75 °C termination limit is 65 amperes, which is higher, so the final ampacity is 52.2 amperes. A 50-ampere overcurrent device is appropriate.

Ampacity and overcurrent protection are related but distinct. The overcurrent device protects the conductor from overload and short-circuit current, and its rating is generally not permitted to exceed the conductor's ampacity. NEC 240.4(B) permits the next higher standard rating in limited cases where the conductor ampacity falls between standard device ratings, the circuit is not a multioutlet receptacle circuit supplying portable loads, and the device rating does not exceed 800 amperes.

NEC 240.4(D) imposes additional limits on small conductors. The overcurrent device is capped at 15 amperes for 14 AWG copper, 20 amperes for 12 AWG copper, and 30 amperes for 10 AWG copper, with corresponding limits for aluminum. These caps apply even though the 75 °C column lists higher values. A 12 AWG copper conductor at 75 °C is listed at 25 amperes, but it may not be protected by a 25-ampere breaker in a typical branch circuit.

The small-conductor rule is a cap on the overcurrent device, not a guarantee that the conductor is adequately sized. If derating reduces a 12 AWG conductor's ampacity below 20 amperes, the 20-ampere breaker no longer protects it, and the installer must reduce the breaker rating or increase the conductor size.

Aluminum conductors have lower ampacity than copper conductors of the same size, as the table shows. At 75 °C, 2/0 copper is rated 175 amperes and 2/0 aluminum is rated 135 amperes. Aluminum is nevertheless widely used for service entrance conductors and large feeders because it costs less per ampere and is lighter.

Aluminum requires terminations listed for the material. The conductor should be landed on lugs marked AL or AL/CU, torqued to the manufacturer's specification, and treated with an antioxidant where the manufacturer requires it. Modern AA-8000 series alloys perform well in these applications. Small aluminum branch-circuit conductors installed before the 1970s are a separate concern and should be evaluated by a qualified electrician before being disturbed.

NEC 310.12 provides a residential service allowance that is often confused with the table values. For a single-phase dwelling service, the service conductors may have an ampacity of at least 83 percent of the service rating.^[2] This is why a 200-ampere residential service can use 4/0 aluminum (180 amperes at 75 °C) or 2/0 copper (175 amperes), even though neither reaches 200 amperes in Table 310.16. The allowance is specific to dwelling service and feeder conductors and does not apply to a 200-ampere feeder to a detached shop, although the next-size-up rule in 240.4(B) may still permit a 200-ampere device where the calculated load does not exceed the conductor ampacity.

Ampacity is a thermal limit. Voltage drop is a performance limit. A conductor can be thermally adequate and still deliver unacceptable voltage to a load at the end of a long run. The NEC's informational notes suggest limiting voltage drop to 3 percent on a branch circuit and 5 percent for the feeder and branch circuit combined, although these figures are not mandatory in most editions and local amendments may differ.

For single-phase circuits, voltage drop can be estimated as:

VD = (2 × K × I × L) / CM

where K is approximately 12.9 for copper and 21.2 for aluminum, I is the current in amperes, L is the one-way length in feet, and CM is the conductor area in circular mils. A 50-ampere load on 6 AWG copper (26,240 circular mils) at 150 feet gives a drop of about 7.4 volts, or 3.1 percent at 240 volts. Moving to 4 AWG reduces the drop to about 4.6 volts, or 1.9 percent.

Example 1: 30-ampere continuous heater. A 7,200-watt, 240-volt heater draws 30 amperes and is a continuous load. The required conductor ampacity is 37.5 amperes. An 8 AWG copper conductor at 75 °C provides 50 amperes, so 8 AWG copper is selected. The overcurrent device is sized at 125 percent of 30 amperes, or 37.5 amperes, and the next standard rating is 40 amperes. The equipment grounding conductor is 10 AWG copper per Table 250.122.

Example 2: 48-ampere electric vehicle charger. A charger with a 48-ampere maximum output requires a circuit rated at 125 percent of that value, or 60 amperes, under Article 625. A 6 AWG copper conductor at 75 °C provides 65 amperes and is protected by a 60-ampere breaker. At 120 feet, the voltage drop is about 5.7 volts at 240 volts, or 2.4 percent, which is acceptable for a branch circuit.

Example 3: 100-ampere subpanel feeder. A 100-ampere feeder to a detached garage can use 2 AWG copper at 75 °C (115 amperes) or 1/0 aluminum at 75 °C (120 amperes). The equipment grounding conductor is 8 AWG copper or 6 AWG aluminum per Table 250.122. If the run is long enough that voltage drop exceeds 3 percent, the conductors are increased and the equipment grounding conductor is increased proportionally under NEC 250.122(B).

• Using the 90 °C column for final ampacity because the insulation is marked 90 °C, while the terminals are only rated 75 °C.
• Using the 75 °C column for NM-B cable, which is limited to 60 °C by NEC 334.80.
• Multiplying the conductor ampacity by 1.25 instead of multiplying the continuous load by 1.25.
• Forgetting ambient correction in attics, mechanical rooms, or rooftop conduit.
• Counting equipment grounding conductors when applying the conductor-count adjustment.
• Sizing the overcurrent device from the 75 °C column for 14, 12, or 10 AWG copper without applying NEC 240.4(D).
• Ignoring voltage drop on long runs because the ampacity calculation passed.

The NEC is a model code. It becomes enforceable only when a jurisdiction adopts it, and many jurisdictions amend it. Local requirements for conduit fill, burial depth, grounding, arc-fault protection, and permits can be more restrictive than the model code. The table values in this article are from the 2023 edition; earlier editions may differ, and the adopted edition in a given location governs.

Work on electrical systems involves shock, fire, and arc-flash hazards. The information here is intended to support education and planning, not to replace a permit, an inspection, or the services of a licensed electrician. Readers should verify all values against the code edition in force and consult the authority having jurisdiction before installing or modifying a circuit.

• Wire ampacity chart based on NEC Table 310.16 — copper and aluminum values at 60 °C, 75 °C, and 90 °C, with correction and adjustment tables.
• Wire size for 50 amps — worked lookup for a common branch-circuit and feeder rating.