Conductors, ampacity and derating

Conductor sizing questions appear throughout electrical exams, on their own and as steps inside larger problems on feeders, services and motors. The basic idea is simple: a conductor must carry its load without overheating its insulation. In practice, the answer depends on the conductor material, insulation temperature rating, installation method, ambient temperature, the number of current-carrying conductors grouped together, and the temperature ratings of the terminations at each end. Exam questions test whether you apply all of these in the right order. This lesson reviews conductor types and markings, the ampacity tables, ambient temperature correction, adjustment for grouped conductors, which conductors count as current-carrying, termination limits, aluminum conductors and small-conductor protection limits.

Why ampacity is limited

Current through a conductor produces heat in proportion to the square of the current times the resistance (\(I^2R\)). The conductor's temperature rises until heat escapes to the surroundings as fast as it is produced. Insulation has a maximum continuous operating temperature, commonly 60, 75 or 90 °C, above which it ages rapidly, becomes brittle and eventually fails. Ampacity is the current a conductor can carry continuously under stated conditions without exceeding its insulation's temperature rating. Anything that makes it harder for heat to escape, such as a hot room, many conductors bundled together, thermal insulation or a buried location, reduces ampacity.

Conductor types and markings

Common building wire types in the United States include THHN and THWN-2 (thermoplastic insulation with nylon jacket, 90 °C dry and, for -2, wet), XHHW-2 (cross-linked polyethylene, 90 °C wet and dry), and USE-2 for underground service entrance. Cables include NM-B non-metallic sheathed cable (90 °C conductors, but ampacity limited to the 60 °C column), UF-B underground feeder cable, MC metal-clad cable and AC armored cable. Table 310.4(1) lists conductor types, insulation, temperature ratings and applications. The letters encode properties: T for thermoplastic, H for 75 °C heat resistance, HH for 90 °C, W for wet locations and N for a nylon jacket.

The ampacity tables

Table 310.16 gives ampacities for not more than three current-carrying conductors in a raceway, cable or earth (directly buried), based on an ambient temperature of 30 °C (86 °F), with columns for 60 °C, 75 °C and 90 °C insulation for both copper and aluminum. Table 310.17 gives ampacities for single conductors in free air. Other tables cover conductors in specific conditions, and 310.14(B) permits engineering supervision to calculate ampacity using the Neher–McGrath method in Annex B.

Ambient temperature correction

Where the ambient temperature differs from the 30 °C basis of the tables, the table ampacity is multiplied by a correction factor. Higher ambient temperatures reduce ampacity because the conductor has less temperature margin before reaching its rating. The factor depends on both the ambient temperature and the conductor's insulation rating; 90 °C conductors lose a smaller fraction of their ampacity in hot locations than 60 °C conductors.

In the NEC, correction factors for ambient temperatures other than 30 °C are given in Table 310.15(B)(1)(1) (Table 310.15(B)(1) in earlier editions). For example, at 36–40 °C (97–104 °F) the factor is 0.91 for 90 °C insulation and 0.88 for 75 °C insulation. Raceways and cables exposed to sunlight on rooftops may require an additional temperature adder where the code specifies one; check the edition you are using.

Adjustment for more than three conductors

When more than three current-carrying conductors share a raceway or cable, or are bundled together over a significant length, each conductor's heat makes it harder for its neighbors to cool, so ampacity is reduced further.

Both correction and adjustment factors, when they apply, are multiplied together with the table ampacity. They are usually applied to the ampacity in the conductor's own insulation column, for example the 90 °C column for a 90 °C conductor, and the result must then also satisfy the termination limit described below.

Which conductors count

Not every conductor in a raceway counts as current-carrying for adjustment purposes. Equipment bonding or grounding conductors never count. A neutral that carries only the unbalanced current of the circuits it serves, such as the neutral of a three-wire single-phase circuit or a four-wire three-phase wye circuit supplying linear loads, generally does not count, because the phase currents largely cancel. A neutral does count when it carries significant current that does not cancel: for example, the neutral of a two-wire circuit, the neutral of a circuit with two phases and the neutral of a three-phase wye system, and a neutral serving mainly non-linear loads such as electronic equipment and LED drivers, whose triplen harmonic currents add in the neutral. Control and signal conductors carrying negligible current may be excluded where the code allows. Counting correctly is a frequent exam trap.

Termination temperature limits

Section 110.14(C) recognizes that conductors connect to equipment terminals tested at a particular temperature. Unless the equipment is listed and marked otherwise, terminations for circuits rated 100 A or less, or for 14 through 1 AWG conductors, are limited to the 60 °C column, and terminations for circuits over 100 A or conductors larger than 1 AWG to the 75 °C column. Much modern equipment is listed for 75 °C terminations, permitting the 75 °C column. A 90 °C conductor therefore offers advantages mainly when derating is needed, because derating starts from its higher 90 °C value.

Aluminum conductors

Aluminum conductors have about 61 % of the conductivity of copper, so a larger aluminum conductor is needed for the same ampacity, typically two AWG sizes larger for building wire. They are lighter and cheaper, and widely used for feeders and services. Aluminum requires connectors and terminals rated for it (marked AL or AL/CU), proper torque, and often an antioxidant compound, because aluminum oxidizes, creeps under pressure and expands more with temperature than copper, all of which can loosen connections. Older small aluminum branch-circuit wiring has a history of overheating at terminals, and modern codes and practices reflect those lessons. Exam questions may ask for an aluminum size using the aluminum tables or columns, or test connection requirements.

Small conductors and overcurrent limits

Even where Table 310.16 allows more, 240.4(D) limits overcurrent protection for small conductors: 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper, with corresponding limits for aluminum, except where specific articles such as motor circuits (430) or air-conditioning equipment (440) permit otherwise. Exam questions often give a derated ampacity that seems to permit a larger breaker; this rule caps it.

Where a conductor's ampacity does not match a standard overcurrent device rating, the code generally permits the next higher standard rating for circuits up to a certain size, provided the conductor does not supply multi-outlet branch circuits for portable loads and other conditions are met. Questions involving this rule require careful reading of the conditions.

Parallel conductors

Large feeders are often run with two or more conductors in parallel per phase. To share current equally, parallel conductors must be the same length, material, size, insulation type and terminated in the same way, and in most codes they must be 1/0 AWG or larger. When parallel sets run in separate raceways, each raceway must contain one conductor of each phase and the neutral, so that magnetic fields cancel and the raceways do not heat by induction. The ampacity of the set is the sum of the individual ampacities, after any adjustment for the total number of current-carrying conductors in each raceway.

Working backwards from a load

Many questions give a load and ask for the minimum conductor size, which requires working backwards through the derating. Divide the required current by the correction factors to find the minimum table ampacity, then choose the smallest conductor whose table ampacity meets it, and finally check the termination column. For example, a 42 A load on conductors in a 40 °C space with six current-carrying conductors needs a table ampacity of at least 42 ÷ (0.91 × 0.80) = 57.7 A in the 90 °C column, so 8 AWG copper (55 A) is too small and 6 AWG (75 A) is needed; the 75 °C value for 6 AWG, 65 A, also exceeds 42 A, so 6 AWG copper is the answer.

Wet, damp and dry locations

Conductor insulation must suit the location as well as the temperature. Dry locations are not normally subject to moisture. Damp locations are partially protected but subject to moderate moisture, such as some basements and covered porches. Wet locations include underground installations, concrete slabs in contact with earth, and locations exposed to weather or water spray. The interior of raceways in wet locations, including underground raceways, is itself considered a wet location, so conductors installed there must be rated for wet use. Exam questions may ask whether a particular insulation type may be installed in an underground conduit or an outdoor raceway; the answer depends on its wet-location rating.

Conductor identification

Codes require identified conductors to be distinguishable. The neutral or grounded conductor is identified by white or gray insulation, or by approved marking for larger sizes, and bonding or equipment grounding conductors by green, green with yellow stripes, or bare copper. Ungrounded conductors may be any other color, and many installations follow conventional colors for phases and voltage systems. On systems with a high-leg delta, where one phase has a higher voltage to ground, that phase must be identified, usually with orange. Questions may ask which colors are permitted for each function or when re-identification with tape is allowed.

Conductor materials and sizes

Conductor sizes in North America use the American Wire Gauge for smaller conductors, in which larger numbers mean smaller conductors, and thousands of circular mils (kcmil) for conductors larger than 4/0 AWG. A useful rule of thumb is that a decrease of three gauge numbers doubles the cross-sectional area and halves the resistance. Copper-clad aluminum and compact stranded conductors are also used, each with its own ampacity and termination considerations.

Conductors in thermal insulation and buried

Cables surrounded by thermal insulation, such as cables running through insulated walls and attics, dissipate heat poorly, and codes reduce their permitted ampacity or require them to use a lower temperature column. Directly buried cables and conductors in underground ducts dissipate heat to the soil, and their ampacity depends on soil thermal resistivity, burial depth, spacing and load factor; for large installations, engineering calculations replace the simple tables.