4.1 Conductor Types, Insulation Markings & Table 310.16

What Table 310.16 Actually Says

The full title matters, because everything in the table is conditional on it:

Table 310.16 Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30 degrees C (86 degrees F)

Three assumptions are baked in:

1. Not more than three current-carrying conductors — a fourth triggers the adjustment factors of Table 310.15(C)(1).
2. 30 degrees C (86 degrees F) ambient — anything else triggers the correction factors of Table 310.15(B)(1).
3. In raceway, cable, or directly buried earth — conductors in free air use the much higher values of Table 310.17.

Note also the 2020 renumbering. What was Table 310.15(B)(16) in the 2014 and 2017 editions is Table 310.16 in the 2020 NEC. Ambient correction is Table 310.15(B)(1) in degrees C and Table 310.15(B)(2) in degrees F. Conductor-count adjustment is Table 310.15(C)(1). Older prep books using the 2017 numbering will send you to the wrong page.

Reading the Insulation Letters

Conductor type designations are built from a small alphabet. Decode them rather than memorizing the whole of Table 310.4(A):

Working through the common types:

Most building wire on the shelf is dual-marked THHN/THWN-2, which means it carries the 90 degrees C rating in both wet and dry locations. Be careful with a conductor marked THHN only: it is not permitted in a wet location, and by 310.10(C) the interior of a raceway installed underground or in a slab in direct contact with earth is a wet location.

Locations — 310.10

310.10(A) Dry Locations. Any insulation specified for the purpose in Table 310.4(A).

310.10(B) Dry and Damp Locations. Types listed in Table 310.4(A) for those locations.

310.10(C) Wet Locations. Insulated conductors in wet locations shall be (1) moisture-impervious metal-sheathed, (2) types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, XHWN, XHWN-2 or ZW, or (3) of a type listed for use in wet locations.

Article 100 defines a wet location to include installations underground or in concrete slabs in direct contact with the earth, locations subject to saturation with water, and unprotected locations exposed to weather. A damp location is protected from weather and not subject to saturation — the underside of a canopy or a cold-storage warehouse.

310.10(D) Exposure to Sunlight. Insulated conductors used where exposed to direct rays of the sun shall be listed and marked sunlight resistant, or covered with insulating material listed as sunlight resistant.

310.10(F) Corrosive Conditions. Conductors exposed to oils, greases, vapors, gases, fumes, liquids, or other substances having a deleterious effect shall be of a type approved for the application, or shall be protected by suitable sheaths.

310.10(H) Direct-Burial Conductors. Conductors used for direct burial applications shall be of a type identified for such use.

Conductor Constructions — 310.4 and 310.6

310.4(A) contains the master table of conductor applications and insulations — type letters, maximum operating temperature, application provisions, insulation, thickness, and outer covering.

310.6 Conductor Identification. Grounded conductors comply with 200.6; equipment grounding conductors comply with 250.119; ungrounded conductors comply with 310.6(C), which requires that where a premises has more than one nominal voltage system, each ungrounded conductor be identified by phase or line and system at all termination, connection, and splice points, with the method documented and readily available.

310.3(B) Conductor Minimum Size. The minimum size of conductors for 0 to 2000 volts is 14 AWG copper and 12 AWG aluminum or copper-clad aluminum, except as permitted elsewhere in the Code — fixture wires, flexible cords, Class 2 circuits, and similar applications have their own smaller minimums.

Multiple Ampacities Along One Circuit — 310.14(A)(2)

Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 10 feet or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

This is the "10-foot / 10-percent" rule. A feeder that passes through a short high-ambient zone — over a boiler, through a mechanical chase — does not have to be sized for that zone if the hot section is short enough to satisfy the exception. Note that the allowance is the lesser of the two figures, so on a 60-foot run the allowance is 6 feet, not 10.

Temperature Limitation — 310.14(A)(3)

No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.

This is the general prohibition that 110.14(C) implements at terminations, and it is also the reason ampacity correction and adjustment exist at all.

Ampacities to Have at Your Fingertips

On an open-book exam you will look these up, but recognizing them speeds sanity checks. Copper, Table 310.16:

Notice that 14, 12, and 10 AWG copper show 15, 20, and 30 A in the 60 degrees C column — the same numbers as the 240.4(D) overcurrent limits, which is a coincidence of the table, not the reason for the rule. Their 75 degrees C values are 20, 25, and 35 A, which are higher than the permitted overcurrent devices. Section 4.4 works through why.