Using NEC Tables
Teaching module — how to read, apply, and navigate NEC tables on the exam. Filter by topic using the buttons below.
Why More Than Three Conductors Derate
When more than three current-carrying conductors share a raceway or cable (or are bundled together for more than 24 in.), they trap each other's heat, so ampacity is reduced by an adjustment factor based on how many current-carrying conductors are present.
What Table 310.16 Is For
Table 310.16 gives the allowable ampacity of insulated conductors when there are not more than three current-carrying conductors in a raceway, cable, or directly buried, at an ambient of 30 C (86 F). It is the starting point for almost every conductor-sizing problem. If your installation differs from that baseline (hotter ambient, or more than three current-carrying conductors), you begin here and then apply correction and adjustment factors.
What Box Fill Protects Against
Box fill ensures a box is large enough for everything inside it so conductors are not crowded or damaged. You add up volume allowances (in cubic inches) for conductors, devices, clamps, and grounding conductors; the total cannot exceed the box's marked volume.
Which Ampacity Table to Use
Table 310.16 is not the only ampacity table, and choosing the right one is a tested skill. Use 310.16 for not more than three current-carrying conductors in a raceway, cable, or directly buried. Use the free-air tables when conductors are in open air: 310.17 (single-insulated in free air), 310.18 and 310.19 (higher-temperature single-insulated), 310.20 (conductors on a messenger), and 310.21 (bare or covered conductors in free air). Free-air ratings are higher because heat escapes more easily.
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