The first time an apprentice opens to Table 310.16, the obvious question is also the right one: there are three ampacity numbers for every wire size — which one do I use? A 10 AWG copper conductor is listed at 30, 35, and 40 amps depending on the column. Pick the wrong column and you either undersize a conductor or talk yourself into a circuit that will not pass inspection. So before anything else, this table is worth understanding column by column, because once the columns make sense, most wire-sizing questions answer themselves.

What ampacity even is

Ampacity is the maximum current a conductor can carry continuously without its temperature climbing past what its insulation can survive. It is, at heart, a heat limit. Current through resistance makes heat; insulation has a temperature it can take before it degrades; ampacity is the current where those two meet. That is why everything about the table is organized around temperature.

Table 310.16 specifically gives ampacities for insulated conductors rated up to 2000 volts, with not more than three current-carrying conductors in a raceway, cable, or buried in earth, at an ambient of 30°C (86°F). Those conditions matter — change them and you have to correct the numbers — but for the everyday "what wire for this load" question, this is the table.

The three columns are three insulation ratings

The three numbers for each size are not "good, better, best." They are three different insulation temperature ratings: 60°C, 75°C, and 90°C. Each column tells you how much current that conductor can carry if its insulation is rated for that temperature.

  • The 60°C column is for conductors with 60-degree insulation — older or basic types.
  • The 75°C column is the workhorse. Most modern building wire — THWN-2, XHHW, the common stuff — carries a 75-degree (or higher) rating, and this column is where a great deal of real-world sizing lands.
  • The 90°C column holds the highest numbers, for conductors with 90-degree insulation. THHN and THWN-2, for instance, are rated 90°C dry.

Here is the catch that trips up every beginner: having 90-degree wire does not mean you get to use the 90-degree column for sizing the load. The 90°C column is mostly a starting point for derating math, not the number you size your breaker and conductor to. The column you are actually allowed to use is decided somewhere else entirely — at the ends of the wire.

The rule that decides the column: 110.14(C)

Conductors do not exist in isolation; they land on terminals — breaker lugs, the screws on a receptacle, the terminals inside equipment. Those terminations have their own temperature ratings, and a connection is only as good as its weakest end. Section 110.14(C) sets the principle: the ampacity of a circuit is governed by the lowest temperature rating of any connected termination, conductor, or device. You are limited by the weakest link in the chain.

In practice that resolves into a rule beginners can lean on:

  • For circuits rated 100 amps or less, or using conductors 14 AWG through 1 AWG, you generally size from the 60°C column — unless the equipment and terminals are listed and marked for 75°C, in which case you may use the 75°C column.
  • For circuits above 100 amps, or larger conductors, the 75°C column is the typical governing column.

So a 90°C conductor on a breaker whose terminals are rated 75°C is sized using the 75°C column — the wire's higher rating does not buy you more amps when the lug cannot take the heat. The 90-degree rating still earns its keep: it gives you headroom for derating, which is the other half of the table's job.

Where the 90°C column actually pays off

When you have to reduce a conductor's ampacity — because the ambient is hotter than 30°C, or because more than three current-carrying conductors share a raceway — you apply correction and adjustment factors. And you are permitted to start that derating from the 90°C column value if your conductor is 90-degree rated, even though you must still finish at or below the termination-limited ampacity. Starting from the bigger number means you can lose some to derating and still land at a workable ampacity. That is the whole reason 90-degree wire is worth having: not bigger circuits, but more room to absorb heat penalties before the final number drops below what the load needs.

A worked example

Suppose you are feeding a 40-amp continuous load on a 75°C-rated breaker with THWN-2 copper (a 90°C conductor), three current-carrying conductors, normal 30°C ambient.

First, the termination governs the column: the breaker is 75°C-rated, the circuit is under 100 amps, so you size from the 75°C column. Second, a continuous load brings in another rule — conductors and overcurrent devices are generally sized at 125 percent of a continuous load, so 40 amps becomes 50 amps of required capacity. You then read down the 75°C copper column for a conductor at or above 50 amps. The 90-degree rating of the wire did not change the column you read; it would only have mattered if you had derating to do. This is the shape of nearly every sizing problem: pick the column from the terminations, adjust the load for continuity and derating, then read the table.

The honest limits of a primer

This is the logic of 310.16 and 110.14(C) in plain language, but conductor sizing has more moving parts than one article holds — small-conductor overcurrent limits, the specific correction and adjustment factors, motor and special-equipment rules, and the exact ampacity values that you must read from the current table itself. Treat this as the mental model, not the final authority. Always size from the edition of the code your jurisdiction has adopted and have a borderline circuit confirmed by a licensed electrician; ampacity is a fire-safety calculation, and the table is the law, not the summary.

Carrying the table in your pocket

The genuine difficulty with 310.16 is not the concept — it is holding the column logic, the continuous-load multiplier, and the derating factors in your head at the same time, in a panel room, without the codebook open. That is exactly what Voltly's ampacity tool is for: it pulls the 310.16 values, applies temperature correction and conductor-fill adjustment, and shows you the governing ampacity with the table cited, so the three-column question answers itself. It runs fully offline, and the NEC reference — with each answer traceable to its article — lives right beside the calculator. If you are still learning to read the table and want a tool that shows its work, take a look at Voltly.