NEC Table 310.16 Ampacity Cheatsheet

Table 310.16 — Copper conductors

Ampacity for not more than 3 current-carrying conductors in raceway, cable, or earth. Ambient temperature 30°C (86°F).

Table 310.16 — Aluminum conductors

Same conditions as copper table above. Aluminum conductors are typically 2 sizes larger than copper for the same ampacity.

Ambient temperature correction factors

Table 310.16 is based on 30°C (86°F) ambient. In hotter environments — attics, outdoor conduit in Florida, industrial spaces near equipment — derate ampacity using these multipliers.

Conduit fill derating — more than 3 current-carrying conductors

Table 310.16 assumes 3 current-carrying conductors. Run more and you must derate — the conductors can't shed heat as easily when packed together.

Note: Equipment grounding conductors do not count toward the fill derating. Neutral conductors count only if they carry unbalanced load (which is most of the time in mixed-load circuits).

Applying both factors at once

Apply both correction factors when both conditions are present. Always use the corrected ampacity to select conductor size — not the table value.

How to use the table — the right way

Step 1: Determine the termination temperature rating

Most equipment (panelboards, breakers, switches) is listed for 75°C terminations. Use the 75°C column to select your conductor, even if the wire insulation is THHN (rated 90°C). The 90°C column is only used for derating math (see below).

Step 2: Apply derating factors using the 90°C column

When derating for ambient temperature or conductor fill, start with the 90°C column (highest rated), apply the correction factors, and then verify that the final derated ampacity doesn't exceed the 60°C or 75°C column for your termination temperature.

Step 3: Apply continuous load rule

Loads that operate continuously for 3 hours or more require the conductor to be sized at 125% of load. A 60A continuous load requires a conductor rated for at least 75A. Size the conductor first, then verify it meets the 125% overcurrent rule for the OCPD (NEC 210.19, 215.3).

Step 4: Select the OCPD

The OCPD (breaker or fuse) must not exceed the conductor ampacity — or if the conductor is derated below the standard breaker size, you use the conductor's derated ampacity to select the OCPD. Always use the lower of the conductor's allowable ampacity and the OCPD standard size that doesn't exceed it.

Common conductor sizing errors

• Using the 90°C column for conductor selection with 75°C terminations. THHN is rated 90°C, but if your panelboard is rated for 75°C terminations, you must use the 75°C column. The conductor's insulation rating doesn't override the equipment's termination rating.
• Forgetting the continuous load factor. HVAC equipment, receptacle circuits with computers, lighting — these are all continuous loads. If it runs for 3 hours, it's continuous. Size at 125%.
• Not counting neutrals. In circuits with non-linear loads (variable frequency drives, electronic ballasts), the neutral carries third-harmonic currents and must be counted as a current-carrying conductor for fill derating.
• Ignoring ambient temperature in mechanical rooms and attics. A THWN conductor in an attic in summer (140°F+) is severely derated. Run the numbers before assuming a wire that "looks big enough" is actually big enough.

The practical conductor sizing quick rule

Electricians carry these in their head for quick field estimates. They're not substitutes for the full calculation, but they'll get you in the right neighborhood fast:

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