Ampacity and Derating Explained (NEC 310.16)
A wire's rating on the table is only the starting point. Heat from the surroundings and from bundling with other conductors chips away at it — and if you skip the derating, you can overload a conductor that looks perfectly legal on paper. Here's how it really works.
Written by Steven Rotan
Licensed Journeyman Electrician — Oregon #883LR
Ampacity trips people up because the number on Table 310.16 feels like the answer — when it's really just the starting point. In the real world, heat eats into that number, and the conductor that looks fine on paper can be overloaded in a hot, crowded raceway. Here's the whole picture.
What ampacity actually is
Ampacity is the current a conductor can carry continuously without its insulation exceeding its temperature rating. It depends on the conductor size, the insulation type (60, 75, or 90 degrees C), and — critically — the conditions around it. Table 310.16 gives the base values at a 30 degree C ambient with no more than three current-carrying conductors. Change those conditions and you must adjust.
The two corrections that shrink it
1. Ambient temperature correction. The table assumes 30 degrees C. Run conductors through a 50 degree C attic and they can't shed heat as well, so their ampacity drops — you multiply by the correction factor from Table 310.15(B)(1). Hotter ambient, smaller factor.
2. Bundling / conductor-count adjustment. More than three current-carrying conductors in the same raceway or cable (or bundled over 24 inches) means they heat each other. Table 310.15(C)(1) gives the adjustment: 4 to 6 conductors = 80%, 7 to 9 = 70%, and down from there.
These stack. A hot attic and a full raceway apply both multipliers, and the result can be far below the table number. This is exactly where "legal on paper" installations get into trouble.
The termination catch (110.14(C))
Here's the detail that catches even experienced hands: even when the 90 degree C column offers a bigger number, 110.14(C) usually limits you to the 60 or 75 degree C ampacity at the terminations, because that's what the breakers and lugs are rated for. Common practice is to use the 90 degree column for the derating math but never exceed the 75 degree value at the ends. And remember 240.4(D)'s small-conductor rules still cap #14/#12/#10 regardless.
Verify, then trust
Our ampacity calculator applies the 310.16 table, the ambient correction, and the bundling adjustment together so you can see the adjusted number instantly. Use it to move fast on a job — but understand the layers underneath, because an inspector (or the journeyman exam) will expect you to explain why the number dropped, not just what a tool said.
Bottom line
Start at the table, then derate for heat — ambient and bundling, stacked — and respect the termination temperature. Do that and your conductors run cool and legal in the real conditions, not just on paper.
Frequently asked questions
What's the difference between ampacity and the breaker size?
Ampacity is how much current a conductor can safely carry continuously given its insulation and conditions. The overcurrent device (breaker/fuse) protects the conductor and generally can't exceed its adjusted ampacity (with the standard rounding rules and small-conductor limits of 240.4). You size the wire's ampacity to the load and conditions first, then protect it appropriately — not the other way around.
When do I have to apply the temperature and bundling corrections?
Ambient temperature correction applies whenever the surroundings differ from the table's 30 degrees C base. Bundling adjustment (310.15(C)(1)) applies when you have more than three current-carrying conductors in a raceway or cable, or where they're bundled longer than 24 inches. Both are multipliers you apply to the table ampacity — and they stack, so a hot attic full of bundled conductors can cut ampacity dramatically.
What's the 'termination temperature' catch?
Even if the 90 degree C column gives you a high number, 110.14(C) usually limits you to the 60 or 75 degree C column at the terminations, because that's what the breakers and lugs are rated for. So you often calculate derating in the 90 degree column (for the correction math) but can't exceed the 75 degree ampacity at the ends. It's the detail that trips people up most.
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