Wire derating is what happens between the number in the ampacity table and the number you are actually allowed to use. Bundle several current carrying conductors in one raceway and they cannot shed heat, so the allowed current drops. Put them somewhere hot and it drops again. Then the temperature rating of the terminals at each end can cap the result no matter how good the cable is. The table value is where the calculation starts, not where it ends.
The gap between the formula and the wall
Every ampacity table, and every online calculator built on one, describes a conductor in a specific set of conditions: a small number of conductors, an ambient temperature around 86 degrees Fahrenheit, and terminals that can handle the heat. Change any of those and the number changes with it.
That is not a flaw in the tables. It is how they are written. The problem is that most calculators quietly present the ideal case as the answer, and a homeowner has no way to know that three more corrections still have to be applied.
The result is a conversation electricians have constantly. The calculator said 12 AWG is fine for 20 amps. The inspector says it is not. Both are reading the same code. Only one of them applied all of it.
Correction one: How many wires can you put in one conduit?
A conductor gets rid of heat by giving it to whatever surrounds it. Put nine of them in the same conduit and they have nothing to give it to but each other. NEC 310.15(C)(1) handles this by cutting the allowable ampacity as the count goes up.
| Current carrying conductors in one raceway | Percent of table ampacity |
|---|---|
| 4 to 6 | 80 percent |
| 7 to 9 | 70 percent |
| 10 to 20 | 50 percent |
| 21 to 30 | 45 percent |
Two details decide whether this applies to you. Only current carrying conductors count, so the equipment grounding conductor is not in the tally, and a neutral that only carries unbalanced current on a normal 240 volt circuit is not either. And the correction only kicks in past three conductors, which is why a single cable in a wall never triggers it.
Where it does bite: a panel feeding several circuits through one conduit, a bundle of cables stacked tightly through a joist, or a run of homeruns leaving a subpanel together. That is exactly where a nine conductor bundle turns 12 AWG rated at 30 amps into 21 amps of usable capacity, and suddenly 20 amps is not comfortable at all.
Correction two: Why is my wire getting hot?
Ampacity tables assume roughly 86 degrees Fahrenheit around the cable. An attic in an American summer runs far hotter, and NEC 310.15(B) has a correction table for it. At 105 degrees a 90 degree rated conductor keeps about 87 percent of its ampacity. At 130 degrees it keeps about 67 percent.
NEC 310.15(B)(2) goes further for cable in sunlight on a rooftop, where the code adds a temperature adder because the air right above dark roofing is much hotter than the shade temperature.
The two corrections multiply. Nine conductors in a conduit in a hot attic is 70 percent of 87 percent, which is 61 percent of what the table promised. Nothing was installed badly. The physics simply stacked up.
Correction three: Is 12 AWG enough for 20 amps?
This is the one that catches people who already know about the first two. NEC 110.14(C) says the circuit is limited by the lowest temperature rating in it, and that includes the terminals of the breaker and the device at the far end.
Most residential equipment is rated for 60 or 75 degrees Celsius. That means a 90 degree rated conductor does not get to use its 90 degree ampacity, even when it truly is a 90 degree cable.
The useful part is the way the two get combined. You are allowed to start from the 90 degree column when applying the bundling and temperature corrections, and only then check the result against the 60 or 75 degree column. So the higher rating is not wasted. It gives you headroom for derating. It just cannot be the final answer.
NM cable, the common trap. Standard NM cable in a residential wall contains 90 degree rated conductors, but NEC 334.80 requires it to be used at the 60 degree ampacity. Which means 14 AWG is 15 amps and 12 AWG is 20 amps, full stop, and there is no spare capacity to absorb derating. Start bundling that cable, and you are already at the limit before you begin.
A worked example
A subpanel feeds four 20 amp circuits, and all four homeruns share one conduit through a hot attic. Twelve AWG copper, THHN, terminals rated 75 degrees.
Start at the 90 degree column: 12 AWG THHN is 30 amps.
Eight current carrying conductors in the raceway, so 70 percent applies: 21 amps.
The attic reaches 105 degrees, so 87 percent applies as well: 18.3 amps.
Then check against the 75 degree column, which is 25 amps for 12 AWG. The derated figure is lower, so the derated figure wins.
Final answer: 18.3 amps of usable ampacity on a 20 amp breaker. It does not pass. Split those homeruns across two conduits, or move up to 10 AWG, and it does.
Nothing in that example is unusual. It is an ordinary subpanel in an ordinary attic, and the calculator that only reads the table would have said yes.
What voltage drop calculators leave out
The same gap exists on the voltage drop side. Every voltage drop formula, including the one behind our own calculator, uses the resistance of copper at a normal operating temperature. Copper resistance rises with temperature at roughly 0.4 percent per degree Celsius, so a conductor running hot in a hot space actually drops more voltage than the calculation predicts.
The other thing a formula cannot see is the length you did not measure. A trench goes around trees and driveways, and the wire follows the trench, so a 300 foot straight line is often a 380 foot run.
Both of those cut the same way, which is why the honest approach is to leave margin. Run your numbers in the voltage drop calculator, then check the result against the length you will actually pull, not the length on the map. For the load side, the circuit load calculator shows how much of a breaker a given appliance uses before any derating is applied.
What this means for you
If you are hiring the work out, this is worth one question: ask whether the conductor size accounts for the number of conductors in the raceway and the ambient temperature where they run. An electrician who has done it will answer in one sentence. It is a fair question, not a challenge.
If you are pulling wire yourself, the practical version is short. Do not bundle more than three current carrying conductors in one raceway unless you have derated on paper. Treat attics as hot, not average. Assume 60 degree terminations on residential equipment unless you can read otherwise on the label. And when a result lands within a couple of amps of the limit, go one size up, because every one of these corrections goes the same direction.
Article references are to the National Electrical Code as commonly adopted. Percentages and correction factors are summarized here for explanation and are not a substitute for the tables themselves, which your jurisdiction may have amended. Confirm with your local authority having jurisdiction and a licensed electrician.
Who writes this
FIXMIRO is written by Rahmouni, an industrial engineer who spent fifteen years inspecting electrical installations and seven inspecting gas. What you read here is what he used to explain to homeowners while standing at their panel, written out properly.
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