Derating is the arithmetic that puts it back. There are two corrections, they are independent, and they multiply. Neither is difficult; what causes trouble is forgetting that they apply at all, or applying them to the wrong starting number. This guide covers both, in order, and then the ceilings that no amount of derating can lift.
The conditions the table was written for
The published ampacities on this site are stated at a 30°C ambient - 86°F - with no more than three current-carrying conductors in a raceway or cable. That is the basis. It describes a temperate indoor space with a lightly loaded pipe, and it is a perfectly reasonable default. It is also not the attic, not the rooftop, not the panel feed shared with four other circuits, and not the equipment room that sits at 40°C all summer.
Ampacity is fundamentally a thermal budget. The conductor generates heat proportional to the square of the current, and the table figure is the current at which that heat, added to the surrounding heat, leaves the insulation at its rated temperature and no hotter. Raise the ambient and less of the budget is left for the conductor's own heat. Add more loaded conductors to the same pipe and each of them has less room to shed what it makes.
That is why both corrections are multiplicative reductions of the table figure rather than adjustments to the load. Nothing about the load changed. What changed is how much current the conductor can carry before its insulation runs out of margin.
Correction one: ambient temperature
The ambient correction factor is a published table indexed by the actual ambient temperature and by the temperature column you are working in. Below 30°C the factors are greater than one - a cold space genuinely lets a conductor carry more. Above 30°C they fall away, and they fall away fastest in the 60°C column, because that insulation had the least headroom to begin with.
The columns diverge sharply at the top end. Past a certain ambient the 60°C column simply stops: there is no factor, because that insulation cannot be used at that temperature at all. The 75°C column runs out later, and the 90°C column later still. A blank in the correction table is not a rounding artefact, it is a prohibition.
Two practical notes. The ambient that matters is the one the conductor actually experiences along its whole length, which for a run passing through a hot space is that space, not the room the panel is in. And the site's tables are in °C; the calculators accept °F and convert, but the published factors are indexed on the Celsius bands.
Correction two: more than three current-carrying conductors
The second factor depends only on how many current-carrying conductors share the raceway or cable. Three or fewer needs no adjustment. From four upward the factor steps down in bands, and it steps down hard: a moderately full pipe costs a fifth of the ampacity, and a genuinely crowded one costs half.
The word doing the work is current-carrying. This is a count of conductors carrying load current, not a count of wires in the pipe. An equipment grounding conductor is not current-carrying under normal conditions and is not counted. Whether a neutral counts depends on what the circuit is doing with it. This site does not model that determination, and it is not a judgement to make from a table - the code text sets it out, and getting it wrong changes the factor.
The adjustment is also the reason conduit fill and ampacity are related questions without being the same question. Fill is a geometry rule about how much conductor cross-section a raceway may contain. The conductor-count adjustment is a thermal rule about how many of those conductors are loaded. A raceway can pass the fill calculation comfortably and still carry a heavy derating penalty.
Applying both, in the right order
Start from the published ampacity in a column your conductor's insulation supports. Multiply by the ambient correction factor. Multiply by the conductor-count adjustment factor. The product is the conductor's ampacity under your conditions, and it is that figure - not the table figure - that has to cover the load.
The starting column is where the 90°C column earns its keep. A conductor with 90°C insulation may begin the calculation from the 90°C value even though it can never be selected there, and doing so leaves a materially better result than starting from the 75°C value. The ampacity lookup on this site shows every step, so you can see the two paths diverge.
The example below is 6 AWG copper in a 40°C space with six current-carrying conductors. Starting hot gives 54.6 A; starting at the termination column gives 45.8 A. Both are well under the 65 A that a 75°C termination would permit, so in this case the derating is what governs and the termination ceiling never comes into play.
NEC 2023Every figure in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
| Starting column | Base | Ambient factor | Count factor | Result |
|---|---|---|---|---|
| 90°C | 75 A | 0.91 | 0.80 | 54.6 A |
| 75°C | 65 A | 0.88 | 0.80 | 45.8 A |
The ceilings derating cannot lift
Two limits sit above the arithmetic and are unaffected by it. The first is the termination rating: whatever the derated figure comes to, the conductor may not be loaded past what the equipment at each end is listed to accept. Derating can only reduce; it never licenses the 90°C figure as a final answer.
The second applies to the smallest conductors, which carry an overcurrent-protection ceiling of their own regardless of what any column says. Twelve AWG copper with six current-carrying conductors derates from 30 A in the 90°C column to 24 A - and the small-conductor ceiling of 20 A still binds, so 20 A is the usable figure. Starting from the hot column gained nothing there, because a different limit was already lower.
The general rule is that several limits apply simultaneously and the lowest one governs. Derating produces one of them. It does not replace the others, and a calculation that reports only the derated number has answered half the question.
- Ambient correction and conductor-count adjustment multiply - apply both, not the worse of the two.
- A blank in the ambient table means that insulation is unusable at that temperature, not that no correction applies.
- Count current-carrying conductors, which is not the same as counting wires in the raceway.
- The derated figure is still capped by the termination rating and, for small conductors, by their own overcurrent ceiling.
Frequently asked questions
Do I apply the ambient factor or the conductor-count factor, or both?
Both, multiplied together, whenever both conditions depart from the table basis of 30°C ambient and no more than three current-carrying conductors. They correct for different physical effects, so applying only the harsher of the two understates how much ampacity the installation actually costs.
Does the equipment grounding conductor count toward the conductor count?
No. The adjustment counts current-carrying conductors, and an equipment grounding conductor does not carry load current under normal conditions. Whether a neutral counts depends on the circuit, which is a determination the code text sets out and this site does not model.
Can I start derating from the 90°C column?
Yes, if the conductor's insulation is genuinely rated 90°C. That is the column's main legitimate use. The result of the calculation is still capped by the termination temperature rating of the equipment, which for ordinary equipment is 60°C or 75°C.
What happens if the ambient table has no factor for my temperature?
It means that insulation rating cannot be used at that ambient. The 60°C column runs out first, the 75°C column later, and the 90°C column later still. A missing factor is a prohibition rather than a gap, and the calculators return no result rather than guessing.
Is conduit fill the same thing as derating?
No. Fill is a geometric limit on how much conductor cross-section a raceway may contain; the conductor-count adjustment is a thermal correction based on how many of those conductors carry current. A raceway can pass the fill check and still carry a heavy ampacity penalty.