A conductor's ampacity, the current it can carry safely, is really a temperature rating in disguise. The copper can handle far more current than the table allows; the limit is how hot the insulation gets. A single cable in free air sheds heat easily. Bundle several current-carrying conductors together in a conduit or a tight cable tray and they heat each other, because each conductor's waste heat becomes its neighbors' ambient temperature. Derating is the formal way of accounting for that mutual heating: you reduce each conductor's allowed current based on how many conductors share the bundle. Skip the derating and the insulation cooks slowly, for years, until it fails.
Why bundles run hotter
Every conductor carrying current dissipates heat proportional to the square of the current times its resistance. In free air, convection and radiation carry that heat away and the conductor settles at a modest temperature rise. In a bundle, the inner conductors are surrounded by other heat sources instead of cool air, so their heat has nowhere to go except through their already-warm neighbors. The result is that a group of conductors each carrying a current that would be fine alone can collectively push the innermost insulation past its temperature rating. This is not a marginal effect. With enough conductors in a conduit, the allowed current per conductor can drop to roughly half of the single-conductor value. The physics does not care that each circuit was designed independently.
How the derating factors work
Electrical codes handle this with adjustment factors keyed to the number of current-carrying conductors in the bundle. A small handful of conductors gets a mild reduction, around eighty percent of the base ampacity. As the count grows, the factor steps down through seventy, sixty, and fifty percent, with further reductions for very large bundles. The factors apply to the conductor's base ampacity at its insulation temperature rating, and the final derated value must also respect the temperature limits of the terminations, which are often the real bottleneck. Count only current-carrying conductors: neutrals that carry only unbalanced current in certain configurations and dedicated grounding conductors generally do not count, but a neutral carrying harmonic currents in a nonlinear load absolutely does. Counting correctly is half the battle.
Walking through an example
Suppose you are pulling six current-carrying conductors through one conduit, each a gauge whose base ampacity at its insulation rating is thirty amps. With six conductors the adjustment factor is eighty percent, so each conductor is good for twenty-four amps in that bundle. If the breakers protecting those circuits are twenty amps, you are fine. Now suppose the design grows to nine conductors in the same conduit. The factor drops to seventy percent, giving twenty-one amps per conductor, still above the twenty-amp protection. But at ten or more conductors the factor falls to fifty percent, or fifteen amps, and suddenly those twenty-amp circuits need larger wire, fewer conductors per conduit, or an additional conduit. This is the characteristic surprise of derating: adding just a couple more circuits to an existing conduit can invalidate the whole pull.
Practical ways to reduce the count
When derating bites, the cheapest fixes reduce the number of current-carrying conductors sharing the heat. Splitting circuits across two conduits is the straightforward answer and often the cheapest once you count the cost of upsizing all the wire. Multiwire branch circuits, where one neutral serves two opposite-phase hots, can reduce conductor count in some layouts, though harmonic loads complicate the counting. Another approach is simply using larger wire: sometimes one size up restores compliance without touching the conduit layout. What you should not do is assume the load will never reach the breaker rating and skip the derating on that basis. Codes derate for the protection rating, not the expected load, because loads change over the life of a building and the next owner will not know about your assumption.
Ambient temperature stacks on top
Conduit in a hot attic or above a boiler room starts with a handicap before bundling is even considered. Electrical codes apply a separate correction factor for ambient temperature, and it multiplies with the bundling adjustment. A bundle that is merely tight at room temperature can become noncompliant in a hot environment, because both factors push the same direction. Measure or honestly estimate the worst-case ambient where the conduit runs, not the temperature on the mild day you pulled the wire. Rooftop conduit in direct sun deserves particular respect. When both factors apply, the combined reduction surprises even experienced installers, which is exactly why the calculation exists.
Safety and the long game
Derating violations rarely fail dramatically. They fail over years as insulation bakes, hardens, and cracks, eventually exposing conductors inside walls and conduits where nobody can see it. That slow failure mode is why this calculation matters more than it feels like it should during rough-in. Do the count, apply the factors honestly, and size the wire for the bundle it actually lives in. If the result forces an extra conduit or a size up, that is the code telling you something true about heat. Listen to it.
Conductors in a bundle heat each other, so each one is allowed less current than it would get alone. Count the current-carrying conductors, apply the adjustment factors, stack the ambient correction, and verify against the protection rating. It is a few minutes of arithmetic that buys decades of cool, intact insulation.
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