Bundled conductor derating is not a technicality for commercial installations. When several current-carrying conductors are packed together, each one is heated by its neighbours and none can shed heat to open air. The ampacity table assumes a conductor in free air or in a defined arrangement; bundle the cables and that assumption no longer holds.
Ampacity is a thermal limit, not a property of copper
It is tempting to think of a conductor size as having a fixed current rating, the way a bolt has a fixed strength. It does not. The limit is the temperature its insulation can tolerate, and the current that produces that temperature depends entirely on how quickly heat escapes. A single cable stapled to a joist in a ventilated basement loses heat easily. The same cable in the middle of a bundle of nine, buried under blown-in attic insulation, loses heat very slowly. Same copper, same insulation, dramatically different safe current. Derating factors are how the code translates that physics into numbers you can apply.
Only the current-carrying conductors count
This is the part people get wrong in both directions. The adjustment applies to conductors that actually carry current in normal operation. An equipment grounding conductor does not normally carry current and is not counted. A neutral that carries only the unbalanced current of a properly arranged multiwire circuit is treated differently from one that carries the full load. Conversely, people often forget that a shared neutral on a circuit feeding significant nonlinear load can carry substantial current and may need to be counted. Get the count right before applying any factor, because starting from the wrong number of conductors makes the whole calculation meaningless.
Where bundling happens in a house
The classic location is the stack of cables leaving a panel. Every circuit exits through the same general area, often strapped together neatly up a wall or through a common set of bored holes, and that grouping can run for a considerable distance before the cables fan out. Another is a remodel where a run of new circuits is fished through a single joist bay. A third is cable passing through thermal insulation, where the problem is not adjacent conductors but the absence of any cooling path at all. Wiring rules treat cable installed in contact with thermal insulation as a distinct condition with its own limitations, so read the requirement for your specific wiring method.
How to design around it rather than calculate through it
The honest answer for most home projects is to avoid the condition instead of computing your way out of it. Spread cables out where they leave the panel rather than strapping them into a single bundle for the whole run. Use separate bored holes rather than forcing five cables through one. Keep grouped runs short, since the adjustment rules recognise that a short bundled section behaves differently from a long one, subject to the specific length limits in the code. Where a bundle is unavoidable, upsizing conductors is straightforward and inexpensive at the time of installation and impossible afterwards.
Derating and ambient corrections stack
A point that trips up people who have learned one rule in isolation: the bundling adjustment and the ambient temperature correction both apply, and they apply together. A bundle of cables in a hot attic gets both. So does a group of conductors in a conduit run along a sun-exposed exterior wall, where the raceway itself can add a further temperature adjustment. The starting ampacity also depends on which temperature column of the table you may use, and terminations at the breaker and the device frequently limit that. Doing one correction and ignoring the others produces a number that looks rigorous and is not.
What to check before you act
- Count only the conductors that carry current in normal operation.
- Check whether the bundled section exceeds the length threshold in your code edition.
- Apply ambient temperature correction as well as the bundling adjustment.
- Separate cables at the panel instead of strapping them into one long bundle.
Common questions
Does derating apply to cables that just touch each other?
The adjustment applies to conductors bundled, stacked or otherwise installed without maintaining spacing over a defined length. Incidental crossing is not the same as a bundled run.
Do I derate cables lying loose on top of attic insulation?
Cable resting on insulation and cable buried in it are treated differently, and the ambient temperature in an attic is a separate issue. Check both conditions for your wiring method.
The practical takeaway
Ampacity describes a conductor in a stated condition, and bundling changes the condition. Where several circuits leave a panel or share a joist bay, plan the routing so cables are not grouped for long distances, and apply both the bundling and ambient corrections when they are.
References and further reading
- NFPA: the NFPA 70 standard development page covering the National Electrical Code
- OSHA: conductor protection and wiring design requirements
- CDC NIOSH: overview of electrical hazards and how overheating conductors contribute
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
