When does the adjustment apply?

It applies whenever more than three current-carrying conductors run in the same raceway or cable, or are bundled longer than 24 inches without spacing. Three or fewer conductors get a factor of 1.0 — no adjustment.

What are the factor bands?

NEC Table 310.15(C)(1) gives 0.8 for 4–6 conductors, 0.7 for 7–9, 0.5 for 10–20, 0.45 for 21–30, 0.4 for 31–40, and 0.35 for 41 and more current-carrying conductors.

Which conductors do I count?

Count phase conductors and any neutral that carries current under normal balanced operation (for example on systems with major harmonic or single-phase loads). A balanced-system neutral that carries only unbalanced current and all equipment grounding conductors are not counted.

Do I combine this with temperature correction?

Yes when both apply. The temperature factor from Table 310.15(B)(1) and the bundling factor from Table 310.15(C)(1) are independent and multiply together to give the final corrected ampacity.

What if the derated ampacity is below my load?

Upsize the conductor to the next standard size and recheck. The tool flags this automatically when you enter a load current that exceeds the adjusted ampacity.

Bundling (Adjustment Factor) Derating Calculator (NEC 310.15(C))

Why crowding a raceway lowers each conductor’s rating

When several current-carrying conductors share a conduit, each one is heated not only by its own current but by its neighbours. The bundle traps heat, so the same individual current pushes the insulation hotter than it would in an open three-conductor run. NEC Table 310.15(C)(1) compensates by multiplying the base ampacity by an adjustment factor that shrinks as the conductor count grows. This is one of the two derating steps — temperature being the other — that turn a table ampacity into a usable one.

How it works

The adjusted ampacity is:

adjusted = base_ampacity × adjustment_factor

The factor is read straight from the conductor count. Four through six conductors use 0.8; seven through nine use 0.7; ten through twenty drop to 0.5; and it keeps falling to 0.35 above forty conductors. The tool maps your count to the right band, applies the factor, and — if you give it a load current — compares the result to the load so you immediately see whether the conductor is adequate or needs upsizing.

Example and notes

Eight current-carrying THWN conductors in one conduit, each on a circuit with a 30 A base ampacity, fall in the 7–9 band with a factor of 0.7. The adjusted ampacity is 21 A, so a 24 A load would require stepping up to the next size. Two reminders: only count current-carrying conductors — a balanced neutral and the equipment grounds are excluded — and combine this factor with the temperature-correction factor when the run is also in a hot location. The two factors multiply, and ignoring either one is a common reason conductors run hotter than the code intends.