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.

Worked example

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.

For a combined derating scenario — suppose the same raceway also runs in a 50°C ambient instead of the 30°C reference — the temperature correction factor from Table 310.15(B)(1) for THWN (75°C rated) at 50°C ambient is 0.75. The two factors multiply: 30 A × 0.7 (bundling) × 0.75 (temperature) = 15.75 A adjusted ampacity. A 16 A load would fail to meet the code minimum, and you would need to upsize the conductor before applying both correction factors.

The NEC 310.15(C)(1) adjustment factor table

Current-carrying conductors	Adjustment factor
4–6	0.80
7–9	0.70
10–20	0.50
21–30	0.45
31–40	0.40
41 and above	0.35

Three or fewer conductors carry a factor of 1.0 — no adjustment required.

Which conductors to count

Only current-carrying conductors are counted for the bundling adjustment. The NEC provides two important exclusions:

Equipment grounding conductors are never counted.
Grounded (neutral) conductors carrying only the unbalanced current of a balanced three-phase or single-phase system are not counted. However, a neutral conductor on a system with significant harmonic loads — such as switch-mode power supplies or variable-frequency drives — may carry substantial neutral current even when the phases are balanced, because triplen harmonics add in the neutral rather than cancelling. In that case, the neutral is counted.

A 4-wire panel feeder (three phases plus a balanced neutral) therefore has three current-carrying conductors, not four, for bundling purposes under a balanced load.

Combining bundling with temperature correction

When a raceway meets both conditions — more than three current-carrying conductors and an ambient temperature above the 30°C reference — both the bundling adjustment factor and the temperature correction factor from Table 310.15(B)(1) apply. They multiply together against the base ampacity to give the final corrected ampacity. It is a common design error to apply one and ignore the other; this tool flags when load current enters the calculation, but you must supply the temperature factor separately and multiply it yourself when both conditions apply.

When spacing eliminates the bundling requirement

NEC 310.15(C)(1) applies to conductors bundled or in the same raceway. Conductors maintained at more than one cable diameter of separation from each other for their entire length are exempt from the bundling adjustment because the spacing allows adequate heat dissipation. Cable tray installations using spacers or open trays designed to maintain separation may qualify. Consult the full NEC section for the specific spacing and installation requirements.