Cable Ampacity Derating Guide: NEC 310.15 Temperature & Bundling

Q: When does NEC 310.15(B)(1) ambient temperature derating apply?

Whenever the ambient temperature surrounding the conductor exceeds 30°C (86°F), the base used in NEC Table 310.16. This is common in mechanical rooms, rooftop conduit runs exposed to sun, boiler rooms, and outdoor installations in hot climates. The table provides correction factors that reduce the published ampacity as ambient temperature rises.

Q: How many current-carrying conductors trigger bundling derating?

NEC 310.15(C)(1) requires an adjustment factor whenever more than 3 current-carrying conductors are bundled together in a raceway or cable for more than 24 inches without maintaining spacing. Neutral conductors carrying only the unbalanced current of a 3-phase, 4-wire circuit and grounding conductors generally don't count toward this total.

Q: Do you apply temperature and bundling derating factors together?

Yes. Both correction factors apply simultaneously when conditions for each are met. Multiply the conductor's base ampacity by the ambient temperature correction factor, then multiply that result by the bundling adjustment factor. The final number must still be equal to or greater than the load's calculated current.

Why Table 310.16 Ampacity Isn't the Whole Story

NEC Table 310.16 lists conductor ampacity at a 30°C (86°F) ambient temperature, based on no more than 3 current-carrying conductors in a raceway or cable. Real installations rarely match those exact conditions. Conduit runs across a hot rooftop, conductors sharing a tray with a dozen other circuits, or a panel inside an unventilated mechanical room all change the conductor's actual operating temperature - and therefore how much current it can safely carry before its insulation overheats. NEC 310.15 provides two correction mechanisms to account for this: ambient temperature correction and conductor bundling adjustment.

Ambient Temperature Correction Factors (NEC Table 310.15(B)(1))

These factors apply when the ambient temperature around the conductor differs from the 30°C base. The table below shows common factors for 90°C-rated insulation (THHN, XHHW-2):

Ambient Temp (°C)	Ambient Temp (°F)	Correction Factor (90°C insulation)
26 - 30	78 - 86	1.00
31 - 35	87 - 95	0.96
36 - 40	96 - 104	0.91
41 - 45	105 - 113	0.87
46 - 50	114 - 122	0.82
51 - 55	123 - 131	0.76
56 - 60	132 - 140	0.71

Note that 75°C and 60°C rated insulation use different correction factor columns from the same table - always match the factor to the conductor's actual temperature rating, not the system voltage or termination rating.

Bundling Adjustment Factors (NEC 310.15(C)(1))

When more than 3 current-carrying conductors are installed together in a raceway, cable, or bundled without maintained spacing for a distance greater than 24 inches, an additional adjustment factor applies:

Number of 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

Only count current-carrying conductors. A 3-phase, 4-wire circuit's neutral generally doesn't count if it only carries unbalanced load, and equipment grounding conductors never count. Cable trays have separate, more favorable rules under NEC 392 for certain configurations - see our cable tray vs cable duct guide for tray-specific ampacity rules.

Worked Example: Combining Both Factors

A 90°C-rated 2 AWG THHN conductor has a Table 310.16 base ampacity of 130A. It's installed in a rooftop conduit at 41°C ambient, bundled with 8 other current-carrying conductors (9 total):

Ambient correction factor at 41°C (90°C insulation): 0.87
Bundling adjustment factor for 9 conductors: 0.70
Adjusted ampacity = 130A × 0.87 × 0.70 = 79.2A

That's a 39% reduction from the table value. If the connected load draws 90A, this conductor fails - despite being correctly sized by the base table. The fix is to upsize the conductor, reduce the conduit fill, relocate the run out of direct sun, or split the circuits across multiple raceways to stay under the bundling threshold.

Common Mistakes Engineers Make

Using the wrong insulation column - applying 75°C correction factors to 90°C-rated cable (or vice versa) gives the wrong adjusted ampacity
Forgetting neutral conductor rules - counting a neutral that doesn't carry continuous unbalanced current, over-derating unnecessarily
Ignoring rooftop conduit exposure - NEC 310.15(B)(3) requires adding extra temperature adders for conduit run on or above rooftops exposed to direct sunlight (up to +33°C ambient adder depending on height above roof)
Applying termination temperature rating incorrectly - per NEC 110.14(C), the final ampacity must also not exceed the lowest temperature rating of any connected termination, even if the conductor itself is 90°C rated
Skipping bundling derating in cable trays with covers - covered trays change the applicable ampacity rules versus open ladder tray

Frequently Asked Questions

When does NEC 310.15(B)(1) ambient temperature derating apply?

Whenever ambient temperature around the conductor exceeds the 30°C (86°F) base used in Table 310.16. Common triggers: mechanical rooms, rooftop conduit in direct sun, boiler rooms, and hot-climate outdoor installations.

How many current-carrying conductors trigger bundling derating?

NEC 310.15(C)(1) requires an adjustment factor once more than 3 current-carrying conductors are bundled in a raceway or cable for more than 24 inches without maintained spacing. Unbalanced neutral and grounding conductors generally don't count toward this total.

Do you apply temperature and bundling derating factors together?

Yes - multiply base ampacity by the ambient correction factor, then by the bundling adjustment factor. Both apply simultaneously whenever their respective conditions are met, and the final adjusted ampacity must still meet or exceed the calculated load current.