NEC 310.15 · Conductor derating

How to apply NEC 310.15 correction & adjustment factors

Which column to start from, what counts as a current-carrying conductor, how the two multipliers combine, and the mistakes that undersize conductors — with a free link to the official tables.

Need the actual tables? Look up Tables 310.15(B)(1)(1) and 310.15(C)(1) in the free NFPA viewer →

How to derate a conductor, step by step

Step	What you do	Where it comes from
1	Start with the conductor's Table 310.16 ampacity in its insulation-rating column (often 90 °C for THHN/THWN-2)	Table 310.16
2	Find the ambient temperature range the conductors pass through, and read the factor for the insulation rating	Temperature correction table in 310.15(B)(1)
3	Add any rooftop temperature adder before step 2 if the raceway is in sun close to the roof	310.15(B)(2)
4	Count current-carrying conductors in the raceway or cable and read the percentage	Table 310.15(C)(1)
5	Multiply: table ampacity × temperature factor × adjustment percentage	—
6	Compare with the termination-temperature ampacity (usually 75 °C) and the small-conductor OCPD limits; the lowest controls	110.14(C), 240.4(D)

Our own summary, not the code text. Look up Tables 310.15(B)(1)(1) and 310.15(C)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.

Worked example — nine 10 AWG Cu THHN/THWN-2 in one conduit at 40 °C (104 °F)

Input / step	Value	Source
Starting ampacity, 90 °C column	40 A	Table 310.16
Temperature factor, 90 °C wire at 36–40 °C	0.91	Table 310.15(B)(1)(1)
Adjustment, 7–9 current-carrying conductors	70%	Table 310.15(C)(1)
Adjusted ampacity	40 × 0.91 × 0.70 = 25.5 A	Calculation
75 °C termination limit	35 A	Table 310.16
Small-conductor OCPD limit	30 A	240.4(D)
Controlling value	25.5 A	Lowest of the three

Worked example values only. Look up the full tables in the free NFPA viewer.

What counts as a current-carrying conductor (paraphrased from 310.15(E)–(F))

Conductor	Count it?
Phase (ungrounded) conductors	Yes
Spare conductors pulled into the raceway	Yes
Neutral carrying only the unbalanced current of its own circuit	No
Neutral of a 4-wire wye circuit serving mainly nonlinear loads (LED drivers, electronics)	Yes
Neutral of a 3-wire circuit taken from two phases of a 4-wire wye	Yes
Equipment grounding and bonding conductors	No

Our own summary, not the code text. Look up Tables 310.15(B)(1)(1) and 310.15(C)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.

Common field mistakes

Mistake	What to do instead
Using the 90 °C ampacity as the final answer with 75 °C terminals	Use 90 °C only as the derating starting point; the result can't exceed the 75 °C (or 60 °C) value
Forgetting the bundling adjustment when circuits share a conduit	Count every current-carrying conductor in the raceway, including spares
Applying only one factor when both conditions exist	Hot ambient and more than three conductors: multiply both factors
Counting the EGC as current-carrying	Grounding and bonding conductors are never counted
Ignoring the small-conductor limits after derating	14–10 AWG copper still have fixed maximum OCPD sizes under 240.4(D)
Using the 30 °C-based factors with a 40 °C-based ampacity table	In 2023, match the correction table to the ampacity table's base ambient

Our own summary, not the code text. Look up Tables 310.15(B)(1)(1) and 310.15(C)(1) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the NEC edition adopted by your AHJ.

Where these tables live, by edition

Table	2017 NEC	2020 NEC	2023 NEC
Ambient temperature correction (30 °C base)	310.15(B)(2)(a)	310.15(B)(1)	310.15(B)(1)(1)
Adjustment for more than three conductors	310.15(B)(3)(a)	310.15(C)(1)	310.15(C)(1)

The 2020 NEC reorganized 310.15; the 2023 NEC split the temperature table into a 30 °C-based and a 40 °C-based version. Verify against your adopted edition.

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Derating, in plain language

Look up Tables 310.15(B)(1)(1) and 310.15(C)(1) in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the tables; it does not reproduce them.

The ampacities in NEC Table 310.16 assume a 30 °C (86 °F) ambient and no more than three current-carrying conductors. Change either condition and you derate. The temperature correction table gives a multiplier by ambient range and insulation rating — above 1.0 when it’s cooler than 30 °C, below 1.0 when hotter, and blank where the insulation can’t be used at all. The adjustment table gives a percentage that drops as more current-carrying conductors share a raceway or cable.

Worked example. Nine current-carrying 10 AWG copper THHN/THWN-2 conductors share one conduit through a 40 °C (104 °F) mechanical room. Start at the 90 °C column: 40 A. The correction factor for 90 °C wire at 36–40 °C is 0.91, and nine conductors take 70%. 40 × 0.91 × 0.70 = 25.5 A. That is below both the 75 °C termination value (35 A) and the 240.4(D) limit for 10 AWG copper (30 A), so 25.5 A controls — the most load these conductors may carry. On a multi-outlet receptacle circuit that means a 25 A breaker; to keep a full 30 A circuit you’d upsize the conductors and rerun the math.

A few habits that save rework: count conductors per raceway before picking sizes, not after; treat the 90 °C column as a starting point only; and check long runs with the voltage drop calculator. Once the wire is set, size the raceway with the conduit fill chart and pick the device with the standard breaker sizes guide.

Edition note: these tables moved in the 2020 reorganization of 310.15, and the 2023 NEC split the temperature table into a 30 °C-based and a 40 °C-based version — use the one that matches the ampacity table you started from.

FAQ

How do I apply both correction and adjustment factors?+

Multiply them together. Start with the Table 310.16 ampacity for the conductor's insulation rating, multiply by the ambient temperature correction factor, then by the adjustment percentage for the number of current-carrying conductors. Then check that the result doesn't exceed the termination-temperature ampacity or the 240.4(D) small-conductor limits. Look up the tables in the free NFPA viewer at nfpa.org/freeaccess.

Does the neutral count as a current-carrying conductor?+

Not when it carries only the unbalanced current of its own circuit. It does count on a 4-wire wye circuit serving mainly nonlinear loads, and on a 3-wire circuit taken from two phases of a wye system. Equipment grounding conductors never count.

Why start from the 90 °C column if my terminals are rated 75 °C?+

The Code lets you derate from the conductor's insulation rating. The final answer is the lower of the derated value and the termination-temperature value. On heavily derated runs, starting at 90 °C often keeps you from upsizing — in the example on this page, 10 AWG copper derates from 40 A to 25.5 A, which is below its 75 °C value of 35 A.

Is there still a rooftop temperature adder?+

Yes, but it is narrower than it used to be. Since the 2017 NEC it applies only to raceways or cables in direct sunlight very close to the roof surface, and certain conductor types are exempt. Check 310.15(B)(2) in the edition your AHJ enforces.

Do the bundling factors apply to short nipples?+

No — 310.15(C)(1) has exceptions, including one for short raceway nipples, plus special cases for certain cable installations. Read the exceptions in your adopted edition before derating a short run.

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