Conductor derating accounts for two different installation conditions: ambient temperature and the number of current-carrying conductors sharing a raceway or cable. When both apply, multiply the base ampacity by the temperature correction factor and the conductor-count adjustment factor, then check the applicable termination temperature limit. In the hypothetical example below, nine current-carrying conductors at 40°C reduce the 75 A starting ampacity of a 6 AWG copper conductor with 90°C insulation to 47.775 A before other circuit requirements are evaluated.

Temperature and Fill Adjustment: Inputs and Example Result

Decision item	Hypothetical example	Condition or verification needed
Conductor and installation	6 AWG copper, 90°C-rated insulation, installed in a raceway	The insulation rating must apply to the actual installation conditions
Base ampacity	75 A	Table 310.16, 90°C column; reference conditions include 30°C ambient and no more than three current-carrying conductors
Ambient temperature	40°C / 104°F	Use the temperature surrounding the conductor installation, not conductor operating temperature
Temperature correction factor	0.91	The 36–40°C row and 90°C insulation column
Current-carrying conductor count	9	Count under the applicable NEC rules, not simply the number of wires present
Conductor-count adjustment factor	0.70	The 7–9 current-carrying conductor range
Combined factor	(0.91 \times 0.70 = 0.637)	Apply the factors multiplicatively
Corrected and adjusted ampacity	(75 \times 0.637 = 47.775\text{ A})	This is a calculation result, not an approved breaker rating
Assumed termination limit	65 A	Assumes all relevant terminations are identified for 75°C use; 6 AWG copper is 65 A in that column
Ampacity after the termination check	47.775 A	The lower of 47.775 A and 65 A; other applicable requirements still need evaluation

Numerical table values above are verified against HELUKABEL’s reference explicitly labeled NFPA 70: NEC–2023. The example assumes an installation to which conductor-count adjustment applies, with no applicable exception.

As of October 11, 2026, NFPA identifies the 2026 NEC as the current edition. That does not make it the enforceable edition everywhere: state and municipal adoption can differ. The tables in this article retain their verified 2023 reference basis rather than presenting older source material as a newly verified 2026 table; confirm the adopted edition and amendments with the local authority having jurisdiction, or AHJ.

Why Temperature Correction and Fill Adjustment Are Different

Ambient temperature correction

Ampacity is conditional. Table 310.16 values use a 30°C / 86°F ambient reference, and the correction table provides factors for other ambient temperatures. A hotter environment reduces the temperature margin available before the conductor reaches its insulation temperature rating, so the corresponding factor reduces the table ampacity.

Choose the correction factor using both the installation’s ambient temperature and the conductor’s applicable insulation rating. At 40°C, the factor is 0.82 for 60°C insulation, 0.88 for 75°C insulation, and 0.91 for 90°C insulation. Those columns are not interchangeable.

Current-carrying conductor adjustment

The conductor-count adjustment addresses the thermal conditions created when additional current-carrying conductors share an installation. Table 310.15(C)(1) provides progressively lower factors as the count increases beyond three. Its application can also extend to cables installed without maintaining spacing, subject to the conditions and exceptions in the adopted code.

“Fill adjustment” is useful shorthand, but the ampacity factor is based on the applicable conductor count—not a conduit’s percentage of occupied area. Physical raceway fill and current-carrying conductor adjustment require separate checks; adding space does not, by itself, change the conductor-count factor.

Temperature Correction Reference Table

The following selected values use a 30°C ambient reference. They are verified against the 2023 NEC-based manufacturer table, Table 310.15(B)(1)(1), which is used with Tables 310.16 and 310.17 as applicable. Use the correct correction table for the base ampacity table you selected.

Ambient temperature, °C	60°C insulation factor	75°C insulation factor	90°C insulation factor
26–30	1.00	1.00	1.00
31–35	0.91	0.94	0.96
36–40	0.82	0.88	0.91
41–45	0.71	0.82	0.87
46–50	0.58	0.75	0.82
51–55	0.41	0.67	0.76

All factors in this table are dimensionless multipliers, not ampere ratings.

Read the temperature range first, then move across to the appropriate insulation column. For example, 40°C belongs in the 36–40°C row; 41°C belongs in the next row and produces a different factor. Do not select the 75°C correction column merely because the equipment has 75°C terminals: qualifying 90°C insulation can provide the starting point for correction and adjustment, with the termination limitation checked separately.

Current-Carrying Conductor Adjustment Reference Table

Applicable current-carrying conductor count	Adjustment factor	Percentage of temperature-corrected table ampacity
4–6	0.80	80%
7–9	0.70	70%
10–20	0.50	50%
21–30	0.45	45%
31–40	0.40	40%
41 or more	0.35	35%

These values are verified against the 2023 NEC-based Table 310.15(C)(1). For an otherwise qualifying Table 310.16 installation with no more than three current-carrying conductors, no more-than-three conductor adjustment is required.

Count the conductors before choosing the factor

Nine physical wires do not necessarily mean nine current-carrying conductors. Grounding and bonding conductors are excluded from this adjustment count under 310.15(F), while neutral treatment depends on the circuit arrangement and load characteristics.

Examples that require attention include:

• A neutral in a two-wire line-to-neutral circuit counts as current-carrying.
• A neutral in a three-wire circuit taken from a four-wire, three-phase wye system counts.
• A neutral in a four-wire, three-phase wye circuit counts when the major portion of the load consists of nonlinear loads.
• A neutral carrying only the unbalanced current of its own circuit may qualify for exclusion under the applicable rule.

Spare conductors also require consideration under the table’s counting provisions. Do not exclude them merely because they are intended for future use, and do not assume every conductor can be excluded because loads are unlikely to operate together. Apply the actual simultaneous-energization provisions.

A common mistake is to use conduit fill percentage as the ampacity adjustment factor, or to subtract the two reductions. Temperature correction and conductor-count adjustment are separate multipliers. For this example, (0.91 \times 0.70 = 0.637); subtracting 9% and 30% from the starting ampacity does not reproduce the table-based calculation.

Calculate Derating Step by Step

1. Identify the applicable code edition and installation method.

Confirm the locally adopted NEC edition and amendments. Then select the ampacity table appropriate to the wiring method; the raceway example here uses Table 310.16, not the free-air Table 310.17.

2. Select the conductor’s starting ampacity.

Match conductor material, size, and applicable insulation temperature rating. For the hypothetical 6 AWG copper conductor rated 90°C under the installation conditions, the Table 310.16 starting value is 75 A.

3. Select the temperature correction factor.

At 40°C ambient, use the 36–40°C row. The 90°C insulation column supplies a factor of 0.91.

4. Determine the adjustment count and factor.

After applying the conductor-count rules, nine current-carrying conductors fall in the 7–9 range, giving a factor of 0.70. Verify that no installation-specific exception changes the requirement.

5. Multiply both factors by the starting ampacity.

Apply temperature correction and conductor-count adjustment to the same eligible base ampacity. The order of multiplication does not change the result.

6. Check termination limits and the remaining circuit requirements.

Compare the result with the ampacity allowed by the applicable termination temperature rating. Then evaluate the load, overcurrent protection, equipment instructions, and other applicable requirements before final selection. A derating calculation alone does not establish a compliant circuit.

Worked Example: Nine Conductors at 40°C

Inputs and assumptions

This is a hypothetical calculation, not a report of an actual installation.

• Conductor: 6 AWG copper with insulation rated 90°C for the actual conditions.
• Wiring method: raceway, using Table 310.16.
• Ambient temperature: 40°C / 104°F.
• Applicable current-carrying conductor count: nine.
• All relevant terminations: assumed identified for 75°C use.
• No applicable exception to the conductor-count adjustment.

The reference ampacities are 75 A in the 90°C column and 65 A in the 75°C column. The factors are 0.91 for ambient temperature and 0.70 for conductor count.

Formula and substitution

Let:

• (I_{\text{base}}) = base table ampacity, in amperes.
• (C_T) = temperature correction factor.
• (C_N) = conductor-count adjustment factor.

Then:

[
I_{\text{adjusted}} = I_{\text{base}} \times C_T \times C_N
]

For this example:

[
I_{\text{adjusted}} = 75\text{ A} \times 0.91 \times 0.70
]

[
I_{\text{adjusted}} = 47.775\text{ A}
]

The displayed result is approximately 47.8 A. Retain the unrounded value when comparing it against a required ampacity; rounding the display is not permission to increase the allowable current.

Termination check

For this simplified example, compare the corrected and adjusted value with the assumed 75°C termination-column limit:

[
I_{\text{after terminal check}}

\min(47.775\text{ A},65\text{ A})

47.775\text{ A}
]

The derated value controls because it is lower than the termination limit. The 90°C insulation rating permits the higher starting column for the calculation, but it does not authorize exceeding the termination limitation.

This result does not automatically authorize a 50 A breaker. Overcurrent protection and applicable load-sizing requirements must be evaluated separately. Small-conductor protection limitations are another reason that a table ampacity or calculated value cannot simply be treated as a breaker-selection chart.

You can use the ampacity calculator to cross-check the arithmetic. Compare its selected code basis, conductor material, insulation column, ambient temperature, conductor count, and termination assumptions with the hand calculation rather than treating its output as approval.

Compare Changes to Temperature and Conductor Count

Keeping the same hypothetical 6 AWG copper conductor, 90°C insulation, and assumed 75°C terminations makes the effect of each input easier to see.

Hypothetical condition	Temperature factor	Count factor	Corrected and adjusted ampacity	After the 65 A termination check
Three current-carrying conductors at 30°C	1.00	1.00	75 A	65 A
Three current-carrying conductors at 40°C	0.91	1.00	68.25 A	65 A
Six current-carrying conductors at 40°C	0.91	0.80	54.6 A	54.6 A
Nine current-carrying conductors at 40°C	0.91	0.70	47.775 A	47.775 A

These are calculated scenarios using the cited 2023 NEC-based reference values, not guaranteed installation outcomes.

The comparison shows why the controlling limit can change. With three current-carrying conductors, the assumed termination limit controls in both temperature scenarios. With six or nine at 40°C, the corrected and adjusted ampacity falls below that limit.

During design, reducing the number of current-carrying conductors sharing a raceway can change the adjustment factor. Merely increasing the raceway diameter while retaining the same count does not change the table’s count range. Any revised arrangement still needs its own raceway-fill, routing, equipment, and code checks. up

Verify the Calculation Against Authoritative Sources

Use NFPA’s NFPA 70 page to access the relevant NEC edition. NFPA provides view-only free access through its website; choose the edition actually adopted for the installation, not automatically the newest edition.

The directly relevant references are:

Reference	What to verify
Table 310.16	Base ampacity, conductor material, temperature column, and installation conditions
Ambient temperature correction provisions in 310.15(B)	Correct reference ambient and correction factor
Table 310.15(C)(1) and associated provisions	Adjustment factor, installation applicability, and exceptions
310.15(E) and (F)	Neutral, grounding, and bonding conductor treatment
110.14(C)	Applicable termination temperature limitation
Applicable overcurrent protection provisions, including 240.4(D) where relevant	Protection limits that cannot be inferred from table ampacity alone

These reference points are supported by the manufacturer table and technical explanations cited throughout this article. Verify their wording and applicability in the adopted edition.

NFPA also offers an ampacity workflow fact sheet containing ampacity, temperature correction, and adjustment tables plus a selection flowchart. Manufacturer reference tables help check arithmetic, but neither a reference chart nor a calculator replaces the adopted NEC, local amendments, equipment instructions, or qualified installation-specific judgment.

Derating Verification Checklist

Use this checklist to review the calculation inputs and code checks described above:

• Confirm the locally adopted NEC edition and amendments.
• Select the ampacity table for the actual installation method.
• Match conductor material, size, and applicable insulation rating.
• Use installation ambient temperature—not the insulation rating—as the temperature input.
• Match the correction table to the base ampacity table’s reference conditions.
• Determine the current-carrying conductor count, including applicable neutral and spare-conductor rules.
• Check installation-specific adjustment provisions and exceptions.
• Multiply correction and adjustment factors; do not add percentage reductions.
• Check every applicable termination temperature limitation.
• Retain calculation precision when comparing against required ampacity.
• Evaluate load sizing, overcurrent protection, physical raceway fill, and manufacturer instructions separately.
• Treat the result as a calculation to verify—not a permit, inspection approval, or authorization to perform energized work.