Wire Derating Explained: How It Changes Ampacity

Wire derating explained simply means understanding why a conductor's allowable ampacity may need to be reduced when installation conditions make it harder for the wire to dissipate heat.

For US electrical installations common factors include ambient temperature and multiple current carrying conductors sharing a raceway or cable. This guide explains the NEC concepts, formulas, adjustment factors and a practical calculation example.

Key Takeaways

Wire derating is not simply choosing a smaller percentage from a chart. The adjustment depends on the actual installation conditions.

• Higher ambient temperatures can require a temperature correction factor.
• Multiple current carrying conductors can require an ampacity adjustment.
• Only applicable current carrying conductors are counted.
• Temperature and conductor count factors can apply together.
• The final allowable ampacity must also satisfy applicable terminal and equipment temperature limitations.

What Does Wire Derating Mean?

Wire derating is the process of reducing or adjusting a conductor's allowable ampacity because the conditions around the conductor can increase its operating temperature.

A basic ampacity table assumes specific conditions. Under the NEC Table 310.16 ampacity values are associated with specified temperature and installation conditions. When those conditions change, correction or adjustment factors may be required.

For example conductors installed together in a raceway can produce more heat than the same conductors installed separately. Likewise a conductor operating in a hot attic has less ability to dissipate heat than one operating at the standard reference ambient temperature. The result is that the conductor may need a lower allowable ampacity.

Why Do Electrical Wires Need Derating?

The main reason is heat. When current flows through a conductor electrical resistance produces heat. If that heat cannot escape efficiently the conductor temperature can rise. Two common conditions are:

Multiple Current Carrying Conductors

When several current carrying conductors share a raceway, cable or similar enclosure the heat produced by the conductors can accumulate. NEC adjustment factors account for this condition when the applicable conductor count threshold is exceeded.

High Ambient Temperature

A conductor installed in a hot environment has less temperature margin available. The NEC provides ambient temperature correction factors for conditions that differ from the standard 30°C (86°F) reference. These are different considerations and should not be treated as the same factor.

Wire Derating vs Ampacity

Ampacity is the amount of current a conductor is permitted to carry under specified conditions without exceeding its allowable temperature. Derating or ampacity adjustment changes that usable value when installation conditions require it.

For example suppose a conductor has a base ampacity of 40 A under the applicable reference conditions. If an 80% adjustment factor applies:

Adjusted ampacity = 40 A × 0.80 = 32 A

The conductor can no longer be treated as having 40 A of usable ampacity under those conditions. This is why a wire size chart should be treated as a starting point rather than an automatic answer for every installation.

NEC Wire Derating Chart for Multiple Conductors

Under NEC Table 310.15(C)(1), the adjustment factor depends on the number of current carrying conductors where the provision applies. The commonly used adjustment factors are:

For example if six applicable current carrying conductors share a raceway the adjustment factor is 80%. The important point is that not every physical wire automatically counts. Equipment grounding conductors are treated differently and neutral conductors have specific rules governing when they are considered current carrying.

How Do You Calculate Wire Derating?

A simplified calculation can be written as:

Adjusted Ampacity = Base Ampacity × Adjustment Factor

When both conductor grouping and ambient temperature require correction:

Adjusted Ampacity = Base Ampacity × Temperature Factor × Adjustment Factor

The starting ampacity and applicable temperature column must be selected according to the conductor, insulation, installation and applicable NEC requirements.

For many common conductors the 90°C column can be useful when applying correction or adjustment factors but it does not automatically become the final permitted ampacity. The final result must be checked against applicable terminal temperature limitations and other requirements.

Example: Derating Wire for Multiple Conductors

Suppose a raceway contains five current carrying #10 AWG copper THHN conductors under conditions where a conductor count adjustment is required.

Assume the applicable 90°C ampacity is 40 A. Five current carrying conductors fall into the 4–6 range giving an 80% adjustment factor.

So:

40 A × 0.80 = 32 A

The adjusted ampacity is therefore 32 A before completing the remaining applicable checks. This does not mean a 32 A breaker should automatically be installed.

The conductor's terminal ratings, overcurrent protection rules, continuous load requirements and other applicable NEC provisions must still be checked.

What About Temperature Derating?

Temperature correction is separate from conductor count adjustment. For example common 90°C correction factors include:

These factors are associated with the conductor's temperature rating and applicable NEC correction table. For example if a 40 A base ampacity is subject to a 0.91 temperature factor:

40 A × 0.91 = 36.4 A

If a conductor count adjustment also applies at the same time both factors may need to be applied.

How to Calculate Wire Derating When Two Factors Apply?

Consider a simplified example using:

The calculation becomes: 40 A × 0.91 × 0.80 = 29.12 A

The resulting adjusted ampacity is 29.12 A before the remaining code and equipment checks. This illustrates why simply looking at the original ampacity value can produce the wrong result when the actual installation has multiple heat related conditions.

• Base ampacity: 40 A
• Ambient temperature factor: 0.91
• Conductor adjustment factor: 0.80

How Many Wires Count for Derating?

This is one of the most important parts of the calculation. The adjustment is based on current carrying conductors, not simply the total number of wires inside a raceway.

For example an equipment grounding conductor is not normally counted as a current carrying conductor for this ampacity adjustment.

Neutral conductors require more careful consideration. The 2026 NEC reorganized Section 310.15(E) to clarify when neutrals are considered current carrying.

The rules include specific conditions involving two wire circuits, certain three wire circuits from four wire three phase wye systems and four wire three phase wye circuits with predominantly nonlinear loads.

Because neutral counting can change the adjustment factor bracket it should be determined from the actual circuit configuration rather than guessed.

Does Conduit Fill Mean the Same as Wire Derating?

No. These concepts are related to installation conditions but they are not identical.

Conduit fill concerns the physical space occupied by conductors inside a raceway.

Ampacity adjustment concerns the thermal effect of multiple current carrying conductors sharing an installation space.

A raceway can satisfy its physical fill requirement while still requiring an ampacity adjustment because of the number of current carrying conductors. Both checks may need to be completed separately.

When Should You Derate Electrical Wires?

Wire derating may become relevant when installation conditions differ from the assumptions behind the basic ampacity table. Common situations include:

The exact requirements depend on the wiring method and applicable electrical code provisions.

• Several current carrying conductors in one raceway
• Multiple conductors bundled together
• High ambient temperatures
• Hot attics or similar spaces
• Certain industrial installations
• Circuits with specific conductor or insulation temperature limitations

Derating and Wire Size Selection

Derating can change which wire size is suitable for a circuit. For example a conductor may have sufficient base ampacity under standard conditions but fall below the required ampacity after adjustment.

In that situation the designer may need to select a larger conductor and repeat the calculation. This is one reason our Wire Size Calculator and Wire Size by Amperage guides treat installation conditions as part of conductor selection rather than relying on amperage alone.

However derating is only one part of wire sizing. A conductor that passes the ampacity calculation may still require a separate voltage drop check on a long circuit. For that calculation see our Wire Voltage Drop Calculator guide.

Common Wire Derating Mistakes

Counting Every Wire in the Raceway

Not every conductor automatically counts as current carrying. The actual circuit configuration needs to be evaluated.

Treating Conduit Fill as Ampacity Adjustment

Physical conduit fill and thermal ampacity adjustment are separate requirements.

Using the 90°C Rating as the Final Ampacity

A 90°C insulation rating does not automatically mean the conductor can be used at its 90°C ampacity at the terminals. Applicable terminal and equipment limitations still matter.

Applying Only One Factor

A hot installation with multiple current carrying conductors may require both ambient temperature correction and conductor count adjustment.

Forgetting Voltage Drop

Derating addresses conductor temperature and allowable ampacity. It does not replace voltage drop calculations. For longer runs check the selected conductor separately using a voltage drop calculation.

Conclusion

Wire derating explained in practical terms comes down to one principle, the ampacity shown under standard conditions may not be the final usable ampacity in every installation. Multiple current carrying conductors can require an adjustment factor while elevated ambient temperature can require a correction factor.

When both conditions apply, their effects can combine. The final conductor selection must also account for applicable terminal ratings, overcurrent protection, continuous loads and other electrical requirements.

For US installations, always verify the calculation against the current NEC edition and any applicable local requirements before using it for an actual installation.

FAQs

What does derating wire mean?

Wire derating means reducing or adjusting a conductor's allowable ampacity because installation conditions such as high ambient temperature or multiple current carrying conductors can increase operating temperature.

How do you calculate the derating of a wire?

Start with the applicable conductor ampacity and multiply it by the required correction and adjustment factors. If multiple factors apply they are normally multiplied rather than averaged.

How do I calculate the derating of cable?

Identify the cable's applicable ampacity, determine the relevant ambient temperature and conductor count factors then apply those factors to the base value. Complete the remaining code and equipment checks afterward.

What is the rule for derating conductors?

Under NEC conductor adjustment rules more than three applicable current carrying conductors can require an ampacity adjustment with the factor depending on the number of conductors. Ambient temperature can require a separate correction factor.

What is a derating factor in electrical work?

A derating factor is a multiplier used to adjust a conductor's ampacity for conditions that affect its allowable operating temperature.

Is wire derating the same as voltage drop?

No. Derating concerns allowable conductor ampacity and temperature. Voltage drop concerns the reduction in voltage along a conductor caused by its electrical resistance.