Conductor Derating Calculator

Calculate adjusted conductor ampacity when bundling more than 3 current-carrying conductors (§310.15(C)(1)) and correcting for elevated ambient temperatures (§310.15(B)(1)).

NEC edition

Inputs

Derating starts from the ampacity of the conductor's own insulation rating — for THHN that is the 90°C column

§110.14(C): the final ampacity may not exceed the ampacity of the conductor at the termination rating

conductors

Count all current-carrying conductors sharing the same raceway or cable. Neutrals carrying only unbalanced current do NOT count.

°C

Installation ambient temperature. Standard is 30°C (86°F).

VoltageLab app

Take this toolkit with you on the job

The full VoltageLab electrician app — wire tables, code lookups and job-site calculators offline, on Android and iOS.

Get it on Google Play Download on the App Store
⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

VoltageLab app

Studying for your electrical exam?

Practice NEC exam questions with full explanations and progress tracking, right on your phone.

Get it on Google Play Download on the App Store

How it works & the NEC rules behind it

When electrical current flows through a conductor, internal resistance produces heat ($I^2R$ losses). When multiple current-carrying conductors are bundled in a raceway or cable, heat dissipation is reduced, requiring ampacity derating to avoid damaging conductor insulation.

Two Types of NEC Derating (§310.15):

  • 1. Bundling Adjustment (§310.15(C)(1)): Where the number of current-carrying conductors in a raceway or cable exceeds 3, the base allowable ampacity from Table 310.16 must be reduced by the percentage shown in Table 310.15(C)(1) (e.g. 4–6 wires = 80%, 7–9 wires = 70%, 10–20 wires = 50%).
  • 2. Ambient Temperature Correction (§310.15(B)(1)): Standard ampacities in Table 310.16 are based on an ambient temperature of 30°C (86°F). For hotter locations (attics, rooftops, industrial boiler rooms), correction factors must be applied.
  • Using the 90°C Column for Derating: If using 90°C rated wire (like THHN), you are permitted to apply derating multipliers to the higher 90°C ampacity value, provided the final derated ampacity does not exceed the 75°C terminal rating!

Formulas & equations

Final Derated Ampacity = Base Ampacity (Table 310.16) × Bundling Factor (§310.15(C)(1)) × Temp Correction Factor (§310.15(B)(1))
Bundling Factors Table 310.15(C)(1):
• 1 to 3 Conductors: 100% (No derating)
• 4 to 6 Conductors: 80% multiplier
• 7 to 9 Conductors: 70% multiplier
• 10 to 20 Conductors: 50% multiplier

NEC reference table

Number of Current-Carrying Conductors Percent of Values in Table 310.16
1 through 3 conductors100%
4 through 6 conductors80%
7 through 9 conductors70%
10 through 20 conductors50%
21 through 30 conductors45%
31 through 40 conductors40%
41 and above35%

*Source: NEC Table 310.15(C)(1) (Adjustment Factors for More Than Three Current-Carrying Conductors).

Worked example, step by step

Example Calculation: (9) #12 AWG THHN Copper in Conduit @ 40°C Ambient

Scenario: Sizing 9 current-carrying #12 THHN Copper conductors pulled in a conduit through an unconditioned commercial ceiling at 40°C (104°F).

  • Step 1 (Base 90°C Ampacity): #12 THHN Copper has a base 90°C rating of 30 Amps (Table 310.16).
  • Step 2 (Bundling Derate): 9 conductors = 70% multiplier (0.70).
  • Step 3 (Temperature Derate): 40°C in the 90°C column = 0.91 multiplier.
  • Step 4 (Calculation): Derated Ampacity = 30A × 0.70 × 0.91 = 19.11 Amps.
  • Result: Protected by a standard 20A Breaker per the §240.4(B) next-size-up rule.

Frequently asked questions

Under NEC §310.15(E)(1), a neutral conductor that carries only the unbalanced current from other conductors of the same circuit (e.g. 3-wire 120/240V or 4-wire 208Y/120V balanced systems) is NOT counted as a current-carrying conductor. However, on 3-phase circuits supplying non-linear loads (computers, LED drivers, VFDs), harmonics cause the neutral to carry substantial current and it MUST be counted (§310.15(E)(3)).

No. Under NEC §310.15(F), equipment grounding conductors (EGCs) do not carry current during normal operation and are NEVER counted as current-carrying conductors for bundling derating calculations.

Related NEC calculators

Other tools in the same install chain:

NEC sizing

Wire Gauge Calculator

Find the minimum conductor size (AWG/kcmil) for any electrical load per NEC §310.16. Supports 60°C, 75°C, and 90°C ratings, continuous loads (125%), and parallel runs.

Open NEC sizing

Conduit Fill Calculator

Calculate raceway fill percentage and minimum conduit size per NEC Chapter 9 Tables 1, 4, and 5. Supports EMT, IMC, RMC, PVC-40, PVC-80, ENT, FMC, and LFMC with mixed wire sizes.

Open NEC sizing

Voltage Drop Calculator

Calculate AC/DC voltage drop and select conductor size per NEC §210.19 and §215.2 recommendations (3% branch, 5% overall). Uses Chapter 9 Table 9 resistance.

Open