Temperature and bundling derating factors
Definition of Ampacity and Correction Factors
Section titled “Definition of Ampacity and Correction Factors”Ampacity is the maximum current-carrying capacity of an electrical conductor, expressed in amperes, that it can withstand continuously without exceeding its allowable operating temperature. This capacity depends on the conductor resistance, insulation type, ambient temperature, and the heat dissipation capability of the environment. When installation conditions differ from the reference conditions (typically 30 °C / 86 °F ambient temperature and a maximum of three current-carrying conductors in a conduit or cable), correction factors are applied to ensure safe operation.
The temperature correction factor adjusts the ampacity due to an ambient temperature different from the reference. The grouping correction factor considers the reduction in heat dissipation capacity when several current-carrying conductors share the same enclosed space, such as a conduit, raceway, or tray.
Both factors are multiplicative and are applied simultaneously to the nominal ampacity to obtain the maximum allowable current under actual installation conditions.
Ambient Temperature Correction Factor
Section titled “Ambient Temperature Correction Factor”When the ambient temperature exceeds the reference value (typically 30 °C / 86 °F), the ability to dissipate the heat generated by the conductor decreases, so the allowable current must be reduced. The correction factors vary depending on the conductor’s insulation class. The following table lists the most common factors for insulations with maximum service temperatures of 60 °C, 75 °C, and 90 °C, in accordance with applicable international standards for low-voltage installations.
| Ambient Temperature Range | Factor (60 °C insulation) | Factor (75 °C insulation) | Factor (90 °C insulation) |
|---|---|---|---|
| 21‑25 °C / 70‑77 °F | 1.08 | 1.05 | 1.04 |
| 26‑30 °C / 78‑86 °F | 1.00 | 1.00 | 1.00 |
| 31‑35 °C / 87‑95 °F | 0.91 | 0.94 | 0.96 |
| 36‑40 °C / 96‑104 °F | 0.82 | 0.88 | 0.91 |
| 41‑45 °C / 106‑113 °F | 0.71 | 0.82 | 0.87 |
| 46‑50 °C / 115‑122 °F | 0.58 | 0.75 | 0.82 |
| 51‑55 °C / 124‑131 °F | 0.41 | 0.67 | 0.76 |
| 56‑60 °C / 133‑140 °F | — | 0.58 | 0.71 |
| 61‑65 °C / 142‑149 °F | — | 0.47 | 0.65 |
| 66‑70 °C / 151‑158 °F | — | 0.33 | 0.58 |
| 71‑75 °C / 160‑167 °F | — | — | 0.50 |
| 76‑80 °C / 169‑176 °F | — | — | 0.41 |
| 81‑85 °C / 178‑185 °F | — | — | 0.29 |
Grouping Correction Factor (Conductors in Conduit or Cable)
Section titled “Grouping Correction Factor (Conductors in Conduit or Cable)”When more than three current-carrying conductors are housed in the same conduit, raceway, or multiconductor cable, the heat buildup reduces the individual dissipation capacity. The ampacity must be multiplied by a grouping factor that depends on the number of active conductors. The following table shows the typical adjustment factors, based on reference standards for electrical installations.
| Number of Current-Carrying Conductors | Grouping Factor (percentage of nominal ampacity) |
|---|---|
| 1 to 3 | 1.00 (100 %) |
| 4 to 6 | 0.80 (80 %) |
| 7 to 9 | 0.70 (70 %) |
| 10 to 20 | 0.50 (50 %) |
| 21 to 30 | 0.45 (45 %) |
| 31 to 40 | 0.40 (40 %) |
| 41 or more | 0.35 (35 %) |
In installations where temperature and grouping corrections coexist, both factors are multiplied sequentially on the base ampacity.
Calculation of Corrected Ampacity
Section titled “Calculation of Corrected Ampacity”The corrected ampacity is obtained by applying all relevant correction factors to the tabulated nominal value. The general expression is:
I_corr = I₀ × f_T × f_A
where:
| Variable | Description | Unit |
|---|---|---|
| I_corr | Corrected ampacity | A |
| I₀ | Nominal ampacity of the conductor under reference conditions (typically 30 °C / 86 °F and a maximum of 3 conductors in conduit) | A |
| f_T | Ambient temperature correction factor (dimensionless) | — |
| f_A | Grouping correction factor (dimensionless) | — |
If other adverse conditions exist (deep burial, solar exposure, etc.), additional multiplicative factors are added.
Practical Example
Section titled “Practical Example”Determine the corrected ampacity of a copper conductor with 90 °C thermoplastic insulation, size 8 AWG, with a base ampacity of 50 A for 3 conductors in conduit at 30 °C / 86 °F. The actual installation has the following conditions:
- Ambient temperature of 45 °C / 113 °F.
- Eight current-carrying conductors in the same conduit.
Step 1 – Temperature factor: for 90 °C insulation and temperature of 41‑45 °C, f_T = 0.87 (according to table). Step 2 – Grouping factor: for 7‑9 conductors, f_A = 0.70. Step 3 – Corrected ampacity:
I_corr = 50 A × 0.87 × 0.70 = 30.45 A.
Therefore, the maximum allowable current under actual conditions is 30.45 A, much lower than the nominal 50 A.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/temperature-derating-factor-steel-pipes-d_1744.html
- southwire.com: https://www.southwire.com/calculators