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Aluminum conductor ampacity

The ampacity of an aluminum conductor is the maximum current, in amperes, that can flow continuously through it without exceeding its rated insulation temperature, under normal installation conditions at an ambient temperature of 30 °C / 86 °F. The value depends on the cross-sectional area, conductor material, insulation type, and heat dissipation conditions.

Determining Factors

Section titled “Determining Factors”

The current-carrying capacity of an aluminum conductor is influenced by:

  • The cross-sectional area and diameter of the conductor.
  • The maximum allowable temperature of the insulation (60 °C / 140 °F, 75 °C / 167 °F, or 90 °C / 194 °F).
  • The ambient temperature of the installation environment.
  • The number of current-carrying conductors grouped in the same conduit or cable.
  • The installation method (in conduit, in free air, direct burial).

Nominal Ampacity Table

Section titled “Nominal Ampacity Table”

The values correspond to a maximum of three aluminum conductors in a conduit, cable, or directly buried, at an ambient temperature of 30 °C / 86 °F, per standard references (NEC).

Size (AWG/kcmil) Nominal cross-section (mm²) Diameter (mm / in) Ampacity 60 °C (A) Ampacity 75 °C (A) Ampacity 90 °C (A)
12 AWG 3,31 mm² 2,05 mm / 0.081 in 15 15 20
10 AWG 5,26 mm² 2,59 mm / 0.102 in 20 25 30
8 AWG 8,37 mm² 3,26 mm / 0.128 in 35 40 45
6 AWG 13,3 mm² 4,11 mm / 0.162 in 40 50 55
4 AWG 21,1 mm² 5,19 mm / 0.204 in 55 65 75
2 AWG 33,6 mm² 6,54 mm / 0.258 in 75 90 100
1/0 AWG 53,5 mm² 8,25 mm / 0.325 in 100 120 135
2/0 AWG 67,4 mm² 9,27 mm / 0.365 in 115 135 150
3/0 AWG 85,0 mm² 10,4 mm / 0.410 in 130 155 175
4/0 AWG 107 mm² 11,7 mm / 0.460 in 150 180 205
250 kcmil 127 mm² 12,7 mm / 0.500 in 170 205 230
300 kcmil 152 mm² 13,9 mm / 0.548 in 195 230 260
350 kcmil 177 mm² 15,0 mm / 0.591 in 210 250 280
400 kcmil 203 mm² 16,1 mm / 0.633 in 225 270 305
500 kcmil 253 mm² 18,0 mm / 0.707 in 260 310 350
600 kcmil 304 mm² 19,7 mm / 0.775 in 285 340 385
700 kcmil 355 mm² 21,3 mm / 0.837 in 310 375 420
750 kcmil 380 mm² 22,0 mm / 0.866 in 320 385 435
800 kcmil 405 mm² 22,7 mm / 0.894 in 330 395 450
1000 kcmil 507 mm² 25,4 mm / 1.000 in 375 445 500

Ampacity Calculation Formula

Section titled “Ampacity Calculation Formula”

The steady-state current capacity can be calculated using the Neher‑McGrath method, which relates the heat generated by Joule effect to the total thermal resistance between the conductor and the ambient. In simplified form:

I = √[ (Tc − Ta) / (Rdc · (1 + Yc) · Rca) ]

Symbol Variable Unit
I Allowable current A
Tc Maximum conductor temperature °C / °F
Ta Ambient temperature °C / °F
Rdc DC electrical resistance per unit length Ω/m
Yc Additional losses factor in alternating current (skin and proximity effect) dimensionless
Rca Effective thermal resistance between conductor and ambient °C·m/W

Correction Factors

Section titled “Correction Factors”

Ambient Temperature Correction

Section titled “Ambient Temperature Correction”

When the ambient temperature differs from 30 °C / 86 °F, the nominal ampacity is multiplied by the indicated factor.

Ambient temperature (°C / °F) Factor for 60 °C Factor for 75 °C Factor for 90 °C
21‑25 °C / 70‑77 °F 1,08 1,05 1,04
26‑30 °C / 79‑86 °F 1,00 1,00 1,00
31‑35 °C / 88‑95 °F 0,91 0,94 0,96
36‑40 °C / 97‑104 °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

Conductor Grouping Correction

Section titled “Conductor Grouping Correction”

When more than three current-carrying conductors share the same conduit, the ampacity is reduced according to the following table.

Number of current-carrying conductors Adjustment factor
4 to 6 0,80
7 to 9 0,70
10 to 20 0,50
21 to 30 0,45
31 to 40 0,40
41 and more 0,35

Calculation Example

Section titled “Calculation Example”

Determine the corrected ampacity of a 350 kcmil (177 mm²) aluminum conductor with 75 °C insulation, installed in a conduit containing five current-carrying conductors, at an ambient temperature of 38 °C / 100 °F.

  1. Nominal ampacity (table): 250 A.
  2. Temperature factor (36‑40 °C, 75 °C): 0,88.
  3. Grouping factor (5 conductors): 0,80.
  4. Corrected ampacity = 250 A × 0,88 × 0,80 = 176 A.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the ampacity of a 2 AWG aluminum conductor with THHN insulation at 75 °C in conduit?

Section titled “What is the ampacity of a 2 AWG aluminum conductor with THHN insulation at 75 °C in conduit?”

The nominal ampacity is 90 A for a maximum of three current-carrying conductors in conduit at 30 °C ambient.

How much is the ampacity of a 1/0 AWG aluminum conductor reduced at 40 °C ambient?

Section titled “How much is the ampacity of a 1/0 AWG aluminum conductor reduced at 40 °C ambient?”

With 75 °C insulation, the correction factor is 0,88; therefore, its ampacity goes from 120 A to 105,6 A.

What is the diameter of a 250 kcmil aluminum conductor?

Section titled “What is the diameter of a 250 kcmil aluminum conductor?”

The nominal diameter is 12,7 mm / 0,500 in for a compact AAC aluminum conductor.

What grouping factor applies with 8 aluminum conductors in the same conduit?

Section titled “What grouping factor applies with 8 aluminum conductors in the same conduit?”

A factor of 0,70 applies, reducing the ampacity to 70 % of the nominal value.

What is the ampacity of a 4/0 AWG aluminum conductor in free air at 75 °C?

Section titled “What is the ampacity of a 4/0 AWG aluminum conductor in free air at 75 °C?”

In free air installation, a single 4/0 AWG aluminum conductor can carry 230 A at 75 °C.

At what maximum temperature can an aluminum conductor with XHHW‑2 insulation operate?

Section titled “At what maximum temperature can an aluminum conductor with XHHW‑2 insulation operate?”

XHHW‑2 insulation is rated for 90 °C / 194 °F in dry condition, although it is typically sized based on the 75 °C column.

Sources

Section titled “Sources”

References

Section titled “References”