Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 195 A | - |
| 75°C | 230 A | THW |
| 90°C | 260 A | THHN / THWN-2, XHHW-2 |
Reference tables · Aluminum ampacity
2023 NEC · Table 310.16
300 kcmil aluminum wire ampacity
At a 75°C termination rating, 300 kcmil aluminum carries 230 A under the published table's basis conditions - 30°C ambient, no more than three current-carrying conductors.
NEC 2023Every ampacity below is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
What 300 kcmil aluminum is used for
300 kcmil aluminum carries 230 A at 75°C. Conductors in the kcmil range are service, feeder, and distribution conductors, frequently run as paralleled sets under rules this reference does not model. It pairs with a 225 A overcurrent device.
Terminations usually decide the practical answer. Breakers and equipment are typically listed for 60°C or 75°C, so even 90°C-rated insulation on this conductor is normally limited to the 230 A column at the terminals. Check the equipment's listing rather than assuming.
Derating
Ampacity in real conditions
NEC 2023Every derated value in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
| Ambient | 3 conductors | 4 conductors | 7 conductors | 10 conductors | 21 conductors |
|---|---|---|---|---|---|
| 20°C | 255.3 A | 204.2 A | 178.7 A | 127.7 A | 114.9 A |
| 30°C | 230 A | 184 A | 161 A | 115 A | 103.5 A |
| 40°C | 202.4 A | 161.9 A | 141.7 A | 101.2 A | 91.1 A |
| 50°C | 172.5 A | 138 A | 120.7 A | 86.3 A | 77.6 A |
NEC 2023Every property and drop figure below is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
Properties
| Circular mils | 300,000 |
|---|---|
| DC resistance | 0.0707 Ω per 1000 ft |
| Ampacity at 75°C | 230 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 225 A |
| Area, THHN / THWN-2 | 0.4608 in² |
| Area, XHHW-2 | 0.4536 in² |
| Area, THW | 0.5281 in² |
Voltage drop over distance
Single-phase 240 V, carrying 230 A on 300 kcmil aluminum.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.34% | Yes |
| 50 ft | 0.68% | Yes |
| 100 ft | 1.35% | Yes |
| 150 ft | 2.03% | Yes |
| 200 ft | 2.71% | Yes |
| 300 ft | 4.06% | No |
FAQ
Frequently asked questions
How many amps can 300 kcmil aluminum wire carry?
230 A at a 75°C termination rating under 2023 NEC Table 310.16, at the table's basis conditions of a 30°C ambient and no more than three current-carrying conductors. The 60°C column reads 195 A and the 90°C column 260 A, but terminations on breakers and equipment usually hold the practical answer to the 75°C column.
What size breaker goes with 300 kcmil aluminum wire?
225 A is the largest standard rating at or below the 230 A this conductor is good for at 75°C under the 2023 NEC. A hotter location or a fuller raceway derates the conductor and can push the breaker down; the derating table above shows how far.
Does 300 kcmil aluminum ampacity change with heat or bundling?
Yes. The published 230 A assumes a 30°C ambient and at most three current-carrying conductors in the raceway or cable. At a 40°C ambient with seven current-carrying conductors, the same 300 kcmil aluminum conductor is usable at 141.7 A after the 2023 NEC correction and adjustment factors. The derating table above covers more combinations.
How far can 300 kcmil aluminum wire run before voltage drop is a problem?
Carrying its full 230 A on a 240 V single-phase circuit, the circular-mil estimate this page uses stays inside a 3% drop out to about 221 ft one way, and reads 0.34% at 25 ft. Lighter loads drop proportionally less. The 2023 NEC treats 3% as a recommendation for efficient operation, not a general requirement.
Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, Table 310.15(B)(1)(1); NFPA 70, National Electrical Code, Table 310.15(C)(1); NFPA 70, National Electrical Code, Chapter 9, Table 8. Published by NFPA.NEC 2023Transcribed from the 2023 NEC (NFPA 70, 2023). Confirm against your locally adopted edition and your authority having jurisdiction (AHJ).
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