2023 NEC · Table 310.16

300 kcmil copper wire ampacity

At a 75°C termination rating, 300 kcmil copper carries 285 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.

Ampacity by temperature rating

300 kcmil copper, at a 30°C ambient with no more than three current-carrying conductors in a raceway or cable. The 90°C column is normally used only as the starting point for derating.
Temperature rating	Ampacity	Typical insulation
60°C	240 A	-
75°C	285 A	THW
90°C	320 A	THHN / THWN-2, XHHW-2

What 300 kcmil copper is used for

300 kcmil copper carries 285 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 250 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 285 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.

300 kcmil copper at a 75°C termination rating, corrected for ambient temperature and adjusted for the number of current-carrying conductors, then capped by any small-conductor limit. Adjustment factors: 4-6 → 80%, 7-9 → 70%, 10-20 → 50%.
Ambient	3 conductors	4 conductors	7 conductors	10 conductors	21 conductors
20°C	316.4 A	253.1 A	221.4 A	158.2 A	142.4 A
30°C	285 A	228 A	199.5 A	142.5 A	128.3 A
40°C	250.8 A	200.6 A	175.6 A	125.4 A	112.9 A
50°C	213.8 A	171 A	149.6 A	106.9 A	96.2 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	0.0429 Ω per 1000 ft
Ampacity at 75°C	285 A
Overcurrent ceiling	Set by ampacity
Largest standard breaker	250 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 285 A on 300 kcmil copper.

Circular-mil estimate at full load. Lighter loads drop proportionally less. Use the voltage drop calculator for your actual current.
One-way distance	Drop	Within 3%?
25 ft	0.26%	Yes
50 ft	0.51%	Yes
100 ft	1.02%	Yes
150 ft	1.53%	Yes
200 ft	2.04%	Yes
300 ft	3.06%	No

FAQ

Frequently asked questions

How many amps can 300 kcmil copper wire carry?

285 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 240 A and the 90°C column 320 A, but terminations on breakers and equipment usually hold the practical answer to the 75°C column.

What size breaker goes with 300 kcmil copper wire?

250 A is the largest standard rating at or below the 285 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 copper ampacity change with heat or bundling?

Yes. The published 285 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 copper conductor is usable at 175.6 A after the 2023 NEC correction and adjustment factors. The derating table above covers more combinations.

How far can 300 kcmil copper wire run before voltage drop is a problem?

Carrying its full 285 A on a 240 V single-phase circuit, the circular-mil estimate this page uses stays inside a 3% drop out to about 293 ft one way, and reads 0.26% 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).

Nearby conductors

250 kcmil copper 350 kcmil copper 300 kcmil aluminum 250 A breaker wire size Wire size calculator Ampacity lookup

Every figure on this page comes 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.

Reference estimate - verify before you build. AmpSizer reports reference estimates transcribed from published tables in the 2023 NEC (NFPA 70, 2023), and from no other edition. It is not a substitute for the code adopted in your jurisdiction. 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. Final conductor sizing, overcurrent protection, and installation must be determined by a licensed electrician and approved by your AHJ.

Jurisdictions adopt editions on their own schedule and frequently amend them, so the edition enforced where you work may not be the one cited here. Confirm which edition and which local amendments apply to your job before relying on any value on this page.