What size wire do I need for a 300 amp breaker?

With 75 °C terminations, 350 kcmil copper or 500 kcmil aluminum (NEC Table 310.16). With 60 °C terminations, 500 kcmil copper. Long runs may need a size up for voltage drop.

Can I use 350 kcmil THHN copper on 300 amps?

Yes: 350 kcmil is the answer at both 75 °C and 90 °C, because the 90 °C column may only be used for derating; the termination (110.14(C)) caps the conductor at its 75 °C value.

How far can 350 kcmil copper run on a 300 amp circuit?

At 80 % load (240 A) and the 3 % informational note, about 203 ft one-way at 120 V and 407 ft at 240 V by the K-factor method. Beyond that, go up a size.

What size wire for 300 amps?

350 kcmil copper or 500 kcmil aluminum with 75 °C terminations, by NEC Table 310.16 at 30 °C with no more than three current-carrying conductors. Commercial and light-industrial feeders; consider parallel sets.

Minimum conductor size

Minimum size · copper350kcmilFITS75 °C insulation on 75 °C terminations · needs 300 A at 30 °C ambient with 3 current-carrying conductors · NEC 2023 Table 310.16

Load current

The load, or the breaker rating you must feed

Conductor

Continuous load

Table 310.16 ampacity, 350 kcmil at 75 °CNEC 2023 Table 310.16, identical to 2017 Table 310.15(B)(16): 310 A
After correction and adjustment× 1.00 for 30 °C (NEC 2023 Table 310.15(B)(1)(1)) × 1.00 for 3 conductors (NEC 2023 Table 310.15(C)(1)): 310.0 A
Termination cap, 110.14(C) at 75 °CCircuits > 100 A or larger than 1 AWG use the 75 °C column: 310 A
Usable ampacitythe smallest of the three above: 310.0 A

Ampacity from NEC 2023 Table 310.16 (identical to 2017 Table 310.15(B)(16)), corrected for ambient and conductor count, capped by the termination rating (110.14(C)) and the small-conductor limits of 240.4(D). Equipment listings, conductor markings and local amendments govern; a licensed electrician makes the final call.

Insulation rating

Termination rating

Ambient temperature

30 °C is the table base; attics and rooftops run hotter

Current-carrying conductors

In the same raceway or cable; 3 or fewer needs no adjustment

Length, one-way (optional)

Adds a voltage-drop check against the 3 % note

Voltage

For the drop check

Phase

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Every column for 300 A

Conductor and terminations	Size	Usable ampacity	Why
Copper, 60 °C (TW, UF; or unmarked equipment ≤ 100 A)	500 kcmil	320 A	Table 310.16, 320 A
Copper, 75 °C (THW, THWN, XHHW; marked 75 °C terminations)	350 kcmil	310 A	Table 310.16, 310 A
Copper, 90 °C insulation on 75 °C terminations (THHN)	350 kcmil	310 A	capped at the 75 °C column (110.14(C))
Aluminum, 75 °C	500 kcmil	310 A	Table 310.16, 310 A
Aluminum, 90 °C insulation on 75 °C terminations (XHHW-2)	500 kcmil	310 A	capped at the 75 °C column (110.14(C))
Copper, 75 °C, continuous load at 80 % of the breaker (240 A × 1.25)	350 kcmil	310 A	210.19(A)(1): a continuous load is sized at 125 %

How far it can run

K-factor method at 80 % load (240 A), single-phase, one-way length for a 3 % drop (an informational note, not a requirement), and the drop at 100 ft.

Conductor	Voltage	3 % reached at	Drop at 100 ft
350 kcmil copper	120 V	203 ft	1.5 %	open
350 kcmil copper	240 V	407 ft	0.7 %	open
500 kcmil aluminum	120 V	177 ft	1.7 %	open
500 kcmil aluminum	240 V	354 ft	0.8 %	open

Questions people ask

What size wire do I need for a 300 amp breaker?: With 75 °C terminations, 350 kcmil copper or 500 kcmil aluminum (NEC Table 310.16). With 60 °C terminations, 500 kcmil copper. Long runs may need a size up for voltage drop.
Can I use 350 kcmil THHN copper on 300 amps?: Yes: 350 kcmil is the answer at both 75 °C and 90 °C, because the 90 °C column may only be used for derating; the termination (110.14(C)) caps the conductor at its 75 °C value.
How far can 350 kcmil copper run on a 300 amp circuit?: At 80 % load (240 A) and the 3 % informational note, about 203 ft one-way at 120 V and 407 ft at 240 V by the K-factor method. Beyond that, go up a size.