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What Size Wire for 300 Amps?

For 300 amps with the displayed 75 C termination defaults, start with 350 kcmil copper or 500 kcmil aluminum. This first large-feeder page is where conductor physical size, lugs, raceway, and possible parallel-set planning become central.
Load
300 A
Distance
0 ft entered
Band
Large service / feeder
Defaults
Non-continuous, 75 C lugs, 86 F, 240 V single-phase
Conductor count
3 current-carrying

Computed Starting Point

The table uses common defaults. Change the calculator inputs for the actual installation.

MaterialComputed sizeOCPDUsable ampacityGoverning constraintDefault check
Copper350 kcmil300 A310 ATermination rated 75 CPasses defaults
Aluminum500 kcmil300 A310 ATermination rated 75 CPasses defaults
What Size Wire for 300 Amps? under common non-continuous defaults: THHN or XHHW-2 insulation, 75 C terminations, 86 F ambient, 240 V single-phase, no more than three current-carrying conductors, and 0 ft route length entered.

What 300 A ChangesAt 300 A, both displayed rows show 310 A usable ampacity under 75 C defaults. That passes the default check, but a 60 C limit, derating, product limits, or long feeder can erase the margin. Parallel conductors may become a design option, but they are not implied by this single-conductor result.

Run the wire size calculator

Adjacent Amp Ratings

See whether stepping one standard size up or down forces a conductor jump under the same defaults.

LoadCopper sizeCopper OCPDAluminum sizeAluminum OCPD
250 A250 kcmil250 A350 kcmil250 A
300 A (this page)350 kcmil300 A500 kcmil300 A
350 A500 kcmil350 A700 kcmil350 A
Copper and aluminum starting sizes for 300 A and the neighboring published amp pages, under the same defaults as the result above.

A plateau means neighboring amp ratings share a conductor size under these defaults. A jump means a standard breaker change often needs a larger wire; confirm with your real ambient, length and terminations.

300 A Starts the Large-Feeder Hardware Check

Large service / feeder context for a 300 A circuit.

A 300 A wire-size search usually means a large service, shop or building feeder, panelboard feeder, or light commercial distribution path. The direct conductor answer is still useful, but the installation can fail on lug range, raceway fill, bend space, or pulling practicality before ampacity is the only concern.

Under the displayed 75 C termination defaults, copper moves from 250 kcmil at 250 A to 350 kcmil here, and aluminum moves from 350 kcmil to 500 kcmil. The next 350 A page moves both materials again, so the adjacent comparison is a real hardware and raceway check.

What Changes This Answer

Checks that can change the starting conductor under the displayed defaults.

  • Do not reuse the 250 A conductors

    Under these defaults, 250 A uses 250 kcmil copper and 350 kcmil aluminum. This 300 A page moves to 350 kcmil copper and 500 kcmil aluminum.

  • Check equipment lug range

    Panelboards, disconnects, meters, and switchgear must accept the selected conductor size and material. The ampacity result is not usable if the lug cannot land it.

  • Raceway fill and bend space are design checks

    Large kcmil conductors can force larger raceways, junction boxes, and equipment clearances. Check conduit fill and bending space before ordering.

  • Do not improvise parallel conductors

    If a single conductor is impractical, parallel sets require matching conductors, equal lengths, correct terminations, and a separate design check outside this direct-answer page.

  • Check 60 C equipment limits

    The displayed 350 kcmil copper and 500 kcmil aluminum results are 75 C defaults. Equipment limited to 60 C will force larger conductors for a full 300 A feeder or service.

300 Amp Wire Size FAQs

What size wire for 300 amps?
Under the 75 C non-continuous defaults on this page, start with 350 kcmil copper or 500 kcmil aluminum. Confirm terminal ratings, conductor product, lug range, raceway fill, voltage drop, and utility or AHJ requirements.
Why does 300 A jump so much from 250 A?
The adjacent 250 A page shows 250 kcmil copper and 350 kcmil aluminum under the same defaults. At 300 A, copper moves to 350 kcmil and aluminum moves to 500 kcmil.
Do I need parallel conductors for 300 A?
Not automatically. This page shows the single-conductor starting point under the displayed defaults. If the equipment, raceway, or pulling plan makes that impractical, parallel conductors need a separate design under the parallel-conductor rules.
Can I use this page as final approval for a 300 A feeder?
No. Treat it as a planning starting point only. Large feeders still need load calculation, conductor product verification, lug checks, raceway fill, grounding and bonding, voltage drop, and utility or AHJ review.