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What Size Wire for 400 Amps?

For 400 amps with the displayed 75 C termination defaults, start with 600 kcmil copper or 900 kcmil aluminum. This is the top published amp-only page, so equipment, utility, and parallel-set planning usually matter more than another simple size jump.
Load
400 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
Copper600 kcmil400 A420 ATermination rated 75 CPasses defaults
Aluminum900 kcmil400 A425 ATermination rated 75 CPasses defaults
What Size Wire for 400 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 400 A ChangesAt 400 A, copper shows 420 A usable ampacity and aluminum shows 425 A under the displayed 75 C defaults. That passes the default check, but equipment limited to 60 C falls below a full 400 A rating, and many real 400 A installations depend on listed service assemblies, utility requirements, raceway strategy, or engineered parallel sets.

Run the wire size calculator

Adjacent Amp Ratings

Compare the prior published amp rating; this is the highest amp-only answer page in the published set.

LoadCopper sizeCopper OCPDAluminum sizeAluminum OCPD
350 A500 kcmil350 A700 kcmil350 A
400 A (this page)600 kcmil400 A900 kcmil400 A
Copper and aluminum starting sizes for 400 A and the prior published amp page, under the same defaults as the result above.

This ceiling comparison shows what changes from the prior published amp rating. For ratings above 400 A, use the calculator with real inputs instead of extrapolating from this page.

400 A Is the Published Amp-Series Ceiling

Large service / feeder context for a 400 A circuit.

A 400 A wire-size search usually means a large residential service, meter-main lineup, building feeder, shop service, or light commercial distribution path. At this rating, the conductor number is only one part of the decision; equipment lineup, utility standards, service product, raceway layout, and installation method can control the accepted design.

Under the displayed 75 C termination defaults, copper moves from 500 kcmil at 350 A to 600 kcmil here, and aluminum moves from 700 kcmil to 900 kcmil. There is no higher published amp-only answer page in this series, so designs above 400 A should go back to the calculator and a project-specific review instead of extrapolating from this page.

What Changes This Answer

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

  • Do not reuse 350 A conductors

    Under these defaults, 350 A uses 500 kcmil copper and 700 kcmil aluminum. This 400 A page moves to 600 kcmil copper and 900 kcmil aluminum.

  • Treat this as the page-series ceiling

    The amp-only answer set stops at 400 A. For larger ratings, use the calculator with real inputs and a project-specific design review rather than extending the adjacent table by guesswork.

  • Check 60 C and equipment listings

    The displayed 600 kcmil copper and 900 kcmil aluminum results are 75 C defaults. Equipment limited to 60 C will force a different conductor or assembly for a full 400 A feeder or service.

  • Verify service gear and utility standards

    Meter-main equipment, service disconnects, CT cabinets, utility standards, grounding, bonding, and AHJ review can require an accepted assembly that differs from a single-conductor table starting point.

  • Do not size parallel sets from this page

    Parallel conductors are common at this scale, but this page does not design them. Parallel sets need matching conductors, equal lengths, listed terminations, and project-specific engineering.

400 Amp Wire Size FAQs

What size wire for 400 amps?
Under the 75 C non-continuous defaults on this page, start with 600 kcmil copper or 900 kcmil aluminum. Confirm the conductor product, lug range, raceway fill, voltage drop, load calculation, utility requirements, and AHJ review before ordering.
Is 400 A the same wire size as 350 A?
No under these defaults. The 350 A page shows 500 kcmil copper and 700 kcmil aluminum, while 400 A moves to 600 kcmil copper and 900 kcmil aluminum.
Do I need parallel conductors for 400 A?
Not automatically from this page alone. Many 400 A services and large feeders use listed assemblies or parallel conductors, but the parallel design is separate from this single-conductor starting point.
Can I extrapolate this page above 400 A?
No. This is the top published amp-only answer page. Larger services and feeders need real calculator inputs, equipment data, utility standards, and project-specific engineering rather than a simple extrapolation.
Can I use this page as final approval for a 400 A service?
No. Treat it as a planning starting point only. Service work still needs load calculation, conductor product verification, equipment lug checks, raceway fill, grounding and bonding, voltage drop, utility requirements, and AHJ review.