What Size Wire for 100 Amp Service?

The correct size still depends on the type of installation, conductor and terminal temperature ratings, adjustment or correction factors, run length, voltage drop, and the NEC edition and amendments adopted locally. Electrical service and panel work should be designed and installed in accordance with local requirements.

The important point is that there is not one universal wire size for every 100A installation. A 100A dwelling service and a 100A subpanel can require different conductor sizes even though both are rated at 100 amps.

100 Amp Wire Size Chart: Copper vs. Aluminum

The table below provides a quick comparison for common 100A applications.

NEC 310.12 permits qualifying dwelling service and feeder conductors rated from 100A through 400A to have an ampacity of at least 83% of the service or feeder rating. For 100A, Table 310.12 identifies 4 AWG copper and 2 AWG aluminum or copper-clad aluminum.

By comparison, under typical 75°C Table 310.16 conditions, 3 AWG copper is rated at 100A while 1 AWG aluminum is rated at 100A. A 4 AWG copper conductor is only 85A under those general conditions, which is why the application matters.

This chart is a starting point rather than a substitute for an installation-specific calculation. Temperature limits, conductor count, wiring method, correction factors, equipment terminals, and local code can change the required size.

What Size Wire Do You Need for 100 Amp Service?

For a typical qualifying single-phase 120/240V dwelling service, NEC 310.12 allows the service conductors to be sized at no less than 83% of the service rating.

For a 100A service:

100A × 83% = 83A required ampacity

That is why a qualifying installation may use:

• 4 AWG copper
• 2 AWG aluminum or copper-clad aluminum

The distinction is important because 4 AWG copper does not have a general 100A ampacity under the common 75°C column. Its 85A rating works in this situation because NEC 310.12 provides a specific dwelling-service allowance.

This allowance is intended for qualifying dwelling services and feeders supplying the dwelling load. It should not automatically be applied to every breaker, branch circuit, feeder, or subpanel simply because the equipment is labeled 100A.

100 Amp Service vs. 100 Amp Subpanel Wire Size

This is where many 100 amp wire size answers become confusing.

A 100A dwelling service and an ordinary 100A subpanel feeder are not necessarily sized under the same NEC provision.

100A Dwelling Service

Where NEC 310.12 applies, a qualifying dwelling service may use:

• 4 AWG copper
• 2 AWG aluminum

The reduced sizing is based on the 83% dwelling-service rule.

100A Whole-Dwelling Feeder

A feeder that qualifies under NEC 310.12 because it supplies the entire load associated with the dwelling can generally use the same:

• 4 AWG copper
• 2 AWG aluminum

Ordinary 100A Subpanel or Feeder

A typical subpanel that serves only part of a dwelling—for example, a garage, workshop, addition, or group of circuits downstream from a larger main service—normally does not qualify simply because it is located in a house.

Under common 75°C general ampacity conditions, the starting sizes are instead:

• 3 AWG copper
• 1 AWG aluminum

This distinction explains why searches for what size wire for 100 amp service sometimes return #4 copper while searches for 100 amp sub panel wire size return #3 copper. They are not necessarily contradictory answers; they can be answers to different installations.

What Size Wire for a 100 Amp Breaker?

For a standard 100 amp breaker protecting an ordinary feeder or subpanel, 3 AWG copper or 1 AWG aluminum is a common starting point when the conductors and equipment terminations are evaluated using the 75°C ampacity column of NEC Table 310.16.

However, the breaker rating alone does not determine the final wire size. If the 100A breaker is associated with a qualifying single-phase dwelling service or a feeder supplying the entire dwelling load, NEC 310.12 may permit 4 AWG copper or 2 AWG aluminum instead.

That is why the answer to what size wire for a 100 amp breaker depends on what the breaker is protecting:

These sizes are starting points rather than universal answers. Terminal temperature ratings, conductor type, ambient temperature, adjustment factors, calculated load, continuous loads, voltage drop, and locally adopted electrical-code requirements can require a different or larger conductor.

Copper vs. Aluminum Wire for 100 Amp Service

Both copper and approved aluminum conductors can be used in 100A installations when correctly sized and installed. The important difference is that aluminum generally requires a larger AWG size to provide equivalent ampacity.

Copper Wire

Copper has higher conductivity than aluminum, so a smaller copper conductor can generally carry the same current.

For the applications discussed above:

• 4 AWG copper can serve a qualifying 100A dwelling service under NEC 310.12.
• 3 AWG copper provides 100A of ampacity under common 75°C general ampacity conditions.

Copper also allows a smaller conductor for a given ampacity, which can be useful where conductor size and termination space matter.

However, copper typically costs more than aluminum, especially as conductor size increases.

Aluminum Wire

Aluminum can be a practical alternative for service and feeder conductors, but it normally requires a larger conductor size.

Typical examples include:

• 2 AWG aluminum for a qualifying 100A dwelling service under NEC 310.12.
• 1 AWG aluminum for an ordinary 100A feeder under common 75°C general ampacity conditions.

Modern aluminum building conductors should be used only with equipment and terminations identified for the conductor material and size. Follow the equipment manufacturer's conductor preparation and torque instructions rather than assuming aluminum requires one universal installation method.

100 Amp Wire Size by Distance

Wire length affects voltage drop, but it does not automatically change the conductor's required ampacity.

That distinction matters.

Ampacity determines whether a conductor can safely carry the required current without exceeding its permitted temperature. Voltage drop determines how much voltage is lost along the run.

50-Foot Runs

For relatively short runs such as around 50 feet, the normal code-compliant conductor size may provide acceptable voltage performance in many installations.

Do not automatically increase the wire size simply because a run reaches an arbitrary distance. Consider:

• actual load current;
• system voltage;
• copper or aluminum conductor;
• one-way run length;
• allowable voltage drop.

100-Foot Runs

At around 100 feet, voltage drop becomes more important to evaluate, particularly when the circuit will operate near its design load.

There is no universal rule saying that a 100-foot 100A run must use one specific oversized conductor.

That is why the original idea that a 100-foot 100A service automatically requires something as large as 3/0 copper should not be retained. Instead, calculate voltage drop for the actual installation and increase conductor size only where the calculation or other requirements justify it.

Longer Runs and Voltage Drop

Longer runs may justify moving to a larger conductor even when the original conductor already meets ampacity requirements.

NEC informational guidance has commonly used approximately 3% voltage drop on a feeder or branch-circuit portion and 5% total feeder-plus-branch-circuit drop as a reasonable efficiency target. These values should not automatically be treated as universal mandatory limits; the applicable code, equipment instructions, and local requirements should be checked.

For a long 100A feeder, calculate voltage drop based on the actual system rather than relying on a generic "50 ft / 100 ft / 150 ft" wire-size table.

What Else Affects 100 Amp Wire Size?

The service or breaker rating is only one part of conductor sizing.

Temperature Rating and Terminations

Wire insulation may carry a 75°C or 90°C rating, but that does not mean the highest insulation-temperature ampacity can automatically be used.

The conductor, breaker, panel, terminals, and applicable NEC rules must all support the temperature column used for sizing.

This is particularly important when comparing values such as:

• 4 AWG copper;
• 3 AWG copper;
• 2 AWG aluminum;
• 1 AWG aluminum.

A higher insulation rating can be useful when applying adjustment or correction factors, but it does not automatically override a lower equipment-termination limit.

Current-Carrying Conductors and Derating

Ampacity tables are based on specified installation conditions. Multiple current-carrying conductors installed together can require adjustment factors.

A conductor that appears large enough from a basic chart may therefore require additional evaluation when installed in a crowded raceway or cable configuration.

Ambient Temperature

High ambient temperature can reduce allowable ampacity.

Locations such as hot attics, rooftops, or other high-temperature environments may require temperature correction.

Wiring Method and Insulation

Conductor type and installation method also matter.

THHN/THWN-2 conductors in raceway, service-entrance cable, and other wiring methods are not interchangeable simply because the conductors have the same AWG size. Use the ampacity and installation rules applicable to the actual wiring method.

Calculated Load

A 100A panel rating does not mean every installation continuously carries exactly 100 amps.

Service and feeder sizing must also satisfy the applicable load-calculation requirements. Certain continuous loads and specialized equipment have additional sizing rules.

Local NEC Adoption and Amendments

The NEC is a model code, and jurisdictions adopt particular editions and may add local amendments. NFPA currently lists the 2026 NEC as the current edition, but the edition legally applicable to a project depends on the authority having jurisdiction.

Before changing service conductors or installing a subpanel, confirm the adopted requirements, permitting process, utility requirements, and inspection rules for the installation.

What Size Ground Wire for 100 Amp Service?

The phrase "100 amp ground wire size" can refer to two different conductors, so there is no safe universal answer without identifying which one you mean.

Equipment Grounding Conductor for a 100A Feeder

For an equipment grounding conductor sized from a 100A overcurrent device, NEC Table 250.122 lists:

• 8 AWG copper
• 6 AWG aluminum or copper-clad aluminum

as the table sizes for 100A protection, subject to the rest of the applicable grounding rules.

Grounding Electrode Conductor at the Service

A grounding electrode conductor is different. Its sizing is governed primarily by the size of the largest ungrounded service-entrance conductor and the type of grounding electrode rather than simply by saying "100 amp."

NEC Table 250.66, for example, places copper service conductors 2 AWG or smaller and aluminum service conductors 1/0 AWG or smaller in a row corresponding to an 8 AWG copper or 6 AWG aluminum grounding electrode conductor. However, electrode-specific provisions and exceptions also apply.

In other words, determine first whether you mean the equipment grounding conductor or the grounding electrode conductor before selecting a size.

Related Electrical Guides

If you're comparing conductor sizes at other current levels, continue with:

• What Size of Wire for a 30 Amp Breaker?
• 50-Amp Wire Size: Copper, Aluminum, and Run-Length Guide
• What Is Involved in Upgrading to 200 Amp Service?

Planning for Home Backup?

Correct service and feeder sizing helps your home's electrical system safely distribute utility power, but an outage creates a different problem: the grid supply itself is unavailable.

If you're planning for outages as well as everyday electrical capacity, BLUETTI home battery backup solutions provide a separate path for keeping selected household loads powered when utility service is interrupted. The backup system should be sized and integrated according to the appliances or circuits you want to support rather than being treated as a substitute for correct service wiring.