The correct wire size for garage subpanel 100 amp feeder installations is 2 AWG copper conductors on a 100-amp double-pole breaker at the main electrical panel, or 1/0 aluminum conductors as a cost-effective alternative. The feeder requires 4-wire configuration with two hot legs, one neutral, and one grounding conductor. This article explains the NEC math behind the wire specification, the important choice between copper and aluminum SER cable, why the dwelling service exception does not apply, the difference from 60 amp subpanels, grounding and neutral isolation at the subpanel, and complete installation guidance for a code-compliant 100 amp garage subpanel.

Across thousands of Southwire and Cerrowire orders shipped from IB Lighting nationwide, our electrician-led team advises DIY installers and licensed electricians on the correct wire size for residential garage subpanel installations. Founded in 2013 by a licensed Master Electrician with field experience across DTE Energy and Consumers Energy projects in Michigan, IB Lighting resells the full Southwire and Cerrowire electrical wire and cable lineup. This article is the walkthrough we provide to buyers researching wire size for their 100 amp garage subpanel installation, using specifications aligned with the current National Electrical Code (NFPA 70) and industry practice from the International Association of Electrical Inspectors.

The Fast Answer: Wire Size for Garage Subpanel 100 Amp Feeder

For a 100 amp garage subpanel feeder, use 2 AWG copper conductors or 1/0 aluminum conductors on a 100-amp double-pole breaker at the main electrical panel. The feeder cable is typically SER (Service Entrance Cable, Round) in a 4-wire configuration containing two hot legs, one neutral, and one grounding conductor. The feeder terminates at a 100 amp subpanel with the neutral bus isolated from the grounding bus. For detached garages, an additional ground rod is required at the subpanel location per NEC 250.32. This is the correct wire size for garage subpanel 100 amp feeder installations in both attached and detached garage applications.

The complete feeder specification

The complete specification for a wire size for garage subpanel 100 amp feeder installation includes: 2 AWG copper conductors with 115 amp ampacity per NEC Table 310.16 (or 1/0 aluminum with 120 amp ampacity as alternative), 100-amp double-pole breaker at the main electrical panel, 240-volt feeder from a two-pole 120/240V residential panel, 4-wire configuration with separate neutral and grounding conductors per NEC 250.32(B), neutral bus isolated from grounding bus at the subpanel, and 8 AWG copper grounding electrode conductor to a ground rod for detached garage installations.

Common cable configurations for 100 amp feeders

The two most common cable configurations for 100 amp subpanel feeders are SER 2-2-2-4 copper (two 2 AWG copper hots and 2 AWG copper neutral plus a 4 AWG copper grounding conductor) and SER 2/0-2/0-2/0-1 aluminum (two 1/0 aluminum hots and 1/0 aluminum neutral plus a 1 AWG aluminum grounding conductor). Both configurations satisfy the 100 amp ampacity requirement with appropriate margin.

Standard breaker sizing for a 100 amp subpanel feeder

A 100-amp double-pole breaker at the main electrical panel is the standard overcurrent protection for a 100 amp garage subpanel feeder. The subpanel itself may include a 100 amp main breaker for local disconnect capability, or be “sub-fed” without a local main breaker relying on the main panel breaker for protection. Local jurisdictions and installation preference determine whether a local disconnect is required or preferred.

The NEC Math Behind the 2 AWG Copper Answer for a 100 Amp Subpanel

Understanding the calculation that drives the 2 AWG copper requirement for a wire size for garage subpanel 100 amp installation prevents common sizing mistakes and clarifies the copper vs aluminum decision.

Step 1: Verify the small conductor rule does not apply

NEC 240.4(D) small conductor rule limits 14, 12, and 10 AWG copper to specific maximum breaker sizes regardless of ampacity. This rule does not apply to wire sizes larger than 10 AWG. For a 100 amp subpanel feeder using 2 AWG copper or 1/0 aluminum, the small conductor rule is not a limiting factor and standard NEC Table 310.16 ampacity applies.

Step 2: Apply NEC Table 310.16 ampacity requirements

The feeder wire must have ampacity equal to or greater than the breaker size. For a 100 amp breaker at 75 degrees Celsius per NEC Table 310.16: 3 AWG copper has 100 amp ampacity (exact match, no margin), 2 AWG copper has 115 amp ampacity (15 amp margin), 1 AWG copper has 130 amp ampacity (30 amp margin), 1/0 aluminum has 120 amp ampacity (20 amp margin), and 2/0 aluminum has 135 amp ampacity (35 amp margin).

Step 3: Select 2 AWG copper for standard installations

2 AWG copper is the standard recommendation for 100 amp subpanel feeders because the 115 amp ampacity provides meaningful safety margin over the 100 amp breaker rating while remaining reasonably priced. 3 AWG copper with exact 100 amp ampacity provides no margin and is generally not recommended by professional installers. 1 AWG copper provides more margin but adds significant cost without meaningful benefit for typical residential applications.

Step 4: Consider 1/0 aluminum as cost-effective alternative

1/0 aluminum SER is significantly less expensive than 2 AWG copper SER for the same 100 amp application, with cost savings that can exceed 40 to 60 percent on long feeder runs. The 120 amp ampacity provides better margin than 2 AWG copper. Aluminum requires specific termination methods (antioxidant compound and torque-specified connections) but is a mature, widely-accepted material for subpanel feeders.

Step 5: Verify local code amendments

Some local jurisdictions have amendments to NEC that may prohibit aluminum for subpanel feeders or require specific installation methods. Some jurisdictions require 1 AWG copper for all 100 amp feeders regardless of NEC 240.4 rules. Always check local code requirements before finalizing wire purchase because local amendments override NEC minimum specifications when they are more restrictive.

Pro tip: When choosing between copper and aluminum for a 100 amp subpanel feeder, calculate the total cost including materials, installation labor, and any specific tooling required. Aluminum termination requires antioxidant compound and torque-wrench installation at specified values. If you are hiring an electrician, ask whether they price copper and aluminum installations differently. Some electricians charge more for aluminum installations because of the specialized termination requirements, which can partially offset the material cost savings of aluminum.

Copper vs Aluminum SER Cable for 100 Amp Subpanel Feeders

The choice between copper and aluminum for the wire size for garage subpanel 100 amp feeder is a significant decision that affects material cost, installation complexity, and long-term reliability.

Copper SER 2-2-2-4 characteristics

Copper SER cable in the 2-2-2-4 configuration (two 2 AWG copper hots and 2 AWG copper neutral plus 4 AWG copper grounding) provides the highest reliability and easiest installation for 100 amp subpanel feeders. Copper conductors terminate with standard mechanical connectors without special preparation, resist corrosion at connection points, and have longer track record in residential installations.

Aluminum SER 2/0-2/0-2/0-1 characteristics

Aluminum SER cable in the 2/0-2/0-2/0-1 configuration provides equivalent electrical capacity to copper at significantly lower material cost. Aluminum conductors require antioxidant compound at all terminations to prevent oxidation-related connection failures, specific torque values for termination screws to ensure adequate mechanical connection, and periodic re-tightening of terminations during initial service life.

Cost comparison at typical distances

At current wire prices, 100 feet of copper SER 2-2-2-4 costs approximately 300 to 500 dollars while 100 feet of aluminum SER 2/0-2/0-2/0-1 costs approximately 150 to 250 dollars. On a 100 foot feeder run, the cost savings from choosing aluminum is meaningful. On very long runs (200 to 300 feet or more), the aluminum savings become significant.

Termination reliability considerations

Aluminum-to-aluminum terminations at both ends (main panel breaker lug and subpanel main terminal) provide the most reliable long-term connections. If the main panel breaker is copper-only rated, the transition creates additional termination complexity. Modern circuit breakers rated CU-AL accept both copper and aluminum conductors, but verify breaker ratings before purchasing aluminum SER.

When to choose copper vs aluminum

Choose copper SER for shorter runs where cost savings are minimal, installations where the electrician charges more for aluminum installation, applications where warranty or insurance requirements specify copper, or personal preference for the more traditional material. Choose aluminum SER for longer runs where cost savings are significant, budget-conscious installations, and installations where the entire electrical system already uses aluminum service entrance.

Why NEC 310.12 Dwelling Service Exception Does Not Apply to Garage Subpanels

One of the most common misconceptions about the wire size for garage subpanel 100 amp installation is confusion about the NEC 310.12 dwelling service exception that allows smaller wire for certain residential applications.

What NEC 310.12 actually says

NEC 310.12 provides an exception to standard ampacity requirements for “services and feeders that supply the entire load associated with a one-family dwelling.” Under this exception, 4 AWG copper is permitted for 100 amp services and feeders that supply the entire dwelling load, even though standard NEC Table 310.16 ampacity for 4 AWG copper is 85 amps (below the 100 amp breaker rating).

Why the exception doesn’t apply to garage subpanels

The critical phrase in NEC 310.12 is “supplying the entire load associated with a one-family dwelling.” A garage subpanel supplies only the garage load, not the entire dwelling load. Therefore the dwelling service exception does not apply and standard NEC Table 310.16 ampacity requirements must be followed. This means 2 AWG copper minimum for 100 amp garage subpanel feeders, not 4 AWG copper.

Common mistake based on this misconception

Some installers see 4 AWG copper working on 100 amp residential services and assume the same wire size applies to any 100 amp application. This mistake is understandable because the visual difference between a service feeder and a subpanel feeder is not obvious, and the same 4 AWG copper cable products appear appropriate in electrical wholesalers. However, using 4 AWG copper on a 100 amp garage subpanel is a code violation and fails inspection.

Verifying the classification for your installation

The garage subpanel is a feeder from the main panel, not from the service equipment (meter). This means the dwelling service exception does not apply. The distinction is: the service conductors go from the utility to the main panel and get the exception, while feeder conductors go from the main panel to subpanels and do not get the exception. If in doubt, use the more conservative 2 AWG copper specification which is always compliant.

60 Amp vs 100 Amp Garage Subpanel: When Each Applies

Choosing between a 60 amp and 100 amp garage subpanel affects wire cost, installation complexity, and future flexibility for adding garage equipment.

60 amp subpanel typical applications

A 60 amp garage subpanel is appropriate for typical residential garages with modest electrical demand: general lighting and outlets, garage door opener, one Level 2 EV charger, and occasional workshop tools. The 60 amp size uses less expensive 6 AWG copper wire and fits in most existing electrical service configurations without service upgrades.

100 amp subpanel typical applications

A 100 amp garage subpanel is appropriate for garages with higher electrical demand: multiple Level 2 EV chargers (two chargers plus other loads), permanent workshop with multiple tools including welder and large air compressor, garage-based home business equipment, or extensive additional lighting for large spaces. The 100 amp size uses larger 2 AWG copper wire but accommodates future load additions without another upgrade.

Future-proofing considerations

Many installers recommend installing a 100 amp subpanel initially even when current demand is modest because the incremental cost of the larger wire is much lower than the cost of upgrading from 60 amp to 100 amp later. Adding a second EV charger, converting to induction cooking, or expanding workshop equipment all become easier with 100 amp subpanel capacity available.

Main panel capacity constraints

A 100 amp subpanel requires the main electrical panel to have 100 amp of capacity available for the feeder. Homes with 200 amp main service typically have this capacity available after accounting for other loads. Homes with 100 amp or 150 amp main service may not have adequate capacity for a 100 amp subpanel and require main service upgrade before installation. Perform a NEC 220 load calculation before choosing between 60 amp and 100 amp subpanel sizes.

The 4-Wire Feeder Requirement for All New Subpanels

All new subpanel feeders including 100 amp installations must use 4-wire configurations per NEC 250.32(B) since the 2008 National Electrical Code adoption.

The 4-wire requirement applies regardless of subpanel size

The 4-wire feeder requirement applies to all new subpanel installations regardless of amperage. Whether the subpanel is 60 amp, 100 amp, 200 amp, or larger, separate neutral and grounding conductors are required. The 3-wire feeder configuration is no longer permitted for new installations for any subpanel size in any location.

How the 4-wire configuration works for 100 amp subpanels

The 100 amp 4-wire feeder includes two hot conductors (two poles of the 240 volt supply), one neutral conductor (returning 120 volt loads to the main panel neutral), and one equipment grounding conductor (providing fault current path to the main panel grounding system). All four conductors are sized appropriately for the 100 amp feeder capacity.

Grounding conductor sizing within the feeder cable

The equipment grounding conductor within an SER 2-2-2-4 cable is 4 AWG copper (the “4” in the cable designation). This is smaller than the hot and neutral conductors because equipment grounding conductor sizing per NEC 250.122 is based on the overcurrent device rating rather than the ampacity of the phase conductors. For a 100 amp breaker, NEC 250.122 requires minimum 8 AWG copper equipment grounding conductor, and the 4 AWG in SER cable comfortably exceeds this minimum.

Grandfathered pre-2008 installations

Existing 3-wire subpanel feeder installations installed before 2008 are grandfathered and legal to continue using as installed. Grandfathering ends when any major electrical modification is made to the subpanel or its feeder. Upgrading from a 60 amp to a 100 amp subpanel always requires new wire and triggers the 4-wire requirement for the replacement installation.

Attached Garage vs Detached Garage Grounding Requirements

The attached vs detached distinction that affects 60 amp subpanel installations applies equally to 100 amp installations and remains one of the most important planning decisions.

Attached garage 100 amp subpanel installations

Attached garages share a common structure with the main house and use the 4-wire feeder without additional grounding electrode installation at the subpanel. The subpanel grounding bus is bonded to the enclosure and connected to the equipment grounding conductor in the feeder, which returns to the main panel grounding electrode system. No additional ground rod is required for attached garage 100 amp subpanels.

Detached garage 100 amp subpanel installations

Detached garages require the same 4-wire feeder plus installation of a ground rod at the subpanel location per NEC 250.32. Because 100 amp subpanels use larger feeder conductors than 60 amp subpanels, the grounding electrode conductor from subpanel grounding bus to the ground rod is also larger, sized per NEC 250.66 based on the ungrounded (hot) feeder conductor size.

Larger buried feeder for detached garage 100 amp

Detached garage 100 amp subpanel installations typically require larger buried cable than 60 amp installations. SER 2-2-2-4 copper or SER 2/0-2/0-2/0-1 aluminum can be direct-buried per NEC requirements for the underground portion. The larger cable is heavier to install and requires wider trenches, but the installation approach is otherwise similar to smaller feeder installations.

Verifying detached vs attached classification

The same borderline classification issues that apply to 60 amp subpanels apply to 100 amp subpanels. Attached garages connected only by a breezeway, converted spaces, or structures with shared roofs but separate walls all require case-by-case classification by the local electrical inspector. Always verify classification before beginning wire purchases because the grounding requirements differ significantly.

Grounding and Neutral Isolation at the 100 Amp Subpanel

Neutral and grounding isolation at the subpanel is critical for all subpanel sizes, and the requirement becomes more consequential at 100 amp because higher currents flow through the isolated bus bars.

The neutral bar must be isolated

At the 100 amp subpanel, the neutral bar must be electrically isolated from the subpanel enclosure. Most 100 amp subpanel enclosures ship with a neutral bar that is bonded to the enclosure through a green bonding screw or bonding strap. For subpanel installation, this bonding must be removed or the neutral bar must be isolated using the specific method provided by the subpanel manufacturer.

The grounding bar must be bonded

The grounding bar in the 100 amp subpanel must be bonded to the enclosure. 100 amp subpanels typically include a separate grounding bar as standard equipment, or use the enclosure directly as the grounding termination point. The bonding of grounding bar to enclosure is verified during electrical inspection.

Feeder connections at the 100 amp subpanel

The 100 amp feeder terminates at the subpanel main lugs (or main breaker if included). The two hot conductors connect to the two-pole main lugs, the neutral connects to the isolated neutral bar, and the grounding conductor connects to the bonded grounding bar. All connections use appropriately-rated lugs matched to the specific wire size and type (copper or aluminum).

Torque requirements at 100 amp terminations

100 amp connections require higher torque values than smaller subpanel connections because of the larger conductor size. Follow the specific torque values specified on the subpanel and breaker labels using a properly calibrated torque wrench. Under-torqued connections create loose connections that overheat during operation, while over-torqued connections can damage the conductor or terminal.

Grounding Electrode Conductor Sizing for 100 Amp Subpanels

The grounding electrode conductor sizing for detached wire size for garage subpanel 100 amp installations is larger than for 60 amp installations because it is sized based on the ungrounded feeder conductor size.

NEC 250.66 grounding electrode conductor sizing

NEC 250.66 specifies minimum grounding electrode conductor sizes based on the ungrounded (hot) feeder conductor size. For 2 AWG copper hot conductors, the minimum grounding electrode conductor is 8 AWG copper. For 1/0 aluminum hot conductors, the minimum grounding electrode conductor is 6 AWG copper.

Ground rod installation for detached 100 amp subpanels

The ground rod for a detached 100 amp garage subpanel is typically a single 8-foot copper-clad ground rod driven at least 8 feet into the earth per NEC 250.53. The top of the rod is flush with or below finished grade. The 8 AWG copper (or 6 AWG for aluminum feeder installations) grounding electrode conductor connects the subpanel grounding bus to the ground rod using an approved acorn clamp or listed grounding connector.

Two-rod requirement for high-resistance soil

If a single ground rod cannot achieve 25 ohms or less resistance to earth per NEC 250.53(A)(2), two ground rods must be installed at least 6 feet apart, connected together by a grounding electrode conductor. This requirement applies to all subpanel sizes including 100 amp installations. Most residential installations satisfy the resistance requirement with a single rod.

Grounding electrode conductor routing

The grounding electrode conductor must run in a continuous piece without splices except at approved accessible connection points. The conductor typically routes from the subpanel grounding bus through a knockout in the subpanel enclosure, down through the wall or exterior of the structure, to the ground rod connection point. Protect the conductor from physical damage where it passes through walls or exposed areas.

Wire Type Selection for 100 Amp Subpanel Feeders

Wire type selection for the wire size for garage subpanel 100 amp feeder depends on installation environment, physical protection requirements, and cost considerations.

SER cable for standard installations

SER (Service Entrance Cable, Round) is the most common wire type for 100 amp subpanel feeders because it is manufactured specifically for feeder applications, is available in the exact 4-conductor configurations needed, and can be used in most installation environments including indoors, outdoors above ground, and in some cases direct-buried (depending on specific SER type). SER 2-2-2-4 copper and SER 2/0-2/0-2/0-1 aluminum are the standard configurations.

Individual THHN or THWN conductors in conduit

For 100 amp subpanel feeders installed in conduit, use four individual 2 AWG THHN or THWN copper conductors (or 1/0 aluminum): two hots (black and red), one neutral (white), and one green or bare copper grounding. Conduit installations require 1-1/4 inch or 1-1/2 inch conduit depending on wire size and fill calculations per NEC 314.16. Larger conduit costs more but simplifies wire pulling.

USE-2 aluminum for direct burial applications

Some detached garage installations use USE-2 (Underground Service Entrance) aluminum cable for the direct-buried portion of the 100 amp feeder. USE-2 is specifically rated for direct burial and provides an alternative to SER cable for underground applications. USE-2 is commonly available in aluminum configurations that provide cost-effective installations.

NM-B rarely used at 100 amp

NM-B (Non-Metallic sheathed cable, Type B) is technically available in the wire sizes needed for 100 amp subpanel feeders, but is rarely used at this size because SER is the more common product for feeder applications. If NM-B is used, verify it is rated for the intended installation environment because NM-B cannot be used outdoors, underground, or in wet locations.

Choosing the Right Wire Product for Your 100 Amp Subpanel Feeder

Understanding which specific wire products fit the wire size for garage subpanel 100 amp specification helps buyers select the correct product for their installation.

Southwire SER 2-2-2-4 copper

Southwire manufactures a comprehensive line of SER cable including 2-2-2-4 copper configurations for 100 amp subpanel feeders. Southwire SER 2-2-2-4 copper is available in standard reel lengths from 25 feet to 500 feet. For typical garage subpanel installations with feeder runs from 30 to 150 feet, 50-foot or 100-foot reels provide enough wire with modest excess.

Southwire SER 2/0-2/0-2/0-1 aluminum

Southwire aluminum SER 2/0-2/0-2/0-1 provides cost-effective alternative to copper for 100 amp subpanel feeders. Aluminum is significantly less expensive per foot than copper, with savings that become substantial on longer feeder runs. Southwire aluminum SER meets the same UL standards as copper SER and is accepted by electrical inspectors nationwide.

Cerrowire alternatives

Cerrowire manufactures comparable lines of SER 2-2-2-4 copper and SER 2/0-2/0-2/0-1 aluminum. Cerrowire and Southwire products are electrically equivalent and meet the same standards. Selection typically depends on regional availability, pricing, and personal preference. Both brands are widely accepted for 100 amp subpanel feeder installations.

THHN individual conductors for conduit installations

For 100 amp subpanel feeders installed in conduit, purchase four individual 2 AWG THHN or THWN copper conductors (or 1/0 aluminum): one black, one red, one white, and one green (or bare copper) for grounding. Stranded THHN is essential at this wire size because 2 AWG solid conductor is impractical to pull through typical conduit runs with bends. Larger conduit (1-1/4 to 1-1/2 inch) accommodates the larger conductors.

Wire and cable pricing considerations

Wire costs for 100 amp subpanel installations vary significantly based on copper vs aluminum choice and total wire length. Copper SER 2-2-2-4 costs approximately 3 to 5 dollars per foot in typical retail quantities. Aluminum SER 2/0-2/0-2/0-1 costs approximately 1.50 to 2.50 dollars per foot. For a typical 75 foot installation, copper wire costs 225 to 375 dollars while aluminum wire costs 115 to 190 dollars.

100 Amp Subpanel for Multiple EV Chargers and Workshops

The primary drivers for choosing the wire size for garage subpanel 100 amp specification over 60 amp are multiple EV chargers and dedicated workshop equipment.

Multiple EV charger accommodation

A 100 amp garage subpanel comfortably accommodates two Level 2 EV chargers plus typical garage loads. Two 40 amp EV charger circuits totaling 80 amps of installed capacity fits within the 100 amp subpanel with 20 amp remaining for lighting, outlets, and garage door opener. Some installations use load management systems to increase effective capacity even further.

Workshop equipment capacity

Serious residential workshops with welders, air compressors, dust collection systems, table saws, planers, and multiple outlets easily exceed 60 amp subpanel capacity. A 100 amp subpanel provides room for a 40 or 50 amp welder circuit, 30 amp compressor circuit, multiple 20 amp outlet circuits, dedicated 20 amp lighting circuit, and dedicated circuits for major tools without concern about simultaneous demand exceeding subpanel capacity.

Home business installations

Garages converted to home business use (auto repair, woodworking studio, small manufacturing, gym) often require 100 amp subpanel capacity to support commercial-grade equipment. Some equipment may require 240 volt circuits at 30 to 60 amp ratings which are more easily accommodated by 100 amp subpanel capacity than 60 amp.

Future-proofing for evolving needs

Many installers recommend 100 amp subpanels for new construction and major renovations because household electrical demand has been increasing over time. EV adoption, electric appliance conversion (induction cooking, heat pump water heaters), and workshop equipment expansion all drive higher garage electrical demand. Installing 100 amp initially avoids future upgrade costs.

Voltage Drop Considerations for Long 100 Amp Feeder Runs

Voltage drop for the wire size for garage subpanel 100 amp feeder is more critical than for smaller feeders because higher current increases drop for the same wire size and distance.

The 3 percent voltage drop guideline

Voltage drop for feeder circuits should target the 3 percent maximum per NEC 210.19 informational note. For a 240 volt 100 amp feeder, this means the drop should not exceed 7.2 volts at maximum feeder load. Excessive voltage drop on the subpanel feeder affects all downstream branch circuits and connected loads.

2 AWG copper distance limits at 100 amp load

For a 100 amp feeder at 240 volts, 2 AWG copper stays within the 3 percent voltage drop guideline for runs up to approximately 80 to 100 feet depending on actual load conditions. Most attached garage installations stay within this distance limit. Detached garage installations with buried feeders sometimes approach or exceed this limit and require larger wire.

1/0 aluminum distance limits

1/0 aluminum has slightly different voltage drop characteristics than 2 AWG copper because of different conductor cross-sectional area and different resistance per foot. 1/0 aluminum stays within the 3 percent voltage drop guideline for runs up to approximately 90 to 110 feet at 100 amp load. Voltage drop is actually slightly better for aluminum than copper at this ampacity because the aluminum conductor is larger.

When to upgrade wire size for long runs

For 100 amp feeder runs from 100 to 150 feet, upgrade to 1 AWG copper or 2/0 aluminum to maintain voltage drop under 3 percent. For runs beyond 150 feet, upgrade to 1/0 copper or 3/0 aluminum. These upgrades add material cost but ensure the 100 amp subpanel receives full voltage at maximum load.

5 Common Mistakes When Wiring a 100 Amp Garage Subpanel

The mistakes below cause the most frequent buyer questions about wire size for garage subpanel 100 amp installations.

Mistake 1: Using 4 AWG copper based on the dwelling service exception

The most common mistake is using 4 AWG copper on a 100 amp garage subpanel feeder based on the NEC 310.12 dwelling service exception. This exception only applies to services and feeders supplying the ENTIRE dwelling load, not to subpanels supplying only a garage. Garage subpanel feeders must use standard NEC Table 310.16 ampacity requirements, meaning 2 AWG copper minimum for 100 amp applications.

Mistake 2: Skipping antioxidant compound on aluminum terminations

Aluminum conductors require antioxidant compound (also called “no-ox” compound) applied to the stripped conductor before termination. Skipping this step allows aluminum oxide to form at the connection, which increases resistance and causes overheating during operation. Aluminum termination failures from missing antioxidant compound are one of the most common electrical fire causes.

Mistake 3: Under-torquing terminations on larger conductors

100 amp connections require specific torque values that are higher than smaller subpanel connections. Under-torqued connections create loose electrical connections that overheat during operation. Always use a properly calibrated torque wrench and follow the specific torque values on the subpanel and breaker labels. This is more critical at 100 amp than at 60 amp because higher current increases the consequences of loose connections.

Mistake 4: Undersizing the grounding electrode conductor for detached installations

For detached garage 100 amp subpanel installations, the grounding electrode conductor must be sized per NEC 250.66 based on the ungrounded (hot) feeder conductor size. For 2 AWG copper hots, minimum is 8 AWG copper. For 1/0 aluminum hots, minimum is 6 AWG copper. Using smaller wire is a code violation. Larger sizes are acceptable but the minimums must be met.

Mistake 5: Leaving the neutral bar bonded at the subpanel

Same as 60 amp subpanel installations, 100 amp subpanels ship with the neutral bar bonded to the enclosure and this bonding must be removed for subpanel installation. This mistake happens on both 60 amp and 100 amp installations, but the consequences are greater at 100 amp because larger unbalanced current can flow through the equipment grounding conductor when neutral and ground are improperly bonded at the subpanel.

Conclusion

The correct wire size for garage subpanel 100 amp feeder installations is 2 AWG copper conductors on a 100-amp double-pole breaker at the main electrical panel, or 1/0 aluminum conductors as a cost-effective alternative. SER cable is the most common product type with SER 2-2-2-4 copper or SER 2/0-2/0-2/0-1 aluminum as standard configurations. The NEC 310.12 dwelling service exception does not apply to garage subpanel feeders.

All new 100 amp subpanel installations require 4-wire feeders with separate neutral and grounding conductors, isolated neutral bus at the subpanel, and grounding bus bonded to the subpanel enclosure. Detached garage installations require additional ground rod at the subpanel location per NEC 250.32 with 8 AWG copper grounding electrode conductor for typical copper feeder installations. 100 amp subpanels accommodate multiple EV chargers and workshop equipment that would exceed 60 amp subpanel capacity.

This article was last reviewed and updated in June 2026 to reflect current National Electrical Code requirements and residential garage subpanel installation best practices. Wire sizing and grounding recommendations are based on the current NEC (NFPA 70) and should be verified against your local code amendments. For complex installations or when in doubt, consult a licensed electrician. Subpanel installations involve significant electrical modifications that require permits and inspection in most jurisdictions.