Feeders Must Be Sized for the Load They Supply

Quick Answer

To size a feeder to a dwelling subpanel, calculate the load the subpanel will actually serve, apply the permitted residential demand factors, then choose conductors and overcurrent protection with enough ampacity after accounting for continuous-load requirements, temperature, bundling, and termination limits. The subpanel bus rating does not determine the feeder size — the load calculation does. Inspectors expect to see the feeder breaker, conductor size, four-wire arrangement, and proper grounding and bonding all line up together, not just a big cabinet with a large label on it.

What IRC 2021 Actually Requires

IRC 2021 Chapter 37 covers feeders serving less than 100 percent of the dwelling load. Feeders serving 100 percent of the dwelling load follow Chapter 36 rules. For a typical garage subpanel, workshop panel, in-law suite panel, or room addition panel, Chapter 37 applies.

Section E3704 ties feeder conductor sizing to the calculated load. The calculated feeder load cannot be less than the sum of the supplied branch-circuit loads after permitted dwelling demand factors are applied, per E3704.5. This is not a rule about picking wire by feel — the calculation is the starting point, and the conductor ampacity must meet it. If the feeder serves continuous loads, E3701.2.1 requires those loads to be sized at 125 percent, just as branch circuits are. The demand factors that reduce load for dwelling-unit calculations — notably the standard calculation method for general lighting and small-appliance loads — can reduce the calculated feeder load meaningfully, but they must actually be applied and documented, not assumed.

The feeder must also include the required conductors: two ungrounded conductors, one grounded conductor (neutral), and one equipment grounding conductor for a standard dwelling subpanel. The neutral must be isolated from the enclosure at the downstream panel — the bonding strap or screw that comes factory-installed in many panelboards must be removed in a subpanel. Equipment grounding conductors bond to the enclosure; the neutral does not. This distinction is one of the most consistently violated aspects of garage and outbuilding panel installations.

Conductor material — copper or aluminum — affects the required wire size. Many residential feeders use aluminum because it is significantly less expensive for runs of 30 feet or more, but aluminum requires listed AL/CU terminals, proper preparation, and manufacturer torque specs. Cutting corners on aluminum terminations is a frequent cause of loose connections, overheating, and fire.

Why This Rule Exists

A subpanel hides overloads behind a neat cabinet. If the feeder is undersized, conductors can overheat inside walls, inside conduit, or in the feeder cable before the overcurrent device trips. Feeder conductors often pass through areas where heat buildup is slow to surface — insulated walls, long conduit runs through hot attics, buried conduit in garages. By requiring the feeder to be sized for the actual calculated load, the code ensures the conductor can safely carry everything the downstream panel might legitimately supply.

The rule also closes the "future flexibility" loophole. The question on forums is constant: "Can I just put a 100-amp breaker on it for future flexibility?" The code answer is only if the conductors and the calculated load support it. Installing a large breaker on undersized conductors because "I might add loads later" is not a permitted design method — it is exactly the conductor protection failure the code prohibits.

What the Inspector Checks at Rough and Final

At rough inspection, the inspector examines the feeder route before walls and finishes close. They check the wiring method — cable type, conduit type, conduit fill, support spacing, and physical protection at penetrations. If the feeder passes through an attic, a crawlspace, an exterior wall, or buried conduit, the inspector verifies the wiring method is appropriate for those conditions. A feeder in PVC conduit buried in the yard has different fill, depth, and conductor requirements than the same feeder in NM cable through an interior wall.

They check for a four-wire arrangement. Three-wire feeders — two hots and a shared neutral-ground — were common in older garage and outbuilding work but are no longer generally permitted for new subpanel installations. If they see only three conductors in the conduit or three conductors in the cable, that is a rough-inspection failure for most current-code subpanel installations. They also verify conductor marking: aluminum conductors should be identifiable as such, and the equipment grounding conductor must be present and appropriately sized.

At final inspection, the inspector checks terminations, labeling, panel interior, and the bonding arrangement. The neutral bar must be isolated. The grounding bar must be bonded to the enclosure. Aluminum conductor terminations must be on listed terminals — antioxidant compound should be applied per the connector manufacturer's instructions. The feeder breaker at the source panel is verified against the conductor ampacity. A 60-amp feeder conductor with a 100-amp breaker protecting it fails. The panel directory at both the source and the subpanel should clearly identify the feeder relationship.

Common final failures include neutral-ground bonding at the subpanel enclosure (the bonding strap left in), aluminum conductors on terminals not listed for aluminum, and feeder breakers that are too large for the conductors installed.

What Contractors Need to Know

Write down the load calculation before ordering materials. That single step prevents the two most expensive feeder mistakes: over-specifying conductors and buying unnecessary copper, or under-specifying and having to repull the feeder. For a garage workshop subpanel serving lights, receptacles, and occasional high-draw tools, the calculation often supports a 60-amp feeder rather than the 100-amp run the homeowner assumes they need. Matching the feeder to the actual load is both code-required and budget-friendly.

Aluminum feeders deserve serious consideration at 60 amps and above. At a 100-amp feeder, aluminum conductors cost roughly 60 to 70 percent less than copper equivalents. The savings are real, and aluminum is perfectly code-compliant when the termination details are handled correctly. Specifically: use listed AL/CU lugs, clean and prepare the conductor ends properly, apply antioxidant compound per the lug manufacturer's instructions, and torque to the specified value. Inspectors who see aluminum conductors without proper terminal listings or with obvious oxidation at the lug will write up the connection regardless of how the rest of the installation looks.

The "big panel for future capacity" request is common. Installing a 100-amp or 125-amp bus panel on a 60-amp feeder is allowed — the bus rating can exceed the feeder protection. But the panel directory and the source panel labeling must accurately reflect the feeder size, not the bus rating. A panel labeled "100A subpanel" when it is fed at 60 amps misleads future owners, service electricians, and inspectors about the actual available capacity.

Coordinate with other trades before finalizing the feeder design. HVAC additions, EV chargers, electric hot tubs, and workshop equipment can change the load calculation dramatically. A 60-amp feeder designed for lights and a few receptacles may be completely inadequate when the homeowner decides to add an EV charger and a table saw. Discussing the full load picture before material is ordered — and before trenching or conduit runs are complete — is far cheaper than a repull.

What Homeowners Get Wrong

The most common mistake on every forum thread about garage subpanels is: "I need a 100-amp subpanel." What the person usually means is they want a lot of circuits. Panel bus rating and feeder capacity are different things. You can install a 200-amp bus panel and feed it at 60 amps; the panel bus rating tells you how many circuits you can have and what the maximum possible capacity is — it does not determine what the feeder must be. Confusing panel size with feeder size leads people to buy oversized conductors they do not need or, worse, to buy a large breaker for the source panel that is too big for the feeder conductors they ran.

The neutral-ground bonding mistake is the most dangerous and most consistent subpanel failure homeowners make. Online discussions in r/electricians and r/DIY reliably feature the same scenario: a homeowner installs a garage panel, sees the factory bonding strap connecting the neutral bar to the enclosure, assumes that is correct, and leaves it in. That arrangement is correct at the main service panel, where the neutral is bonded to ground at the service entrance. It is a violation at every downstream panel, where the neutral must be isolated. When the neutral is bonded at both ends, fault currents can return through the equipment grounding path simultaneously, which is exactly the condition the code is preventing.

A third common mistake is thinking a three-wire feeder from an old sub-feed can be reused for a new subpanel. Old three-wire systems — two hots and a combined neutral-ground — were phased out precisely because they create shock hazards at the downstream panel when the neutral becomes compromised. Current code requires a four-wire feeder for new subpanel installations. Reusing three-wire feeders when adding or replacing subpanels is a routine inspection failure.

Finally, homeowners assume they can figure out feeder sizing from a quick internet search and avoid a permit. The permit and inspection exist because feeder installations — conductor sizing, termination methods, bonding — are exactly the type of work that looks correct until something goes wrong. A poorly terminated aluminum conductor can smolder inside an enclosure for months before becoming a fire.

State and Local Amendments

Some jurisdictions enforce IRC Chapter 37 directly for residential electrical work. Others adopt the NEC as the controlling residential electrical code and use the IRC only for structural and building envelope provisions. In NEC jurisdictions, feeder sizing follows NEC Articles 215 and 220, which cover the same core concepts but may have different demand-factor tables or calculation methods.

Local amendments frequently affect documentation requirements. Some AHJs require a formal load calculation submitted with the permit application for any new subpanel. Others accept a verbal description for small installations. Cities with local electrical codes may have specific rules about aluminum conductors, bonding details for detached structures, minimum feeder sizes, or required disconnect arrangements. The three questions to ask any AHJ before buying materials: which code edition controls, is a load calculation required in the permit file, and what conductor material restrictions apply for this installation type?

When to Hire a Licensed Electrician

Hire a licensed electrician for any new subpanel installation, feeder conductor replacement, service upgrade, or modification of source panel equipment. The combination of load calculation, conductor sizing, conduit routing, four-wire feeder requirements, and grounding and bonding details is exactly the package of tasks where DIY errors are most consequential and most common. If the installation is for a detached structure with its own grounding electrode system, or if the feeder will serve an ADU, in-law suite, or rental space, professional design is not optional — it is necessary for both safety and insurance coverage.

Common Violations Found at Inspection

• Feeder breaker larger than conductor ampacity. The most basic feeder failure: the wire cannot be protected by the overcurrent device installed. Found when inspectors compare the breaker size to the conductor gauge.
• No load calculation in the permit file. The feeder may look clean, but there is no documented basis for the conductor and breaker sizes chosen.
• Neutral bonded to the subpanel enclosure. The factory bonding strap left in place at the subpanel creates a parallel return path and is the most common bonding violation in residential subpanel work.
• Three-wire feeder used where four wires are required. Particularly common in garage and outbuilding work where old conductors are reused.
• Aluminum conductors on terminals not listed for aluminum. Wrong lug rating, improper termination technique, or missing antioxidant compound where required by the manufacturer.
• Feeder undersized for the actual installed loads. Heaters, EV chargers, or workshop equipment added after the original design created an overload condition on the installed conductors.
• Conduit too small, improperly routed, or missing physical protection. Conductor damage risk at penetrations or entry points, particularly where conduit transitions to direct-buried cable.
• Subpanel labeled with bus rating instead of feeder capacity. A "100A subpanel" notation that suggests 100 amps are available when the feeder breaker is 60 amps misleads future users and inspectors.