Detached garages look simple on paper, but feeder design is where a lot of installations go sideways. The common failure mode is not ampacity. It is voltage drop, neutral isolation, grounding details, and forgetting that a 120-volt branch circuit in the garage still has to live inside the total feeder-plus-branch-circuit budget. If the panel is 120 to 200 feet away, the minimum conductor allowed by ampacity tables may produce dim lights, nuisance motor issues, and slow EV charging.

The working baseline in the National Electrical Code is still the familiar 3% recommendation for branch circuits and 5% combined feeder plus branch circuit, referenced in NEC 210.19(A)(1) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2. For international comparison, IEC guidance is often applied in a similar way, with 3% commonly used for lighting circuits and 5% for other loads under IEC 60364-5-52.

“When a detached garage feeder is longer than about 100 feet, design around performance first and minimum ampacity second. A clean 2% to 3% feeder drop usually prevents the callback.”
— Hommer Zhao, Technical Director

The Four Checks That Matter

A detached-garage feeder should be checked in this order:

Ampacity: start with NEC Table 310.16 and the terminal temperature limitations, then apply any ambient or conductor-count adjustments if they are relevant.
Voltage drop: use the full one-way distance and keep the feeder low enough that downstream 120-volt branch circuits still fit inside the 5% combined target.
Grounding and bonding: detached structures generally push you into NEC 250.32 rules for a grounding electrode system, isolated neutrals, and an equipment grounding conductor run with the feeder.
Disconnecting means: NEC 225.31 and 225.32 still matter at the detached structure. If the panel is not arranged correctly, the wire-size decision is not your main problem.

A Fast Rule of Thumb for Garage Projects

For a typical 120/240-volt single-phase detached garage, I usually set a feeder target of about 2% to 2.5%. That leaves room for a 120-volt lighting or receptacle branch circuit inside the garage without blowing past 5% combined drop. This is especially useful when the garage may later get a compressor, mini-split, welder, or EV charger.

Quick design target

If the detached structure is more than 100 feet away, treat 2% feeder drop as the preferred design target. If it is over 150 feet, check both copper and aluminum options early because material choice often changes the economics.

“A garage feeder that looks acceptable at 240 volts can still create a bad 120-volt branch circuit. The feeder has to leave room for the loads you have not installed yet.”
— Hommer Zhao, Technical Director

Practical Voltage Drop Comparison Table

The numbers below use the same resistance model as this site’s calculator: 120/240-volt single-phase, one-way distance, and standard copper or aluminum conductor resistivity. These are not replacements for full ampacity checks, but they are accurate enough for design screening.

Scenario	Conductor	Distance	Load	Drop	Result
60A garage subpanel	6 AWG copper	100 ft	240V / 60A	1.98%	Strong
60A garage subpanel	6 AWG copper	150 ft	240V / 60A	2.96%	Usable, tight
60A garage subpanel	4 AWG copper	150 ft	240V / 60A	1.86%	Preferred
60A garage subpanel	2 AWG aluminum	150 ft	240V / 60A	1.92%	Preferred
100A workshop feeder	1/0 aluminum	180 ft	240V / 100A	2.41%	Good margin
20A lighting circuit	12 AWG copper	110 ft	120V / 20A	5.82%	Too high

That last row is the reason detached garage feeders should not be sized in isolation. A feeder might look acceptable, but a 120-volt branch circuit added later for lights or convenience receptacles can eat the rest of the voltage-drop budget immediately.

Example 1: 60A Detached Garage Subpanel at 150 Feet

Assume a 120/240-volt single-phase feeder from the house panel to a detached garage panel, one-way distance 150 feet, design load 60A. A lot of installers will start by looking at conductor ampacity only, then land on a minimum copper size and move on. That is where the second trip begins.

6 AWG copper produces about 7.11 volts of drop at 240 volts, or 2.96%.
4 AWG copper drops about 4.46 volts, or 1.86%.
2 AWG aluminum drops about 4.60 volts, or 1.92%.

If the garage includes a 20A, 120-volt branch circuit for lighting and bench receptacles, that branch circuit can easily add another 2% to 3%. In practice, 4 AWG copper or 2 AWG aluminum is the cleaner decision, because it keeps the feeder low enough that future 120-volt circuits remain comfortable.

Example 2: 100A Hobby Shop With Welder and Mini-Split

Now assume a detached shop 180 feet from the service equipment. The owner wants a 100A panel for a 240V welder, a 240V mini-split, general receptacles, and LED lighting. A practical aluminum option is 1/0 aluminum for the ungrounded and grounded conductors, with the equipment grounding conductor sized per NEC 250.122 and the grounding electrode conductor sized per NEC 250.66.

At 100A and 180 feet, 1/0 aluminum produces about 5.78 volts of drop, or 2.41% at 240 volts. That is good feeder performance. If you instead tried to force a smaller conductor around the ampacity minimum, the welder and motor loads would show it quickly during startup and under heavy use.

Design note

Long detached-building feeders should be checked for present loads and likely future loads. The cost delta from one conductor size up is often much smaller than the labor cost of replacing a buried feeder.

Grounding and Bonding Errors to Avoid

NEC 250.32 is where detached-garage work gets expensive if you have to revisit it. The most common mistakes are not separating the neutral bar from the equipment grounding bar in the detached building, forgetting the grounding electrode system, or assuming the old three-wire detached building habits still apply universally.

Run an equipment grounding conductor with the feeder. That is the normal modern design path.
Keep the neutral isolated in the detached structure panel. Do not install the main bonding jumper there unless the service disconnect is there, which it is not in a standard detached-garage feeder arrangement.
Install the required grounding electrode system at the detached structure and size the grounding electrode conductor correctly.
Remember that NEC 225.30 generally limits you to one feeder or one branch circuit to the detached structure, except where a listed exception applies.

“I trust a garage feeder only after the neutral bar, grounding electrode system, and disconnect arrangement all tell the same story. Good conductor sizing cannot rescue a bad bonding scheme.”
— Hommer Zhao, Technical Director

Copper vs. Aluminum for Detached Garages

Copper wins on compactness and lower resistance. Aluminum wins on cost, especially as feeder size climbs. For detached garages, aluminum often becomes very attractive at 60A, 100A, and 125A feeders because the trenching and conduit work are already the dominant labor costs.

The correct comparison is not “copper vs. aluminum at the same AWG.” The correct comparison is “which aluminum size gives me equivalent voltage-drop and termination performance for less installed cost.” That is why 2 AWG aluminum can be a better garage-feeder decision than a smaller copper-only minimum. If you want a broader material tradeoff discussion, see our copper vs. aluminum guide.

Field Workflow That Prevents Rework

Confirm the actual one-way route length, not the optimistic sketch distance.
Establish feeder ampacity from NEC Table 310.16 and load calculations.
Check feeder voltage drop at the expected design current.
Reserve branch-circuit margin for 120-volt loads in the garage.
Verify disconnect, grounding electrode, isolated neutral, and equipment grounding conductor requirements.
Run the final numbers through the site’s wire size calculator and compare them against the NEC standards page.

FAQ: Detached Garage Feeder Sizing

What voltage drop should I allow for a detached garage feeder?

A good field target is 2% to 3% on the feeder so the total stays within the NEC 5% feeder-plus-branch recommendation in 215.2(A)(1) Informational Note No. 2 and 210.19(A)(1) Informational Note No. 4.

Can I run a three-wire feeder to a detached garage?

For modern work, the standard answer is no: run four conductors for a 120/240V feeder, including an equipment grounding conductor, then keep the neutral isolated at the detached structure per NEC 250.32.

Does a detached garage need ground rods?

In most feeder-fed detached structure installations, yes. NEC 250.32 requires a grounding electrode system at the building or structure supplied by the feeder, even though the equipment grounding conductor still runs with the feeder.

Is 6 AWG copper enough for a 60A detached garage feeder?

It may be enough for ampacity, but at 150 feet and 60A it drops about 2.96% at 240 volts. That leaves very little room for a 120V branch circuit, so 4 AWG copper or 2 AWG aluminum is often the cleaner design.

Can I oversize the feeder just for voltage drop even if ampacity is already okay?

Yes. That is routine engineering practice. NEC informational notes do not prohibit upsizing, and long-run feeders over 100 feet often need one or two conductor sizes above the ampacity minimum.

What IEC voltage-drop target is comparable for detached garages?

A common IEC 60364-5-52 design convention is 3% for lighting and 5% for other utilization circuits. In practice, many designers still hold detached-building feeders near 2% to 3% so 120V or lighting loads do not suffer.

Next Step

If you are laying out a detached garage or workshop, run the feeder in the calculator before you buy wire. Then compare your numbers against our installation examples and adjust the conductor size before trenching.

Need to check the design before installation?

Use the calculator for the feeder numbers, then send the project details through our contact page if you want a second technical look at conductor sizing, branch-circuit margin, or code references.

Open Calculator Contact the Team

Detached Garage Feeder Sizing: Voltage Drop, Grounding, and NEC Rules