Subpanel feeders are where plenty of otherwise clean jobs start to feel weak. The ampacity answer may be legal, but if the feeder is long, the remote panel begins every downstream branch circuit with less voltage than the loads actually expect. That is when a garage panel looks fine on paper yet produces dim lighting, a mini-split that complains on startup, a compressor that sounds heavy, or an EV charger that quietly reduces output.

For electricians, engineers, and serious DIY users, the workflow should be performance first and minimum conductor size second. Start with the real feeder load, the actual one-way route length, and the conductor material you intend to install. Then check the voltage drop before the trench is backfilled, before the conduit is packed, and before the panel schedule grows. In US work, the familiar design references remain NEC 215.2(A)(1) Informational Note No. 2 for feeders, NEC 210.19(A)(1) Informational Note No. 4 for branch circuits, NEC 310.16 for ampacity, and NEC 250.32 where detached structures are involved. In IEC-style work, the same engineering discipline shows up through IEC 60364-5-52, which keeps voltage drop, installation method, grouping, and conductor sizing on the same sheet.

Keep the big picture in mind: the feeder is only the first segment. A remote panel that already arrives 4% or 5% low leaves almost no margin for the 120V receptacle circuit, freezer circuit, lighting circuit, mini-split branch circuit, or workshop motor circuit connected after it. That is why remote-panel work often feels worse in the field than it looked during a quick breaker-and-ampacity review.

Authority References Worth Keeping Open

Use these authority references alongside the calculator when you review a feeder layout. The National Electrical Code remains the practical baseline for feeder and branch-circuit design in US work, while the International Electrotechnical Commission remains the day-to-day reference point for many IEC projects. For broader context on what the feeder is actually doing in a building system, the electric power distribution overview is also a useful reference.

“When a remote subpanel is more than 125 feet away, I stop asking whether the minimum ampacity works and start asking what voltage the panel will actually see. A feeder target near 3% is usually the safer starting point under NEC 215.2(A)(1).”
— Hommer Zhao, Technical Director

Why Subpanel Feeders Need Their Own Voltage-Drop Review

A feeder drop is not the whole story. Every branch circuit downstream starts with whatever voltage is left at the remote panel.

Garages, barns, detached workshops, mechanical buildings, and backyard offices often add 100 to 250 feet of feeder length before the first receptacle or motor branch circuit is even counted.

Future loads matter. A feeder that serves lights and receptacles today may need to support EV charging, HVAC, a welder, or an air compressor later.

The best feeder size is usually not the smallest legal conductor. It is the smallest conductor that leaves practical margin for the actual panel you are building.

“A 100A workshop feeder that drops 5% before the first 120V branch circuit is already spending the whole voltage budget. That is why 1/0 aluminum at 220 feet often looks cheap first and expensive later.”
— Hommer Zhao, Technical Director

Comparison Table: Common Subpanel Feeder Decisions

These planning numbers are not a substitute for final design review, but they show how quickly voltage drop becomes the real conductor-sizing driver once the route gets long.

Scenario	System and load	Conductor option	One-way distance	Approx. drop	Design read
Garage subpanel	60A at 120/240V	6 AWG copper	150 ft	3.7%	Works on paper, but leaves little branch-circuit margin
Garage subpanel	60A at 120/240V	4 AWG copper	150 ft	2.3%	Cleaner choice when a compressor or EV load may appear
Workshop subpanel	100A at 120/240V	1/0 aluminum	220 ft	5.9%	Too much feeder loss for a panel expected to grow
Workshop subpanel	100A at 120/240V	3/0 aluminum	220 ft	3.0%	Balanced answer for cost and future expansion
IEC remote board	63A at 230V	25 mm2 copper	70 m	2.8%	Much healthier for socket circuits plus motor loads

Worked Examples With Specific Numbers

Example 1: 60A garage subpanel at 150 feet

Suppose a detached garage gets a 60A, 120/240V subpanel 150 feet away, measured one-way from the source panel to the garage lugs. With 6 AWG copper at roughly 0.491 ohm per 1000 feet, the round-trip resistance is about 0.147 ohm. At 60A, the feeder loses about 8.8V. On a 240V feeder that is roughly 3.7%, which may still energize the panel but leaves very little room for a 120V branch circuit feeding a freezer, lighting, or a future charger. Moving to 4 AWG copper cuts the drop to about 5.5V, or roughly 2.3%, which is a much more comfortable feeder answer for real garage use.

Example 2: 100A workshop subpanel at 220 feet

Now take a 100A workshop feeder with lighting, 120V receptacles, a dust collector, and future compressor capacity. If you choose 1/0 aluminum because the ampacity feels reasonable, the voltage-drop math still lands around 14.2V, or about 5.9%, over a 220-foot one-way run. That means the panel itself already arrives weak before the first branch circuit starts. Upsizing to 3/0 aluminum brings the feeder loss down to about 7.1V, or about 3.0%. The wire is larger, but the panel now has room to behave like a real workshop instead of a temporary outbuilding panel.

Example 3: 63A remote IEC board at 70 meters

For a 230V remote distribution board in an IEC environment, a 63A design load over a 70 meter one-way run illustrates the same principle. A 16 mm2 copper cable lands around 10.1V of drop, or about 4.4%. That can be difficult to defend when the board serves socket circuits and a heat pump or other motor load. Moving to 25 mm2 copper cuts the drop to about 6.4V, or around 2.8%. The code language changes between NEC and IEC practice, but the engineering answer does not: long feeders deserve margin before the downstream circuits are ever considered.

“Detached buildings trick people into thinking grounding fixes performance. NEC 250.32 handles grounding and bonding, but only larger conductors fix a feeder that arrives 7 to 10 volts low under real load.”
— Hommer Zhao, Technical Director

Code and Standards Checkpoints That Actually Change the Answer

NEC 215.2(A)(1) Informational Note No. 2: many designers still aim for about 3% on the feeder portion and about 5% total feeder plus branch circuit to the farthest outlet.
NEC 210.19(A)(1) Informational Note No. 4: a remote panel does not cancel the branch-circuit side of the budget. If the feeder already consumes too much drop, the downstream 120V branch circuits get squeezed.
NEC 310.16: conductor ampacity and terminal temperature limits still define the minimum thermally legal feeder. Voltage-drop optimization happens after that, not instead of it.
NEC 250.32: detached structures need the grounding electrode system, neutral isolation, and equipment grounding conductor handled correctly. None of those steps compensate for an undersized feeder.
IEC 60364-5-52: the same practical answer appears through different language: installation method, grouping, ambient conditions, and permissible voltage drop must all be checked before a remote board location is approved.

Five Design Mistakes That Cause Weak Remote Panels

Sizing from breaker habit instead of actual feeder load. The feeder should be checked against the panel you expect to serve, not only the breaker someone wants to install today.
Measuring building-to-building distance instead of real conductor route. Wall offsets, trench bends, and vertical rises can add more length than people expect.
Treating 240V feeder performance as the full answer. A remote panel may still serve long 120V branch circuits, and they need part of the voltage-drop budget too.
Ignoring future loads. The cheapest feeder is often the one that does not need to be replaced when a charger, mini-split, or compressor gets added later.
Confusing grounding and bonding compliance with electrical performance. A correct grounding system is mandatory, but it does not improve delivered voltage at the panel.

Related Tools and Guides

Use the calculator and the related site content together when you compare feeder options:

Wire Size Calculator Detached Garage Feeder Guide Long-Distance Feeders Knowledge Base

FAQ: Subpanel Feeder Voltage Drop

How much voltage drop is acceptable on a subpanel feeder?

A practical design target is about 3% on the feeder so the full feeder-plus-branch-circuit path can stay near the familiar 5% guidance in NEC 215.2(A)(1) Informational Note No. 2 and NEC 210.19(A)(1) Informational Note No. 4.

Should I size a remote subpanel feeder from breaker size alone?

No. Breaker size only tells you the overcurrent limit. The real feeder answer depends on the actual load, the one-way route length, conductor material, and the downstream 120V or 230V branch circuits the panel must support.

When does aluminum make sense for a subpanel feeder?

Aluminum often makes economic sense on longer 100A and 125A feeders, but it usually needs a larger conductor size to match copper voltage-drop performance. That is why 1/0 aluminum can look acceptable on ampacity and still land too weak at 220 feet.

Does a detached garage or barn feeder need a grounding electrode system?

In most modern detached-structure installations, yes. NEC 250.32 requires a grounding electrode system at the separate building, while the feeder still carries an equipment grounding conductor and uses an isolated neutral at the remote panel.

Can a feeder pass ampacity and still perform badly?

Absolutely. A conductor can be thermally legal under NEC 310.16 and still arrive several volts low at the remote panel under real load. The breaker stays happy while lights dim, motors start hard, and chargers quietly reduce output.

At what distance should I start checking feeder voltage drop every time?

There is no single magic number, but once a subpanel route gets beyond about 100 feet, voltage drop should move from optional to routine. At 150 feet and beyond, it often becomes the first-order design issue instead of a last-minute note.

Check the Feeder Before You Pull the Conductors

Enter the real load, voltage, conductor material, wire size, and one-way route length into the calculator before the trench is closed. If the numbers are tight or the detached structure includes future expansion, use the contact page and review the layout before material is locked.

Open Calculator Contact Us

Subpanel Feeder Voltage Drop: Garage, Workshop, and Remote Panel Sizing