Voltage Drop ng Subpanel Feeder: Garage, Workshop at Remote Panel
I-size ang subpanel feeder gamit ang praktikal na voltage-drop numbers para sa garage, workshop at remote panel.
Subpanel feeders are where good projects quietly become expensive callbacks. The ampacity answer may look legal, but if the feeder is long, the remote panel can start every downstream branch circuit with less voltage than the load actually expects. The result is dim lights, unhappy motors, weak EV charging performance, and a panel that feels undersized even when the breaker never trips.
The practical workflow is simple. Start with the real feeder load or design load, measure the actual one-way route length, choose the thermally legal conductor, and then check voltage drop before the trench or conduit fill is fixed. For US work, the usual references are NEC 215.2(A)(1), NEC 210.19(A)(1) informational notes, NEC 310.16 for ampacity, and NEC 250.32 where detached structures are involved. For IEC-style projects, IEC 60364-5-52 is the day-to-day reference for installation method, grouping, and voltage-drop limits.
Why Subpanel Feeders Deserve 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.
Detached garages, barns, sheds, and workshop additions often add 100 to 250 feet of feeder distance before the first receptacle is even considered.
Loads added later such as EV chargers, air compressors, mini-splits, and welders can turn an adequate feeder into a weak one without changing the original breaker.
DIY projects often size only for today's lights and receptacles, while electricians and engineers need to leave room for tomorrow's panel schedule.
Code and Standards Notes Worth Keeping on the Plan
- NEC 215.2(A)(1) informational note: many designers still target about 3% voltage drop on the feeder portion and about 5% total feeder plus branch circuit to the farthest outlet.
- NEC 210.19(A)(1) informational note: a remote panel does not remove the need to keep downstream branch circuits inside a sensible voltage budget.
- NEC 310.16: conductor ampacity and terminal temperature still decide the minimum thermally legal feeder before any voltage-drop optimization is applied.
- NEC 250.32: detached structures need the grounding and bonding arrangement handled correctly, but correct grounding does not fix an undersized feeder.
- IEC 60364-5-52: cable selection still has to account for installation method, grouping, ambient conditions, and the project voltage-drop target before the board location is approved.
Planning Cases for Common Subpanel Feeder Runs
These values are planning numbers, not a substitute for final design review. They are useful because they show how often feeder voltage drop becomes the real conductor-sizing driver on remote panels.
| Scenario | Distance and load | Approx. result | Design note |
|---|---|---|---|
| 60A detached garage subpanel | 120/240V, 150 ft / 46 m one-way | 6 AWG Cu about 3.7%; 4 AWG Cu about 2.3% | Good feeder designs usually leave margin for branch circuits inside the garage |
| 100A workshop subpanel | 120/240V, 220 ft / 67 m one-way | 1/0 Al about 5.9%; 3/0 Al about 3.0% | Aluminum can be economical, but long runs often need two size jumps |
| 125A barn or outbuilding panel | 120/240V, 180 ft / 55 m one-way | 2/0 Al about 3.8%; 4/0 Al about 2.4% | Future welders, compressors, or heaters change the comfort margin quickly |
| 63A IEC remote distribution board | 230V, 70 m one-way | 16 mm2 Cu about 4.4%; 25 mm2 Cu about 2.8% | IEC 60364-5-52 usually pushes the final answer toward the larger cable on longer runs |
Worked Feeder Examples with Recheckable Numbers
60A garage subpanel, 120/240V, 150 ft one-way
Using 6 AWG copper at roughly 0.491 ohm per 1000 ft, the feeder drop lands near 8.8V, or about 3.7% at 240V. That may still run, but it leaves less margin for the branch circuit feeding a compressor or EV charger inside the garage. Moving to 4 AWG copper drops the feeder to about 5.5V, or roughly 2.3%, which is usually a cleaner remote-panel answer.
100A workshop subpanel, 120/240V, 220 ft one-way
At this distance, 1/0 aluminum loses about 14.2V, or roughly 5.9%. That is hard to justify once the shop adds 120V receptacles, lighting, and motor loads. Moving to 3/0 aluminum brings the feeder drop near 7.1V, or about 3.0%, which is far more realistic for a workshop expected to grow.
63A IEC remote board, 230V, 70 m one-way
A 16 mm2 copper feeder can land near 10.1V of drop, about 4.4% at 230V. A 25 mm2 copper feeder brings that down to around 6.4V, or about 2.8%. When the remote board will feed socket circuits plus a heat pump or motor load, the larger cable usually protects the whole installation better.
Field Checklist Before You Approve the Feeder Size
- Measure the real one-way route from source panel to subpanel lugs, not just the building-to-building distance.
- Use the feeder design load or calculated load, then compare it with likely future loads such as EV charging, HVAC, or workshop equipment.
- Choose the minimum thermally legal conductor first, then recheck voltage drop on the actual candidate size.
- Leave room for downstream branch circuits so the total path can still live inside the usual 5% design target.
- For detached structures, keep grounding, bonding, trench, and conductor-sizing decisions on the same worksheet so the feeder is both compliant and practical.
Run the Subpanel Feeder Before You Pull the Conductors
Enter the feeder load, source voltage, conductor material, candidate wire size, and one-way distance so you can compare conductor options before the trench is backfilled or the panel is mounted.
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