Subpanel Feeder Conductor Sizing by Distance
Sizing an electrical feeder to a detached garage, outbuilding, workshop, or second-floor subpanel requires balancing two independent electrical constraints: thermal conductor ampacity (safety) and cumulative voltage drop over distance (equipment performance).
On short runs (under 50 feet), conductor gauge is governed strictly by the overcurrent protection device (breaker rating) per NEC Table 310.16. On longer feeder runs (75 to 300+ feet), conductor resistance causes voltage drop; conductors are frequently upsized 1 to 2 gauge sizes to meet the recommended 3% engineering design guidance (NEC Informational Note 215.2(A)(1)) at the subpanel terminals.
Feeder Circuit Topology & Loss Model
NEC 215.2(A)(1) Feeder Sizing Model01.What Determines Feeder Conductor Size?
A subpanel feeder conductor must safely transport total demand current without overheating insulation or causing excessive voltage sag. Four primary factors dictate conductor selection:
- • Overcurrent Rating:The feeder conductor ampacity must match or exceed the upstream breaker rating (e.g., 60A, 100A, 150A, 200A).
- • Continuous Loads:Loads operating for 3 hours or more (EV chargers, electric heating) require a 125% multiplier per NEC 215.2(A)(1)(a).
- • Terminal Ratings:Most modern residential circuit breakers and panel lugs are rated at 75°C. Even if using 90°C wire (THHN/XHHW-2), ampacity is sized from the 75°C column of NEC Table 310.16.
- • One-Way Distance:As length increases, resistance accumulates linearly, necessitating upsized circular mil area to prevent voltage drop.
02.Thermal Ampacity vs. Voltage Drop
A common point of confusion is treating thermal ampacity and voltage drop as the same calculation. They represent two distinct engineering constraints:
Thermal Ampacity (Mandatory Code Sizing)
Governed by NEC Table 310.16 and Section 215.2(A)(1). Specifies the allowable continuous current a conductor can carry without exceeding its insulation temperature rating (e.g., 75°C THWN-2). Ampacity is independent of distance.
Voltage Drop (Engineering Design Guidance)
Governed by Ohm's Law and NEC Informational Note 215.2(A)(1). Provides non-mandatory design guidance recommending limiting feeder voltage drop to 3.0% (and 5.0% total branch + feeder) for equipment efficiency and motor protection, unless mandated by local energy codes.
03.Why Distance Governs Long Feeder Runs
Electrical resistance in a conductor is proportional to length and inversely proportional to cross-sectional area:
For an 80A calculated operating load on a 100A feeder breaker at 25 feet, a #3 AWG Copper conductor exhibits only 0.98V (0.41%) voltage drop. However, when that same 80A load spans 200 feet to an outbuilding, voltage drop on #3 AWG Copper accumulates to 7.84V (3.27%), exceeding the recommended 3.0% design target. Sizing is therefore upsized to #2 AWG Copper (5.83V, 2.43% drop) or 1/0 AWG Aluminum (6.42V, 2.68% drop).
04.Copper vs. Aluminum Feeder Conductors
For subpanel feeders, aluminum conductors (specifically AA-8000 series aluminum alloy) are standard in residential installations due to significant cost savings on large wire gauges:
| Characteristic | Copper (Cu) Feeder | Aluminum (Al) AA-8000 |
|---|---|---|
| Conductivity & Gauge | Higher conductivity; smaller wire gauge for same ampacity. | Requires ~1 to 2 gauge sizes larger than copper for equivalent rating. |
| Material Cost | Significantly higher cost on large feeder sizes (2 AWG to 4/0). | High cost efficiency; often 60–75% less expensive per foot. |
| Conduit Trade Size | Smaller outer diameter allows smaller conduit trade sizes. | Larger conductor bundle may require upsizing conduit (e.g., 1-1/2″ to 2″). |
| Lug Terminations | Compatible with standard Cu/Al marked lugs. | Requires AL7CU/AL9CU rated lugs and anti-oxidant joint compound where specified. |
05.Worked Engineering Example: 100A Feeder at 150 Feet
Consider a residential subpanel installation with the following baseline design parameters:
Step 1: Determine Minimum Code Ampacity for 100A OCPD (NEC Table 310.16 @ 75°C)
- Minimum Copper Conductor: #3 AWG Copper (rated 100A @ 75°C, 52,620 CM).
- Minimum Aluminum Conductor: #1 AWG Aluminum (rated 100A @ 75°C, 83,690 CM).
Step 2: Calculate Voltage Drop at 150 Feet (80A Operating Load)
Vd = 309,600 ÷ 52,620 = 5.88V (2.45% drop)
V_terminal = 240V - 5.88V = 234.12V
✓ Meets <3% design guidance.
Vd = 508,800 ÷ 83,690 = 6.08V (2.53% drop)
V_terminal = 240V - 6.08V = 233.92V
✓ Meets <3% design guidance.
Distance & Full-Capacity Sensitivity: If the 80A operating load distance increases to 200 feet, #1 AWG Aluminum voltage drop reaches 8.11V (3.38%), requiring an upsize to 1/0 AWG Aluminum (6.42V, 2.68% drop). If the feeder were operated at its full 100A nameplate continuous design capacity at 150 feet, #1 AWG Aluminum would produce 7.60V (3.17% drop), requiring 1/0 AWG Aluminum (6.02V, 2.51% drop).
06.Common Feeder Sizing Mistakes & Critical Code Nuances
Single-phase formulas already include the multiplier of 2 (2 × K × I × L ÷ CM) to account for both supply and return conductors. Always input the single-point one-way distance between panels.
Although conductors like THHN/THWN-2 carry a 90°C insulation rating, circuit breaker lugs on residential panels are typically rated for 75°C. Under NEC 110.14(C), the conductor ampacity must be selected from the 75°C column of Table 310.16. The 90°C rating may only be used as a starting point for ambient temperature or conduit fill derating calculations.
Under NEC 250.122(B), where ungrounded feeder conductors are increased in size for reasons such as voltage drop (rather than standard ambient or conduit fill derating adjustments), wire-type equipment grounding conductors (EGC) must be increased proportionately in circular mil area based on the ratio of the upsized conductor circular mils to the minimum code-required conductor circular mils. Note that specific metallic conduit raceways serving as equipment grounding paths or specialized qualified industrial systems follow distinct statutory provisions.
Under modern NEC standards, subpanels require a dedicated 4-wire feeder where the grounded neutral bus remains completely isolated (floating) from the equipment grounding bus and enclosure (main bonding jumper removed). Detached outbuildings also require a local grounding electrode system bonded to the EGC bus. Note that historic existing 3-wire feeders installed under pre-2008 code editions are subject to narrow legacy exceptions that must be evaluated on-site by a licensed electrician.
Local Code Adoptions & AHJ Verification
Electrical installations must conform to the edition of the National Electrical Code (NEC) adopted by your local Authority Having Jurisdiction (AHJ), including regional amendments. Voltage drop limits (3% feeder, 5% total) are published as informational design recommendations in NEC Informational Note 215.2(A)(1) rather than mandatory prescriptive statutes in base NEC, though certain local energy conservation codes mandate them. Always verify your specific feeder, raceway fill, and grounding schedule with a licensed electrical contractor and municipal building department.
Calculate Feeder Wire Size & Drop
Run real-time voltage drop calculations across 120V, 240V, and 208V circuits with instant copper vs. aluminum comparisons:
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