Voltage Drop Calculation and Feeder Sizing

Why this matters

A 200-foot run from the main panel to a detached garage subpanel using 6 AWG copper for a 40A load looks fine on the ampacity table. Plug it in and the workshop's drill press runs at 84 percent of nominal voltage; the motor draws more current to compensate, heats the winding insulation, and dies after 18 months. The conductor was correctly sized for ampacity (NEC 310.16) but not for voltage drop. NEC 215.2 is an FPN (informational note), not a mandatory rule - which is why this gets missed by contractors who only chase the bare-minimum table values. Customers do not know the difference; they know "the new outbuilding doesn't work right". Voltage drop is the difference between an install that runs forever and one that produces a callback in three years.

The voltage drop formula

For single-phase circuits:

VD = (2 × K × I × L) / CM

Where:

• VD = voltage drop in volts
• K = constant for material (12.9 for copper, 21.2 for aluminum at 75 C)
• I = current in amps
• L = one-way conductor length in feet
• CM = circular mils of the conductor cross-section

For three-phase circuits:

VD = (1.732 × K × I × L) / CM

The factor of 2 in single-phase accounts for the round-trip on both conductors; the factor of 1.732 in three-phase accounts for the wye-or-delta vector relationship.

Circular mils reference

For kcmil (thousand circular mils) sizes, the size is the kcmil value times 1,000.

NEC recommendations vs requirements

NEC 210.19(A)(1) FPN 4 and 215.2(A)(1) FPN 2 recommend:

• Branch circuits: voltage drop not to exceed 3 percent
• Feeders: voltage drop not to exceed 3 percent
• Combined branch + feeder: voltage drop not to exceed 5 percent

These are informational. NEC 110.14(C) is mandatory and limits termination temperature, but it does not specify a voltage-drop limit. Some jurisdictions (and some equipment standards, especially EV chargers and motors) impose stricter limits via the equipment listing.

3 percent is the standard target. Some applications need tighter:

• Motor circuits: NEMA MG-1 recommends under 5 percent at the motor terminals during operation, under 10 percent during start
• EV chargers: most manufacturers recommend under 3 percent
• Long-run lighting circuits: under 3 percent or noticeable lamp dimming
• Sensitive electronics: under 3 percent often advised; UPS protects against worse

Quick-reference voltage drop table

For typical 120V branch circuits at 80 percent of conductor ampacity:

For 240V branch circuits, the distances double (since the same VD is a smaller percentage of 240V).

For 240V feeders:

These are starting points. Recompute with the formula for any run that is unusual (different load percent, special temperature, aluminum).

Worked examples

Example 1: 200 ft to detached garage subpanel, 40A load

Single-phase 240V, copper, 200 ft one-way, 40A actual load.

If using 6 AWG copper (26,240 CM):

VD = (2 × 12.9 × 40 × 200) / 26,240 = 206,400 / 26,240 = 7.86 V

7.86 / 240 = 3.3 percent

This is just over the 3 percent recommendation. Marginal. Upsize to 4 AWG (41,740 CM):

VD = (2 × 12.9 × 40 × 200) / 41,740 = 4.94 V = 2.1 percent

4 AWG is the right choice.

Example 2: EV charger, 250 ft from panel, 40A continuous

Single-phase 240V, copper, 250 ft one-way, 40A continuous (50A breaker, 6 AWG by ampacity).

VD = (2 × 12.9 × 40 × 250) / 26,240 = 9.83 V = 4.1 percent

Manufacturer recommends under 3 percent. Upsize to 4 AWG:

VD = (2 × 12.9 × 40 × 250) / 41,740 = 6.18 V = 2.6 percent

4 AWG copper is required for this run.

Example 3: 600A three-phase feeder, 400 ft

480V three-phase, copper, 400 ft, 600A actual load.

500 kcmil per phase (single set):

VD = (1.732 × 12.9 × 600 × 400) / 500,000 = 10.72 V = 2.2 percent

500 kcmil works for VD; ampacity check (500 kcmil at 75 C: 380A) shows it does not work for ampacity at 600A.

Solution: two parallel sets of 350 kcmil. Ampacity = 2 × 310 = 620 A. VD:

VD = (1.732 × 12.9 × 600 × 400) / (2 × 350,000) = 7.66 V = 1.6 percent

Two parallel sets of 350 kcmil is the right design.

Aluminum derate

Aluminum has higher resistance per circular mil than copper. The K constant is 21.2 versus 12.9. Roughly: aluminum conductor for the same VD must be one size larger than copper.

Quick conversion: if copper says 4 AWG, aluminum should be 2 AWG. If copper says 1/0, aluminum should be 2/0.

The cost-savings of aluminum for large feeders typically dominates over the upsizing cost, which is why utility services and large commercial feeders are almost always aluminum despite the larger size.

Effect of temperature

Conductor resistance increases with temperature. NEC 310.15(B) provides ambient temperature correction factors for ampacity, but the same resistance increase affects voltage drop.

A conductor at 75 C carrying full load has measurably higher VD than the same conductor at 25 C carrying low load. For real-world calculations, the K constant in the formula assumes 75 C. At 25 C, the VD is roughly 75 percent of the table value; at 90 C, roughly 110 percent.

Effect of conduit fill

More current-carrying conductors in a single raceway require derating per NEC 310.15(C). The derating affects ampacity, not directly VD. But: if the conductors are selected after derating, they are typically larger, and the larger conductors have lower VD.

In practice: if you size the conductor for the derated ampacity, the VD usually comes out fine. If you size for ampacity at table value (no derating), the actual operating conductor is smaller and VD is worse.

When VD calculation can be skipped

For runs under 75 ft at 120V or under 100 ft at 240V, VD is essentially never an issue on conductors sized properly for ampacity. The formula will confirm under 1 percent.

References

• NFPA 70 (NEC) 2023, Section 210.19 (Branch Circuit Conductor Sizing).
• NFPA 70 (NEC) 2023, Section 215.2 (Feeder Conductor Sizing).
• NFPA 70 (NEC) 2023, Section 310.15 (Ampacities for Conductors).
• NFPA 70 (NEC) 2023, Section 310.16 (Allowable Ampacities of Insulated Conductors).
• NEMA MG-1 (Motors and Generators), motor terminal voltage tolerance.
• IEEE 141 (Red Book - Recommended Practice for Electric Power Distribution).
• Mike Holt Voltage Drop Calculation reference.
• Manuall internal: Electrical Conductor Ampacity Reference, Electrical Service Panel Upgrade, Electrical EV Charger Load Calc Service Sizing.