Managing Voltage Drop on Long Circuit Runs

Why Voltage Drop Matters

A motor running on low voltage won’t start properly. LED fixtures dim visibly. Equipment operates at reduced efficiency, generating heat and wearing out faster. Voltage drop is silent damage—a 2,000-foot run from the service to a barn doesn’t look wrong, but it’s costing the customer money and reducing equipment life. Understanding and managing voltage drop separates competent electricians from those who create problems for future troubleshooting.

NEC Reference: NEC 210.19(A)(1) informational note recommends not exceeding 3% on branch circuits or 5% combined feeder/branch circuit voltage drop.

The Physics: Voltage Drop Formula

Voltage drop occurs when current flows through a conductor’s resistance. The longer the run or the smaller the wire, the higher the drop.

Basic Voltage Drop Formula

For AC circuits (three-phase or single-phase):

V drop = (2 × L × I × R) / 1000

Where:

V drop = voltage drop in volts
L = one-way length of conductor run in feet
I = current in amperes
R = resistance of conductor in ohms per 1,000 feet (from NEC Table 8 or 9)
2 factor = accounts for both conductors (out and back)
1000 = conversion factor for R values

For Three-Phase AC Circuits

V drop = (1.732 × L × I × R) / 1000

Where 1.732 = √3 (three-phase correction factor).

Practical Formula (Using % Drop)

% Voltage Drop = (V drop / Source Voltage) × 100

For a 240V circuit with 5V drop:

% drop = (5 ÷ 240) × 100 = 2.08%

Conductor Resistance Reference (NEC Table 8)

Resistance values are given in ohms per 1,000 feet at 20°C. Use these for copper conductors (standard in residential/commercial):

Wire Size	Resistance (Ω per 1,000 ft)
#14 AWG	2.50
#12 AWG	1.588
#10 AWG	0.999
#8 AWG	0.628
#6 AWG	0.395
#4 AWG	0.248
#2 AWG	0.156
#1 AWG	0.124
#1/0 AWG	0.0983
#2/0 AWG	0.0780

Higher temperatures increase resistance by ~0.4% per degree Celsius above 20°C, so in a hot attic, resistance increases.

The 3% vs. 5% Rule Explained

NEC 210.19(A)(1) states:

“It is recommended that the voltage drop of the branch circuit should not exceed 3 percent, and the voltage drop of feeder and branch circuit combined should not exceed 5 percent.”

This is an informational note—not mandatory—but it’s the industry standard and is expected by inspectors, engineers, and building code officials.

Breaking Down the Rule

Circuit Type	Recommended Max Drop
Branch circuit only	3%
Feeder only	2%
Feeder + branch combined	5%
Service entrance feeder	1–2% (good practice)

Why these percentages?

At 3% drop on a 120V branch circuit, voltage at the load is 116.4V. Most equipment tolerates this, but motors, heaters, and LEDs begin showing degradation.

At 5% combined, you hit the limit of what equipment can reliably operate without efficiency loss.

Example: A 240V circuit with a 5V drop operates at 235V (2.08% drop)—well within limits.

Practical Voltage Drop Scenarios

Scenario 1: Standard Kitchen Circuit (20A, 60 feet, #12 AWG)

Calculation:

V drop = (2 × 60 × 20 × 1.588) / 1000
V drop = (3,811.2) / 1000 = 3.81V
Source voltage: 120V
% drop = (3.81 ÷ 120) × 100 = 3.18%

Result: This is acceptable but tight. If this were a long run to a garage receptacle with high inrush current (charging equipment), consider #10 AWG.

Scenario 2: 200-Foot Run to a Barn (30A, 240V, #8 AWG Copper)

Calculation:

V drop = (2 × 200 × 30 × 0.628) / 1000
V drop = (7,536) / 1000 = 7.54V
Source voltage: 240V
% drop = (7.54 ÷ 240) × 100 = 3.14%

Result: Acceptable on branch circuit alone, but if there’s also a feeder drop from service to the subpanel feeding this circuit, combined drop might exceed 5%. Recheck overall system drop.

Scenario 3: 300-Foot Feeder to Remote Panel (60A, 240V, #4 AWG)

Calculation:

V drop = (2 × 300 × 60 × 0.248) / 1000
V drop = (8,928) / 1000 = 8.93V
Source voltage: 240V
% drop = (8.93 ÷ 240) × 100 = 3.72%

Result: If this is a feeder only (before branch circuits branch off), 3.72% is borderline. If branch circuits add another 2%, combined drop is 5.72%—over limit. Solution: Use #2 AWG instead.

V drop with #2 = (2 × 300 × 60 × 0.156) / 1000 = 5.62V = 2.34% drop
Combined drop would be: 2.34% + 2% = 4.34% (acceptable)

Wire Upsizing Strategy for Long Runs

When voltage drop is an issue, upsizing the wire is the solution. Here’s a practical approach:

Step 1: Calculate Drop with Planned Wire Size

Use the formula above. If drop exceeds 3% (branch) or 5% (feeder + branch), proceed to step 2.

Step 2: Double the Wire Area (Roughly)

Doubling the wire cross-sectional area roughly halves the voltage drop. Moving from #12 to #10 AWG isn’t quite double, but it’s close. From #6 to #4 is closer.

Wire cross-sectional areas (circular mils):

#14 = 4,107
#12 = 6,530
#10 = 10,380 (1.6× larger than #12)
#8 = 16,510 (1.6× larger than #10)
#6 = 26,240 (1.6× larger than #8)
#4 = 41,740 (1.6× larger than #6)

Each step up ~1.6×, which reduces resistance proportionally.

Step 3: Recalculate and Verify

Rerun the formula with the larger wire size. If drop is now acceptable, proceed. If not, go up another size.

Step 4: Consider Voltage at the Load

Don’t just calculate voltage drop—think about what voltage the equipment will actually see and whether that’s acceptable.

Example: A 240V receptacle 200 feet away with a 7.5V drop operates at 232.5V. A 120V circuit 150 feet away with a 3V drop operates at 117V. The 120V load is more problematic (24.75V drop) because it’s a larger percentage of the supply voltage.

Real-World Upsizing Examples

Example 1: Garage Circuits (20A, 120V, 150 feet)

Initial plan: #12 AWG

V drop = (2 × 150 × 20 × 1.588) / 1000 = 9.53V
% drop = (9.53 ÷ 120) × 100 = 7.94% ❌ Over limit

Upgrade to #10 AWG:

V drop = (2 × 150 × 20 × 0.999) / 1000 = 5.99V
% drop = (5.99 ÷ 120) × 100 = 4.99% ✓ Just acceptable

Better choice: #8 AWG:

V drop = (2 × 150 × 20 × 0.628) / 1000 = 3.77V
% drop = (3.77 ÷ 120) × 100 = 3.14% ✓ Comfortable margin

Example 2: Well Pump Circuit (15A, 240V, 400 feet)

Initial plan: #12 AWG

V drop = (2 × 400 × 15 × 1.588) / 1000 = 19.06V
% drop = (19.06 ÷ 240) × 100 = 7.94% ❌ Way over

Step up to #10:

V drop = (2 × 400 × 15 × 0.999) / 1000 = 11.99V
% drop = 4.99% ✓ Acceptable but tight

Recommended: #8 AWG:

V drop = (2 × 400 × 15 × 0.628) / 1000 = 7.54V
% drop = 3.14% ✓ Good safety margin

For well pumps specifically: Pumps are sensitive to low voltage. Consider going one size larger (#6 AWG) if budget allows, to ensure reliable operation.

Three-Phase Voltage Drop

Three-phase circuits are more efficient at long distances because current is distributed across three conductors. Use the √3 correction factor:

V drop = (1.732 × L × I × R) / 1000

Example: 300-Foot Three-Phase 480V, 100A Feeder, #2 AWG Copper

V drop = (1.732 × 300 × 100 × 0.156) / 1000
V drop = 8.10V
% drop = (8.10 ÷ 480) × 100 = 1.69% ✓ Excellent

This shows why three-phase power is advantageous for long industrial runs—the same wire carries more current with less percentage drop.

Special Cases: Aluminum Conductors

Aluminum has higher resistance than copper. Use NEC Table 9 for aluminum:

Wire Size	Aluminum Resistance (Ω/1,000 ft)
#12	2.55
#10	1.61
#8	1.01
#6	0.634
#4	0.398

Aluminum has ~60% higher resistance than copper for the same size, so expect 60% more voltage drop. Upsize aluminum compared to copper—a #10 aluminum circuit should use copper #12 equivalent.

Temperature Correction Factor

Conductors are hotter in service than the 20°C reference. Resistance increases roughly 0.4% per degree Celsius above 20°C. In a hot attic or conduit exposed to sun, add 10–15% to calculated voltage drop as safety margin.

Adjusted V drop = Base V drop × 1.10 to 1.15

NEC Code References

NEC 210.19(A)(1): Branch circuit voltage drop informational note (3% recommended)
NEC 215.2(A)(1): Feeder voltage drop informational note (combined 5% recommended)
NEC Table 8: Resistance for copper conductors in DC and AC
NEC Table 9: Resistance for copper and aluminum conductors in AC
NEC 310.15(B): Ampacity adjustment for bundled conductors (more heat = more resistance)

Voltage Drop Checklist for Long Runs

Takeaway

Voltage drop on long runs is predictable and manageable with proper wire sizing. Know the formula, use the 3%/5% rule, and don’t skip this step on rural jobs, barn circuits, or any run over 100 feet. A 30-cent/foot upsize in wire cost is far cheaper than replacing a burnt-out motor or having an inspector fail your work because voltage drop wasn’t addressed.