14.3 Voltage Drop and Conductor Sizing

Voltage Drop — Informational Notes, Not Mandatory

The NEC addresses voltage drop in Informational Notes — not enforceable code language. The key references are:

• 215.2(A)(3), Informational Note No. 2: Recommends a maximum 3% voltage drop on feeders.
• 210.19(A)(1), Informational Note No. 4: Recommends a maximum 3% voltage drop on branch circuits.
• Combined recommendation: 5% total voltage drop (feeder + branch circuit).

While not mandatory, voltage drop is a practical design requirement and appears frequently on the exam Calculations part. Inspectors and engineers treat the 3%/5% thresholds as best practice.

The Voltage Drop Formulas

Where:

• Vd = voltage drop in volts
• K = DC resistance constant (12.9 for copper, 21.2 for aluminum)
• I = load current in amperes (at 100%, not 125%)
• L = one-way length of the run in feet
• CM = circular mil area of the conductor (from Chapter 9, Table 8)
• √3 ≈ 1.732

The K values are derived from Chapter 9, Table 8: a 1,000 kcmil copper conductor has 0.0129 Ω per 1,000 ft, so K = 0.0129 × 1,000,000 = 12.9. For aluminum, 0.0212 × 1,000,000 = 21.2.

Chapter 9, Table 8 — Conductor Circular Mil Areas

Circular mil area is the same for copper and aluminum of a given AWG size — only the resistance (and therefore K) differs by material.

Percentage Voltage Drop

%Vd = (Vd ÷ source voltage) × 100

For 240 V: 3% = 7.2 V. For 208 V: 3% = 6.24 V. For 120 V: 3% = 3.6 V.

110.14(C) — Termination Temperature Limitations

The termination temperature rating — not the conductor insulation rating — determines which ampacity column of Table 310.16 you may use for final conductor sizing.

The 90°C column (for THHN, THWN-2, XHHW-2) is used only as a starting point for derating calculations — ambient temperature correction (Table 310.15(B)(1)(1)) and conductor bundling adjustment (Table 310.15(C)(1)). The final ampacity after derating cannot exceed the termination temperature limit from 110.14(C).

240.4(B) — Next Size Up

When a conductor's ampacity does not correspond to a standard OCPD rating in 240.6(A), the next higher standard rating is permitted, provided:

• The conductors being protected are not part of a multioutlet branch circuit supplying receptacles.
• The next higher rating does not exceed 800 A.
• The conductor ampacity is adequate for the load.

Critical distinction: Voltage-drop upsizing is not a 240.4(B) application. Voltage drop drives you to a larger conductor for performance, not for OCPD alignment. You still select the OCPD based on the load current and the conductor's ampacity at the termination temperature column.

240.6(A) — Standard Ampere Ratings

Standard ratings for fuses and inverse time circuit breakers include: 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000.

Note: Both 25 A and 35 A are standard ratings. Do not treat them as non-standard — this is a known exam trap. The 2023 NEC added 10 A as a standard rating for the first time.

Worked Example: 250 ft, 30 A, 240 V Single-Phase Feeder

Given: A 30 A load at 240 V, 250 ft (one-way) from the source, wired with #10 Cu.

Step 1 — Identify Values

• K = 12.9 (copper)
• I = 30 A
• L = 250 ft
• CM = 10,380 (#10 Cu, Chapter 9 Table 8)

Step 2 — Calculate Voltage Drop

• Vd = (2 × 12.9 × 30 × 250) ÷ 10,380
• Vd = 193,500 ÷ 10,380
• Vd = 18.6 V

Step 3 — Check Against 3% Recommendation

• 3% of 240 V = 7.2 V
• 18.6 V > 7.2 V → exceeds the recommendation

Step 4 — Find Required CM for 3% Vd

• CM = (2 × K × I × L) ÷ Vd_max
• CM = 193,500 ÷ 7.2 = 26,875 CM

Step 5 — Select Larger Conductor

• #6 Cu = 26,240 CM → Vd = 193,500 ÷ 26,240 = 7.4 V (3.07%) — still just over 3%
• #4 Cu = 41,740 CM → Vd = 193,500 ÷ 41,740 = 4.6 V (1.93%) ✓
• Select: #4 Cu for voltage drop compliance

Note on ampacity: #10 Cu at 75°C is rated 35 A (Table 310.16), which is adequate for the 30 A load. The upsizing to #4 is driven entirely by voltage drop, not by ampacity. This is a design adjustment — 240.4(B) is not invoked.