Official Voltage Drop & Wire Sizing Report

Compliant with US National Electrical Code (NEC 2023 / 2026)

Report Date:
Source: generatorsizecalculator.com
100% Accurate NEC 210.19(A) & 215.2(A) šŸ‡ŗšŸ‡ø United States Standard (AWG & kcmil)

Voltage Drop Calculator

Calculate electrical voltage loss, percentage drop, terminal voltage at the load, and determine the exact required AWG conductor size for 120V/240V single-phase, 208V/480V 3-phase, and 12Vā€“48V DC circuits.

Table of Contents

1. āš” Calculator Form 2. šŸ“Š Results & Diagnostics 3. šŸ”„ Multi-Gauge Matrix 4. šŸ“ Drop Formulas 5. šŸ“– NEC Guidelines 6. šŸ›’ Recommended Gear 7. šŸ§® Related Tools 8. ā“ FAQs
Quick US Presets:
āš”

Circuit & Conductor Parameters

Fields marked with * are required.

Multiplies by 2.0 for single-phase/DC or āˆš3 (1.732) for 3-phase circuits.

V
Amps
Feet

Copper has lower electrical resistance ($K=12.9$ vs $21.2$).

Calculation Results & Diagnostics

Total Voltage Drop Good (ā‰¤3% NEC Branch)
3.24 Volts | 2.70%
NEC Voltage Drop Scale Compliant (<3.0%)
0% 3% (Branch) 5% (Total) >5% Warning
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12 AWG keeps voltage loss at 2.70% (under the 3% NEC limit for 75 ft @ 15A).

Nominal Source: 120 V
3% Max Allowed: 3.60 V
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Voltage at Load
116.76 V AC
Delivered to load
Conductor Power Loss
48.6 Watts
Wire heat loss
Loop Resistance
0.216 Ī©
Total out & return
Max 3% Distance
83.3 Feet
At 3% drop limit
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Mathematical Formula Applied: NEC Ch 9 Tables 8 & 9
VD = (2 Ɨ K Ɨ L Ɨ I) / CM = (2 Ɨ 12.9 Ɨ 75 Ɨ 15) / 6530 = 3.24 Volts (2.70%)

Wire Size Comparison Matrix

Compare how different AWG conductor sizes perform for this exact circuit load and distance.

Current: 120V, 15A, 75 ft
Conductor Gauge Circular Mils Voltage Drop (V) Voltage Drop (%) Terminal Voltage Power Loss (W) NEC Compliance

Step-by-Step Voltage Drop Formulas & Engineering Rules

Voltage drop in electrical circuits is governed by Ohm's Law ($V = I \times R$) and conductor physical characteristics (length, cross-sectional area, material resistivity, and AC inductive reactance). The US National Electrical Code (NEC) provides standard conductor dimensions in Chapter 9, Tables 8 & 9.

1. Single-Phase AC & DC Formula

Applied for standard 120V / 240V residential and commercial 2-wire circuits:

Vdrop = (2 Ɨ K Ɨ L Ɨ I) / CM
  • 2: Multiplier for out-and-return conductor loop.
  • K: Resistivity factor (12.9 for Copper, 21.2 for Aluminum @ 75Ā°C).
  • L: One-way length of circuit in feet.
  • I: Load current in amperes.
  • CM: Cross-sectional area in Circular Mils (NEC Table 8).

2. Three-Phase AC Formula

Applied for 208V, 480V, and 600V 3-phase commercial / industrial balanced loads:

Vdrop = (āˆš3 Ɨ K Ɨ L Ɨ I) / CM = (1.732 Ɨ K Ɨ L Ɨ I) / CM
  • āˆš3 ā‰ˆ 1.732: Three-phase line-to-line phase relationship factor.
  • Calculates phase-to-phase voltage drop between two active lines.
  • For phase-to-neutral drop, divide result by 1.732.
Standard US AWG Conductor Circular Mils (NEC Ch 9 Table 8)
14 AWG4,110 CM
12 AWG6,530 CM
10 AWG10,380 CM
8 AWG16,510 CM
6 AWG26,240 CM
4 AWG41,740 CM
2 AWG66,360 CM
1/0 AWG105,600 CM
2/0 AWG133,100 CM
4/0 AWG211,600 CM
250 kcmil250,000 CM
500 kcmil500,000 CM

National Electrical Code (NEC) Voltage Drop Guidelines

šŸŸ¢ Branch Circuits (3% Max)

NEC 210.19(A) Note 4: Sizing conductors to provide a maximum voltage drop of 3% at the furthest outlet of power, heating, or lighting.

šŸŸ” Feeder Circuits (2% - 3% Max)

NEC 215.2(A)(1) Note 2: Feeder conductor sizing for maximum 2% to 3% voltage drop from main service panel to subpanel.

šŸ”µ Total System (5% Max)

Overall System: The total combined voltage drop of both feeder and branch circuit conductors must not exceed 5%.

āš ļø Consequences of Excessive Voltage Drop (> 5%):
  • Electric Motors & Compressors: Draw higher current, run significantly hotter, and risk thermal tripping.
  • Lighting: Noticeable flickering, dimming, and shortened driver lifespans.
  • Electric Heaters: Heat output drops by the square of voltage drop (a 10% drop causes ~19% loss in heat).
  • Electronic Equipment: Random reboots, communication errors, and power supply failures.

Frequently Asked Questions (FAQ)

What is the maximum recommended run distance for 120V 15A & 20A circuits before upsizing? ā–¼
For a 120V single-phase circuit loaded to 80% capacity (12A continuous on a 15A breaker), standard 14 AWG Copper wire reaches the 3% voltage drop limit at approximately 50 feet. For a 20A circuit loaded to 16A continuous, standard 12 AWG Copper wire reaches the 3% limit at approximately 60 to 70 feet. For runs exceeding these distances, you should upsize by one wire gauge (e.g. use 12 AWG for 15A or 10 AWG for 20A).
Do I need to install a larger circuit breaker when I upsize wire for voltage drop? ā–¼
No, absolutely not. The circuit breaker must be sized based on the branch circuit rating or connected appliance (e.g., a 15A breaker for 15A outlets, or 20A breaker for 20A outlets). Upsizing the wire (e.g. pulling 10 AWG wire onto a 15A breaker) is done solely to reduce electrical resistance over long distances. If the larger wire doesn't fit standard outlet terminals, pigtail a short 14 AWG or 12 AWG wire in the outlet junction box using a wire nut or WAGO connector.
Why is Aluminum wire sized differently than Copper for voltage drop? ā–¼
Aluminum has an electrical resistivity constant ($K=21.2\,\Omega\cdot\text{cmil/ft}$) that is approximately 64% higher than Copper ($K=12.9\,\Omega\cdot\text{cmil/ft}$). As a rule of thumb, you must use an Aluminum conductor that is one to two AWG sizes larger than a Copper conductor to achieve the exact same ampacity and voltage drop performance.
How does Power Factor (PF) affect AC voltage drop? ā–¼
In AC circuits containing inductive loads (such as electric motors, transformers, or HVAC compressors), the current lags behind voltage (power factor < 1.0, typically 0.80 to 0.90). In large wire sizes and steel conduits, inductive reactance ($X_L$) combines with conductor resistance ($R$) into effective impedance: $Z = R\cos\theta + X\sin\theta$. Lower power factors cause higher total voltage drop for the same real wattage.