Project Overview

A homeowner in San Diego, California contacted an electrical contractor to install a Tesla Wall Connector in their detached garage. The garage, located approximately 200 feet from the main electrical panel in the house, would serve as the primary charging location for their new Tesla Model Y. The customer requested the fastest possible charging speed to fully replenish the vehicle's battery overnight, which meant the installation needed to support the full 48-amp charging capability of the Tesla Wall Connector.

This case study demonstrates the critical importance of voltage drop calculations for long residential circuits and how proper engineering analysis prevents costly problems before they occur. What initially appeared to be a straightforward EV charger installation required careful consideration of conductor sizing, NEC requirements, and real-world performance expectations. The contractor's decision to verify voltage drop calculations before proceeding saved both time and money while ensuring complete customer satisfaction.

Electric vehicle charging installations have become one of the most common residential electrical projects, and they present unique challenges that electricians must understand. Unlike traditional 240V loads such as dryers or ranges, EV chargers operate as continuous loads—drawing their rated current for three hours or more during each charging session. This continuous duty rating has significant implications for conductor sizing and voltage drop calculations.

Understanding the Technical Requirements

The Tesla Wall Connector is one of the most popular Level 2 electric vehicle supply equipment (EVSE) units on the market. When configured for maximum output, it delivers 48 amps of continuous charging current at 240 volts, providing approximately 11.5 kilowatts of power to the vehicle. This allows the Tesla Model Y to gain approximately 44 miles of range per hour of charging—enough to fully replenish the battery overnight even after a long day of driving.

Critical Design Considerations

• Circuit Distance: 200 feet one-way from main panel to detached garage (400 feet total conductor length)
• Load Current: 48A continuous charging current as rated by the EVSE manufacturer
• Continuous Load Factor: NEC requires sizing at 125% for continuous loads = 60A minimum circuit rating
• System Voltage: 240V single-phase residential service
• Conduit Type: Schedule 40 PVC underground between house and garage
• Initial Wire Selection: 6 AWG copper THWN-2 (65A ampacity at 75°C termination rating)

Per NEC Article 625.40, the EV charger circuit must be sized at 125% of the maximum load because EV charging qualifies as a continuous load under the definition in Article 100. A 48-amp charger therefore requires conductors and overcurrent protection rated for at least 60 amps. Based solely on ampacity requirements from NEC Table 310.16, 6 AWG copper conductors with 75°C-rated insulation provide 65 amps of ampacity, which exceeds the 60-amp requirement and would appear to be adequate.

However, the experienced contractor recognized that ampacity alone does not tell the complete story. The long 200-foot run from the panel to the garage creates significant resistance in the circuit, and this resistance causes voltage to drop along the length of the conductors. If the voltage drop is too high, the charger may not receive sufficient voltage to operate at full power, resulting in reduced charging speed and customer dissatisfaction.

Voltage Drop Analysis: Initial 6 AWG Design

Before purchasing materials, the contractor used our professional voltage drop calculator to analyze the proposed 6 AWG installation. The calculation revealed a significant problem that would have gone unnoticed if only ampacity had been considered:

Voltage Drop Calculation: 6 AWG Copper @ 200ft

Wire Resistance: 6 AWG copper = 0.491 Ω per 1000 feet

Formula: Vd = (2 × I × L × R) / 1000

Vd = (2 × 48A × 200ft × 0.491) / 1000

Vd = 9,427.2 / 1000

Vd = 9.43 volts dropped

Voltage at Charger: 240V - 9.43V = 230.57V

Vd% = (9.43 / 240) × 100 = 3.93%

At 3.93%, the voltage drop exceeds the NEC's recommended 3% limit for branch circuits as stated in the Informational Note to NEC 210.19(A)(1). While voltage drop is technically a recommendation rather than a mandatory requirement in most jurisdictions, exceeding this limit can have real consequences for equipment performance and energy efficiency.

The NEC also recommends that the total voltage drop from the service entrance to the final outlet should not exceed 5%. With nearly 4% already consumed by the branch circuit alone, there would be minimal margin remaining for any voltage drop in the service entrance conductors or feeder. During periods of high household electrical demand—such as when the HVAC system, water heater, and other major loads are operating simultaneously—the actual voltage at the charger could drop even further.

The Optimized Solution: Upsizing to 4 AWG

Based on the voltage drop analysis, the contractor recommended upgrading to 4 AWG copper conductors. This larger wire size provides significantly lower resistance, which directly translates to reduced voltage drop. Here's how the improved design performs:

Optimized Calculation: 4 AWG Copper @ 200ft

Wire Resistance: 4 AWG copper = 0.308 Ω per 1000 feet

Formula: Vd = (2 × I × L × R) / 1000

Vd = (2 × 48A × 200ft × 0.308) / 1000

Vd = 5,913.6 / 1000

Vd = 5.91 volts dropped

Voltage at Charger: 240V - 5.91V = 234.09V

Vd% = (5.91 / 240) × 100 = 2.46%

At 2.46%, the voltage drop now comfortably meets the 3% branch circuit recommendation with margin to spare. This leaves adequate headroom for the service entrance and ensures the charger receives sufficient voltage even during periods of high household electrical demand. The additional material cost for upgrading from 6 AWG to 4 AWG was approximately $180 for the 400 feet of wire needed—a small investment compared to the potential cost of a callback, troubleshooting, or customer dissatisfaction.

Cost-Benefit Analysis

Material Cost Comparison

• 6 AWG THWN-2 (400ft): ~$320
• 4 AWG THWN-2 (400ft): ~$500
• Upgrade Cost: $180

Potential Callback Costs

• Return service call: $150-250
• Rewiring labor: $500-1,000
• Customer dissatisfaction: Priceless
• Reputation damage: Significant

Results and Performance Verification

Performance Achieved

• Full 11.5kW charging power delivered
• 2.46% voltage drop (well under 3%)
• 234.1V measured at the charger
• 44 miles of range per hour added
• Full overnight charge capability

Customer Satisfaction

• No charging speed complaints
• Full overnight charge achieved
• Professional documentation provided
• Referral to three neighbors
• 5-star review received

Key Takeaways for EV Charger Installations

Lessons Learned

Always calculate voltage drop for circuits over 50 feet, especially for continuous loads like EV chargers. Ampacity compliance alone is not sufficient.
EV chargers are continuous loads—size conductors at 125% of rated current per NEC 625.40 and factor this into voltage drop calculations.
Material cost increases are minimal compared to callback costs, reputation damage, and lost referral opportunities.
Document your calculations to demonstrate professional engineering and justify your pricing to customers.
Verify with measurement after installation to confirm calculated values and document for the customer record.

Calculate Your EV Installation

Planning an EV charger installation? Use our professional voltage drop calculator to verify your wire sizing meets NEC recommendations and delivers full charging performance to your customers. Our calculator supports single-phase and three-phase calculations, copper and aluminum conductors, and provides instant results with detailed analysis.

Calculate Your Circuit

Residential EV Charger Installation: 200ft Garage Run

Challenge

Solution