Voltage Drop Calculator – NEC, Estimated, and Custom Resistance Methods

This is a calculator for the estimation of the voltage drop of an electrical circuit. The "NEC data" tab calculates based on the resistance and reactance data from the National Electrical Code (NEC). The "Estimated resistance" tab calculates based on the resistance data estimated from the wire size. Click the "Other" tab to use customized resistance or impedance data, such as data from other standards or wire manufacturers.

NEC Data Method
Estimated Resistance Method
Other (Custom Resistance)

Results

Voltage Drop: 0 V
Voltage Drop Percentage: 0%
Voltage at Load: 0 V

Results

Voltage Drop: 0 V
Voltage Drop Percentage: 0%
Voltage at Load: 0 V

Results

Voltage Drop: 0 V
Voltage Drop Percentage: 0%
Voltage at Load: 0 V

Voltage Drop Formula Used in This Calculator

Single-Phase Formula

For single-phase circuits, the voltage drop is calculated using:

VD = 2 × I × R × L

Where:

  • VD = Voltage drop (volts)
  • I = Current (amperes)
  • R = Resistance (ohms per unit length)
  • L = Length (one-way distance)

Three-Phase Formula

For three-phase circuits, the voltage drop is calculated using:

VD = √3 × I × (R cos θ + X sin θ)

Where:

  • VD = Voltage drop (volts)
  • I = Current (amperes)
  • R = Resistance (ohms per unit length)
  • X = Reactance (ohms per unit length)
  • θ = Phase angle (determined by power factor)

Understanding Voltage Drop and Why It Matters

What Is Voltage Drop?

When electrical current moves through a wire, it is pushed by electrical potential (voltage) and it needs to surpass a certain level of contrary pressure caused by the wire. The voltage drop is the amount of electrical potential (voltage) loss caused by the contrary pressure of the wire. If the current is alternating, such contrary pressure is called impedance. Impedance is a vector, or two-dimensional quantity, consisting of resistance and reactance (reaction of a built-up electric field to a change of current). If the current is direct, the contrary pressure is called resistance.

Impact of Voltage Drop

Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. It is recommended that the voltage drop should be less than 5% under a fully loaded condition. This can be achieved by selecting the right wire, and by taking care in the use of extension cords and similar devices.

Acceptable Limits

NEC recommends no more than 3% voltage drop for branch circuits and no more than 5% for feeder circuits. These limits help ensure proper operation of electrical equipment and prevent energy waste.

Causes of Voltage Drop

There are four major causes of voltage drop:

  • Wire Material: The choice of material used for the wire. Silver, copper, gold, and aluminum are among the metals with the best electrical conductivity. Copper and aluminum are the most common materials used for wires due to their relatively low price compared with silver and gold. Copper is a better conductor than aluminum and will have less voltage drop than aluminum for a given length and wire size.
  • Wire Size: Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. In American wire gauge, every 6-gauge decrease doubles the wire diameter, and every 3-gauge decrease doubles the wire cross sectional area. In the Metric Gauge scale, the gauge is 10 times the diameter in millimeters, so a 50 gauge metric wire would be 5 mm in diameter.
  • Wire Length: Shorter wires will have less voltage drop than longer wires for the same wire size. Voltage drop becomes important when the length of a run of wire or cable becomes very long. Usually this is not a problem in circuits within a house, but may become an issue when running wire to an outbuilding, well pump, etc.
  • Current Load: The amount of current being carried can affect voltage drop levels; an increase in current through a wire results in an increased voltage drop. Current carrying capacity is often referred to as ampacity, which is the maximum number of electrons that can be pushed at one time – the word ampacity is short for ampere capacity.

Common Use Cases and Applications

  • Residential wiring
  • Industrial 3-phase motor setups
  • Solar power systems
  • Low-voltage landscape lighting
  • RV or marine wiring

Voltage Drop Knowledge Test

Test your understanding of voltage drop concepts with these 10 real-world scenarios.

Question 1: Long Extension Cord

You're powering a 120V tool 150 ft away using 12 AWG copper wire. The tool draws 10A. What is the voltage drop?

Answer: 6.56 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 10A × 1.588Ω/1000ft × 150ft = 6.56V

The resistance of 12 AWG copper wire is approximately 1.588Ω per 1000 feet. For a 150-foot run (one-way), the total round-trip length is 300 feet. With a 10A current, the voltage drop is 6.56V, which is about 5.47% of the 120V supply.

Question 2: Residential Lighting Circuit

A lighting circuit uses 14 AWG copper wire over 100 ft and carries 5A. What is the voltage drop?

Answer: 2.56 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 5A × 2.525Ω/1000ft × 100ft = 2.56V

The resistance of 14 AWG copper wire is approximately 2.525Ω per 1000 feet. For a 100-foot run (one-way), the total round-trip length is 200 feet. With a 5A current, the voltage drop is 2.56V, which is about 2.13% of a 120V supply.

Question 3: Subpanel Wire Sizing

A subpanel is 250 ft away and will carry 50A at 240V single-phase. What size wire should be used to keep voltage drop under 3%?

Answer: 3 AWG copper

For a 240V system, the maximum allowable voltage drop is 7.2V (3% of 240V).

Let's check each option:

  • 2 AWG: VD = 2 × 50A × 0.5127Ω/1000ft × 250ft = 12.82V (5.34%) - Too high
  • 3 AWG: VD = 2 × 50A × 0.6465Ω/1000ft × 250ft = 16.16V (6.73%) - Too high
  • 4 AWG: VD = 2 × 50A × 0.8152Ω/1000ft × 250ft = 20.38V (8.49%) - Too high
  • 6 AWG: VD = 2 × 50A × 1.296Ω/1000ft × 250ft = 32.40V (13.5%) - Too high

None of the options provided meet the 3% requirement. For this application, you would need at least 1/0 AWG copper wire, which would give a voltage drop of approximately 6.44V (2.68%).

Question 4: Aluminum Wire in Shed

You're running 8 AWG aluminum wire 75 ft to a shed. The current is 20A at 120V. Calculate the voltage drop.

Answer: 3.29 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 20A × 1.096Ω/1000ft × 75ft = 3.29V

The resistance of 8 AWG aluminum wire is approximately 1.096Ω per 1000 feet. For a 75-foot run (one-way), the total round-trip length is 150 feet. With a 20A current, the voltage drop is 3.29V, which is about 2.74% of the 120V supply.

Question 5: 3-Phase Motor Installation

A 3-phase motor uses 30A at 480V over 200 ft of copper wire. What's the voltage drop using NEC values for 4 AWG wire?

Answer: 7.06 V

Using the three-phase formula VD = √3 × I × (R cos θ + X sin θ):

VD = √3 × 30A × (0.8152Ω/1000ft × 0.85 + 0.103Ω/1000ft × 0.527) × 200ft = 7.06V

For 4 AWG copper wire in steel conduit, the resistance is 0.8152Ω/1000ft and the reactance is 0.103Ω/1000ft. With a power factor of 0.85, cos θ = 0.85 and sin θ = 0.527. For a 200-foot run, the voltage drop is 7.06V, which is about 1.47% of the 480V supply.

Question 6: Solar Panel Array

A solar panel array is wired with 10 AWG copper over 60 ft at 12V, drawing 15A. What is the voltage drop?

Answer: 1.18 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 15A × 3.277Ω/1000ft × 60ft = 1.18V

The resistance of 10 AWG copper wire is approximately 3.277Ω per 1000 feet. For a 60-foot run (one-way), the total round-trip length is 120 feet. With a 15A current, the voltage drop is 1.18V, which is about 9.83% of the 12V supply.

This is a significant voltage drop for a 12V system, highlighting why low-voltage systems require careful wire sizing to minimize voltage drop.

Question 7: RV Extension Cable

An RV power supply runs over 50 ft of 10 AWG wire at 30A and 120V. Is the voltage drop acceptable?

Answer: Yes, it's 2.95V (2.46%)

Using the formula VD = 2 × I × R × L:

VD = 2 × 30A × 3.277Ω/1000ft × 50ft = 2.95V

The resistance of 10 AWG copper wire is approximately 3.277Ω per 1000 feet. For a 50-foot run (one-way), the total round-trip length is 100 feet. With a 30A current, the voltage drop is 2.95V, which is about 2.46% of the 120V supply.

Since the voltage drop is less than 3%, it is considered acceptable according to NEC recommendations for branch circuits.

Question 8: HVAC Wiring

An HVAC unit requires 208V and 25A, installed 100 ft away using 6 AWG copper. What is the voltage drop?

Answer: 6.48 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 25A × 1.296Ω/1000ft × 100ft = 6.48V

The resistance of 6 AWG copper wire is approximately 1.296Ω per 1000 feet. For a 100-foot run (one-way), the total round-trip length is 200 feet. With a 25A current, the voltage drop is 6.48V, which is about 3.12% of the 208V supply.

This voltage drop is slightly above the recommended 3% limit for branch circuits. For optimal performance, using 4 AWG wire would reduce the voltage drop to approximately 4.08V (1.96%).

Question 9: Parking Lot Lights

Several parking lot lights are installed 300 ft away and draw 10A total at 277V. What wire size minimizes voltage drop?

Answer: 8 AWG copper (2.46V drop)

Using the formula VD = 2 × I × R × L:

  • 8 AWG: VD = 2 × 10A × 0.8152Ω/1000ft × 300ft = 4.89V (1.77%)
  • 10 AWG: VD = 2 × 10A × 1.296Ω/1000ft × 300ft = 7.78V (2.81%)
  • 12 AWG: VD = 2 × 10A × 2.061Ω/1000ft × 300ft = 12.37V (4.47%)
  • 14 AWG: VD = 2 × 10A × 3.277Ω/1000ft × 300ft = 19.66V (7.10%)

For a 277V system, the maximum allowable voltage drop is 8.31V (3% of 277V). All options except 14 AWG meet this requirement, but 8 AWG provides the lowest voltage drop at 4.89V (1.77%).

Question 10: Custom Resistance Scenario

A wire has a known resistance of 0.4 ohms over 150 ft. With 20A current, calculate the voltage drop using the custom input tab.

Answer: 16.00 V

Using the formula VD = 2 × I × R × L:

VD = 2 × 20A × 0.4Ω = 16.00V

Since the resistance value of 0.4 ohms is already for the round-trip length of 150 feet, we don't need to multiply by the length again. With a 20A current, the voltage drop is 16.00V.

This example demonstrates how to use the "Other" tab in the calculator when you have custom resistance data that doesn't match standard wire sizes or materials.

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Typical AWG Wire Sizes

AWG Diameter (inch) Diameter (mm) Area (mm²) Copper resistance (Ω/km) Copper resistance (Ω/1000ft)
0000 (4/0) 0.4600 11.684 107 0.1608 0.04901
000 (3/0) 0.4096 10.404 85.0 0.2028 0.06180
00 (2/0) 0.3648 9.266 67.4 0.2557 0.07793
0 (1/0) 0.3249 8.252 53.5 0.3224 0.09827
1 0.2893 7.348 42.4 0.4066 0.1239
2 0.2576 6.544 33.6 0.5127 0.1563
3 0.2294 5.827 26.7 0.6465 0.1970
4 0.2043 5.189 21.2 0.8152 0.2485
5 0.1819 4.621 16.8 1.028 0.3133
6 0.1620 4.115 13.3 1.296 0.3951
7 0.1443 3.665 10.5 1.634 0.4982
8 0.1285 3.264 8.37 2.061 0.6282
9 0.1144 2.906 6.63 2.599 0.7921
10 0.1019 2.588 5.26 3.277 0.9989
11 0.0907 2.305 4.17 4.132 1.260
12 0.0808 2.053 3.31 5.211 1.588
13 0.0720 1.828 2.62 6.571 2.003
14 0.0641 1.628 2.08 8.286 2.525
15 0.0571 1.450 1.65 10.45 3.184
16 0.0508 1.291 1.31 13.17 4.016
17 0.0453 1.150 1.04 16.61 5.064
18 0.0403 1.024 0.823 20.95 6.385
19 0.0359 0.912 0.653 26.42 8.051
20 0.0320 0.812 0.518 33.31 10.15

Frequently Asked Questions

What is the maximum allowed voltage drop?

NEC recommends no more than 3% voltage drop for branch circuits and no more than 5% for feeder circuits. These limits help ensure proper operation of electrical equipment and prevent energy waste.

Should I size up the wire to reduce voltage drop?

Yes, increasing conductor size reduces resistance and voltage drop. When voltage drop exceeds recommended limits, using a larger wire size is the most effective solution.

Does distance affect voltage drop?

Yes, longer distances result in more resistance and higher voltage drop. Voltage drop is directly proportional to the length of the wire.

What's the difference between NEC data and estimated resistance?

NEC data is standardized and based on actual measurements of wire resistance and reactance in various conduit types. Estimated resistance is based on typical conductor properties and provides a good approximation for quick calculations.

How does power factor affect voltage drop?

Power factor affects the impedance of AC circuits. A lower power factor (more reactive current) results in higher voltage drop for the same amount of current. This is particularly important in three-phase systems with inductive loads like motors.

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