Voltage Drop Calculator -- NEC Wire Sizing

Calculate Voltage Drop

Enter your circuit parameters below to calculate voltage drop, percentage, power loss, and NEC compliance. Results update instantly.

NEC Wire Ampacity Ratings

Maximum current capacity for copper and aluminum conductors at 75°C insulation rating (NEC Table 310.16). Actual ampacity depends on insulation type, temperature, and conduit fill.

Wire Resistance Reference

DC resistance per 1,000 feet at 75°C for solid and stranded conductors (NEC Chapter 9, Table 8).

The Voltage Drop Formula

Voltage drop is calculated using the wire resistance and circuit parameters:

Single-Phase Circuits

VD = 2 x I x R x L x PF

Three-Phase Circuits

VD = √3 x I x R x L x PF

Where:

• VD = Voltage drop (volts)
• I = Current in amperes
• R = Wire resistance per unit length (ohms/ft or ohms/m)
• L = One-way wire length
• PF = Power factor (1.0 for purely resistive loads)

The factor of 2 in the single-phase formula accounts for the complete circuit -- current flows out on one conductor and returns on the other. For three-phase, the factor is √3 (approximately 1.732) due to the phase relationship between conductors.

Voltage drop percentage = (VD / Source Voltage) x 100

Power loss = VD x I (watts lost as heat in the wire)

What Causes Voltage Drop

Every electrical conductor has some resistance, which converts a portion of electrical energy into heat as current flows through it. This resistance causes the voltage to be lower at the far end of a wire run compared to the source. Several factors affect the magnitude of voltage drop:

• Wire length: Longer runs mean more resistance and more voltage drop. This is the most common cause of excessive voltage drop in residential and commercial wiring.
• Wire gauge: Thinner wire (higher AWG number) has more resistance per foot. Using a larger wire size directly reduces voltage drop.
• Current draw: Higher loads draw more current through the same resistance, increasing voltage drop proportionally.
• Conductor material: Aluminum has about 1.6 times the resistance of copper. For the same voltage drop, aluminum conductors need to be approximately two sizes larger than copper.
• Temperature: Wire resistance increases with temperature. The values used in this calculator are based on 75°C, which is the standard NEC rating for most insulation types.

NEC Voltage Drop Guidelines

The National Electrical Code (NEC) provides voltage drop recommendations in informational notes to sections 210.19(A) and 215.2(A):

• Branch circuits: A maximum of 3% voltage drop is recommended for the branch circuit itself.
• Feeders: A maximum of 3% voltage drop is recommended for the feeder.
• Combined total: The total voltage drop for both the feeder and branch circuit combined should not exceed 5%.

These are recommendations, not mandatory requirements. However, exceeding them can cause:

• Dimming or flickering lights
• Motors running hot and losing efficiency
• Electronic equipment malfunctioning or shutting down
• Increased energy costs from wasted power
• Premature equipment failure

How to Reduce Voltage Drop

• Increase wire size: Going up one or two AWG sizes significantly reduces resistance.
• Shorten the run: Relocating panels or sub-panels closer to the load reduces wire length.
• Use copper instead of aluminum: Copper has lower resistance, though it costs more.
• Increase voltage: For the same power, higher voltage means lower current and less voltage drop (e.g., 240V vs. 120V).
• Add a sub-panel: For long runs, a sub-panel closer to the load can reduce branch circuit lengths.

Frequently Asked Questions

What is voltage drop?

Voltage drop is the reduction in voltage as electrical current flows through a wire. Every conductor has resistance that opposes the flow of current. This resistance converts some electrical energy to heat, resulting in a lower voltage at the end of the wire compared to the source. The longer the wire and the more current it carries, the greater the voltage drop.

What is the acceptable voltage drop per NEC?

The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total for the combination of feeder and branch circuits. For a 120V circuit, 3% equals 3.6V, meaning the voltage at the load should be at least 116.4V. These are recommendations, not mandatory code requirements, but are widely followed as best practice.

How do I calculate voltage drop?

For single-phase circuits: VD = 2 x I x R x L, where I is current in amps, R is wire resistance per foot, and L is one-way wire length in feet. For three-phase: VD = 1.732 x I x R x L. Divide the result by source voltage and multiply by 100 to get the percentage.

Is copper or aluminum wire better for reducing voltage drop?

Copper has approximately 61% of the resistance of aluminum for the same wire gauge, making it better for reducing voltage drop. However, aluminum is significantly cheaper and lighter. The common practice is to use aluminum wire two sizes larger than the copper equivalent (e.g., 2 AWG aluminum instead of 4 AWG copper).

What happens if voltage drop is too high?

Excessive voltage drop causes lights to dim, motors to run hotter and less efficiently, and sensitive electronics to malfunction. It also wastes energy as heat in the wiring. In extreme cases, equipment may not start or may be damaged by operating at reduced voltage.

Does wire length mean one-way or round-trip distance?

In this calculator, enter the one-way distance from the panel to the load. The formula automatically accounts for the round-trip path of current (out and back) by using a multiplier of 2 for single-phase circuits.

What is power factor and when should I change it?

Power factor represents how efficiently electrical power is used. A value of 1.0 (default) represents purely resistive loads like heaters and incandescent lights. Motors, fluorescent lighting, and other inductive loads typically have a power factor between 0.8 and 0.95. For most residential calculations, 1.0 is appropriate.

Does this calculator store my data?

No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.

Related Tools

• Wire Size Calculator -- calculate the right wire gauge for your electrical project
• Ohm's Law Calculator -- calculate voltage, current, resistance, and power
• Resistor Color Code Calculator -- decode resistor values from color bands
• Power Law Calculator -- calculate electrical power relationships
• Capacitor Charge Calculator -- calculate RC circuit charge and discharge times

Privacy & Limitations

Privacy: This calculator runs entirely in your browser. No personal data or calculation inputs are transmitted or stored anywhere.

Limitations: This calculator uses DC resistance values at 75°C and does not account for AC impedance effects (skin effect, proximity effect, conduit material), ambient temperature derating, or conduit fill corrections. For critical installations, consult a licensed electrician and refer to the complete NEC tables. Results are for estimation purposes only.

Voltage Drop Calculator FAQ