Electrical Voltage Drop Calculator (VD)

What is Physics of Line Resistance (NEC 210.19)?

Electricity acts like water in a pipe. As it travels hundreds of feet, the physical resistance of the copper wire causes 'friction' that eats up the voltage before it reaches the machine. If a 120V motor only receives 105V because the wire is too long, the motor will pull excessive amperage to compensate, overheat, and catch fire. The NEC legally mandates a maximum voltage limit drop to prevent catastrophic equipment failure.

Mathematical Foundation

Single-Phase Voltage Drop Formula

VD = \frac{2 \times K \times I \times D}{CM}

K

= Conductor Resistance Constant. Copper = 12.9, Aluminum = 21.2. Aluminum's higher K-Factor directly results in massive voltage drops on long runs.

I

= Load current in amps (Continuous Operating Load).

D

= One-way run distance in feet. The formula multiplies by '2' to account for the complete return trip (hot out, neutral back).

CM

= Circular Mils — the cross-sectional mass of the wire. 10 AWG = 10,380 CM. Larger wire = more CM = less drop.

Three-Phase Voltage Drop Formula

VD = \frac{1.732 \times K \times I \times D}{CM}

1.732

= The square root of 3 (√3). Three-phase power returns through the offset legs rather than a dedicated neutral, mathematically reducing the line resistance.

Laws & Principles

The 3% Branch Rule: NEC Section 210.19(A) Informational Note 4 strictly recommends a maximum voltage drop of 3% for the final branch circuit feeding the machine. Dropping past this voids manufacturers warranties.
The 5% Feeder Rule: The total system drop (the feeder from the pole to the main panel + the branch from the panel to the plug) must not collectively exceed 5%.
Linear Failure: Distance is the ultimate enemy. The relationship is perfectly linear. If 100 feet of wire drops 2%, then 200 feet drops 4% (Failing NEC). To fix this, you must buy physically thicker wire.
Copper vs. Aluminum: Aluminum is a terrible conductor compared to copper. Over long distances, you typically must jump up two full wire sizes (e.g., from 4/0 Copper to 300 KCMIL Aluminum) just to survive the voltage drop.

Step-by-Step Example Walkthrough

" Wiring a 15-amp load on a 120V single-phase circuit using standard 12 AWG Copper wire, running 125 feet out to a detached shed. "

1. Identify K-Factor: Copper = 12.9.
2. Identify CM for 12 AWG: 6,530 Circular Mils.
3. Apply single-phase formula: (2 × 12.9 × 15 Amps × 125 feet run) / 6,530 CM.
4. Calculate Raw Drop: 48,375 / 6,530 = 7.4 Volts dropped along the wire.
5. Calculate Percentage: (7.4V Drop / 120V Source) × 100 = 6.1% Drop.

Final Result: The 120V circuit drops 7.4 Volts entirely within the wire, delivering only 112.6V to the shed (a 6.1% drop). This massively fails the 3% NEC limit. The electrician must upsize from 12 AWG to 8 AWG Copper just to pass inspection.

Quick Answer: What is acceptable voltage drop?

The National Electrical Code (NEC) mandates a maximum acceptable voltage drop of 3% for a branch circuit, and a maximum of 5% for the entire system combined (feeder + branch). For a standard 120V wall outlet, a 3% drop means the lowest legal voltage acceptable at the plug is 116.4 Volts. If you drop below this floor, motors will overheat and sensitive electronics will crash. You fix severe voltage drop by either shortening the wire run, or buying a much thicker gauge of wire (e.g., jumping from 12 AWG to 10 AWG to push more current).

Underlying Mathematics

Voltage Drop = (2 × K × Amps × Distance) ÷ Circular Mils

Formula Variables:

K (Constant). Copper has a K rating of 12.9. Aluminum has a K rating of 21.2. The higher the K, the worse the drop.
Circular Mils. This is the cross-sectional mass of the wire. If the wire isn't thick enough to handle the distance, you divide a large top number by a small bottom number, resulting in a blown voltage drop.

Standard Breaker	Normal Wire Size	Required Size @ 100ft (Copper)
15 Amp (120V)	14 AWG	12 AWG (Failed 3%)
20 Amp (120V)	12 AWG	10 AWG (Failed 3%)
30 Amp (240V)	10 AWG	10 AWG (Passes 3%)
50 Amp (240V)	8 AWG	6 AWG (Failed 3%)

Standard Breaker

Normal Wire Size

Required Size @ 100ft (Copper)

15 Amp (120V)

14 AWG

12 AWG (Failed 3%)

20 Amp (120V)

12 AWG

10 AWG (Failed 3%)

30 Amp (240V)

10 AWG

10 AWG (Passes 3%)

50 Amp (240V)

8 AWG

6 AWG (Failed 3%)

Inspection Violations & Safety Faults

Motor Burnout

A homeowner installs a heavy 20-Amp air compressor in a detached garage 150 feet away using standard 12 AWG wire. The start-capacitor fires up, dragging 20 amps across 150 feet. The voltage drops almost 8% down to 110V. Because the motor cannot get enough voltage to spin up to speed, it stalls. As it stalls, it pulls massive lock-rotor amperage, melts its internal copper windings, and catches fire. The wire size was totally legal for 20 Amps, but illegal for 150 feet.

Parking Lot Lighting Cascades

An electrician wires 5 parking lot light poles on a single 120V circuit stringing out 400 feet. He sizes the wire for the total load (15 amps) and uses 12 AWG. By the time the electricity reaches the final light pole 400 feet away, it only has 98 Volts left. The High Intensity Discharge (HID) lamp ballast detects the low voltage and shuts off. The only way to fix this failure is to dig up 400 feet of concrete and upsize the wire to 6 AWG.

Field Design Best Practices & Pro Tips

Do This

✓Run 240V for Long Distances. Voltage drop is a ratio. If you drop 5 Volts on a 120V line, that's a 4.1% failing grade. If you drop that exact same 5 Volts on a 240V line, it's only a 2% passing grade. Whenever feeding detached buildings, barns, or distant motors, always design for a 240V subpanel feed rather than pushing a 120V branch circuit entirely across the property.

Avoid This

✗Never assume larger breakers fix voltage drop. The breaker does not push the electricity, it only limits it. Putting a 30A breaker on a 120V / 150-foot run will do absolutely nothing to increase the destination voltage, but it will create a massive fire hazard by allowing the wire to melt before the breaker trips.

Frequently Asked Questions

What is acceptable voltage drop under NEC?

The National Electrical Code (NEC) specifies in Informational Note 4 to Section 210.19(A) that voltage drop should not exceed 3% for a branch circuit, and the maximum total drop for combined feeder and branch circuits should not exceed 5%.

Does a higher voltage reduce voltage drop problems?

Yes. Because voltage drop is calculated as a percentage of the source voltage, pushing 240V instead of 120V over the same length of wire mathematically cuts your voltage drop percentage entirely in half. High-voltage transmission lines (like 480V) can push power thousands of feet before violating the 3% restriction.

Why do aluminum wires have worse voltage drop than copper?

Aluminum has a lower conductivity than copper. Copper's resistance factor (K-Factor) is 12.9, while aluminum's is 21.2. Because aluminum resists the flow of electricity nearly 64% more than copper, the voltage drop is much steeper over distance. You must buy significantly thicker aluminum wire (e.g. 4/0 Aluminum vs 2/0 Copper) to achieve the exact same voltage drop.

What happens to electronics if the voltage drop is too bad?

"Brownouts". If a 120V system drops below 110V, heavy inductive motors (like refrigerators and air compressors) will attempt to draw massive amounts of amperage to make up the missing power gap, which melts their windings. Sensitive electronics (like servers or computers) will experience violent switching power supply fluctuations, causing reboot loops and hardware death.

Voltage Drop Analyzer