Voltage Drop Calculator

A voltage drop calculator returns the voltage lost along a wire run, in volts and as a percentage of supply voltage, plus the smallest standard wire gauge that keeps the drop within the commonly cited 3% guideline. It applies the standard formula using the wire's circular mil area, your load current and one-way distance.

Last reviewed September 2026Method & sources Report an error

Voltage Drop (% of supply)

6.58 %

Guideline Check

At 6.58%, this exceeds the commonly cited 3% branch-circuit voltage drop guideline - consider 8 AWG or shortening the run.

Breakdown
Voltage Drop (% of supply)	6.58%
Voltage Drop (volts)	7.90 V
Recommended Minimum Gauge	8 AWG

How to Calculate Voltage Drop

Voltage drop over a wire run follows the standard formula `VD = (2 × K × I × D) ÷ CM` for single-phase. Three-phase uses `√3` in place of `2`. K is a conductor resistivity constant (12.9 for copper, 21.2 for aluminum, in ohm-circular-mils per foot). I is load current, D is the one-way distance, and CM is the wire's circular mil area from NEC Chapter 9, Table 8. Dividing that voltage drop by the system voltage gives the percentage. The calculator then checks that percentage against the 3% figure commonly cited from the NEC's own informational note to Section 210.19(A) for branch circuits. It also works the formula backward. It solves for the minimum circular mil area needed to stay at or under 3%, then finds the smallest standard wire gauge with at least that much area.

Example: 120 V single-phase circuit, 20 A load, 100 ft one-way

A 120 V single-phase circuit, 20 A load, 100 ft one-way, using 12 AWG copper: 1. Voltage drop: (2 × 12.9 × 20 × 100) ÷ 6,530 = 51,600 ÷ 6,530 ≈ 7.90 V 2. As a percentage: 7.90 ÷ 120 × 100 ≈ 6.59% That exceeds the 3% guideline. Solving for the minimum wire size instead: 3. Required circular mils: 51,600 ÷ (120 × 0.03) = 51,600 ÷ 3.6 ≈ 14,333 cmil 4. Smallest standard gauge meeting that: 10 AWG (10,380 cmil) is short. 8 AWG (16,510 cmil) clears it So 12 AWG is undersized for this specific run. 8 AWG copper would bring the drop back under the 3% guideline.

Voltage Drop: Compare Scenarios

Scenario	Voltage Drop (% of supply)	Voltage Drop (volts)	Recommended Minimum Gauge
Short run (25 ft, 12 AWG)	1.65%	1.98 V	14 AWG
Medium run (100 ft, 12 AWG)	6.58%	7.90 V	8 AWG
Long run (250 ft, 12 AWG)	16.46%	19.75 V	4 AWG

What Is Voltage Drop?

A voltage drop calculator estimates how much voltage is lost to a wire's own resistance over a given run length, and whether a chosen wire gauge keeps that loss within the commonly recommended limit. Excess voltage drop shows up as dimmer lights, a motor running hot, or reduced equipment performance at the end of a long circuit run. This calculator also recommends the minimum wire gauge that would bring a given run back under the 3% guideline.

Voltage Drop: Pros and Cons

Pros

Uses the standard, widely published voltage-drop formula rather than an approximation
Also solves the formula backward for the minimum recommended wire gauge, not just the drop for a wire size you already picked
Covers both single-phase and three-phase circuits with the correct multiplier for each

Cons

The 3% figure is a commonly cited planning guideline from the NEC's own informational note, not a hard code requirement in most jurisdictions. Some inspections don't check it directly, though it remains good design practice
The K constant is a standard approximation at typical operating temperature. Actual conductor resistance varies somewhat with real operating temperature and specific conductor construction
Doesn't check the wire's ampacity (how much current it's rated to carry safely). A wire recommended here to control voltage drop must still separately meet or exceed the NEC's ampacity table for the circuit's breaker size

Voltage Drop: What a Pro Checks

Check the wire's ampacity separately from its voltage drop result. A gauge that controls voltage drop still needs to independently meet the NEC's ampacity table for the circuit's actual breaker size.
Remember the 3% figure is a commonly cited planning guideline from the NEC's own informational note, not a hard code requirement checked at every inspection, though it remains good design practice.
The K constant used here is a standard approximation at typical operating temperature. Actual resistance varies somewhat with real conductor temperature and construction.

Frequently Asked Questions

What is an acceptable voltage drop percentage?

The NEC's own informational note to Section 210.19(A) is commonly cited as a guideline of 3% max for a branch circuit alone, or 5% total combining a feeder and branch circuit together. It's not a strict code requirement in most jurisdictions, but standard good design practice.

How do you calculate voltage drop in a wire?

For single-phase: VD = (2 × K × I × D) ÷ CM. Here K is the conductor's resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, D is the one-way distance in feet, and CM is the wire's circular mil area. Three-phase uses √3 in place of the 2.

What causes excessive voltage drop?

A wire that's too small (low circular mil area) for its current and distance combination. The longer the run and the higher the current, the larger the wire needs to be to keep resistance-driven voltage loss under control.

Does aluminum wire have more voltage drop than copper?

Yes, at the same gauge and length. Aluminum's resistivity constant (21.2) is significantly higher than copper's (12.9), which is why aluminum wiring is typically upsized relative to an equivalent copper run to keep both ampacity and voltage drop in check.

Does a wire sized to control voltage drop also meet ampacity requirements?

Not necessarily. Voltage drop and ampacity (a wire's safe current-carrying capacity) are two separate checks. A wire recommended here for voltage drop still needs to independently meet or exceed the NEC's ampacity table for the circuit's actual breaker size.

Sources

Electrical