Voltage Drop Chart

Use this chart to turn a percent drop target into volts, then gather route length, conductor material, load current, and phase for calculator work. A 3% target means 3.6 V at 120 V, 7.2 V at 240 V, and 14.4 V at 480 V; a 5% target means 6.0 V, 12.0 V, and 24.0 V.

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Quick reference table

Voltage drop is a design-performance check, not a simple wire-size lookup. A 3% target equals 3.6 V on a 120 V circuit, 7.2 V on 240 V, and 14.4 V on 480 V; a 5% target equals 6.0 V, 12.0 V, and 24.0 V. After choosing the target, use the calculator with one-way distance, actual load current, conductor material, conductor size, and phase before changing a conductor size.

Percent drop translated to volts

Percent drop translated to volts
Drop target	120 V	208 V	240 V	480 V
1%	1.2 V	2.1 V	2.4 V	4.8 V
2%	2.4 V	4.2 V	4.8 V	9.6 V
3%	3.6 V	6.2 V	7.2 V	14.4 V
4%	4.8 V	8.3 V	9.6 V	19.2 V
5%	6.0 V	10.4 V	12.0 V	24.0 V

Voltage-drop field inputs before using the calculator

Voltage-drop field inputs before using the calculator
Input	Good field note	Why it matters
Route length	One-way measured path with verticals and offsets	Distance drives resistance
Load current	Expected operating amps under normal load	Breaker size can overstate drop
Conductor material	Copper or aluminum from the actual design	Material changes resistance
Circuit type	DC, single-phase, or balanced three-phase	Formula multiplier changes
Load sensitivity	Motor start, LED driver, control load, or general load	Tolerance affects target choice

Formula basis

Single phase: Vdrop = 2 x K x I x D / cmil. Three phase: Vdrop = 1.732 x K x I x D / cmil.

K is the conductor material constant used by the calculator.
I is the expected load current, not automatically the breaker rating.
D is one-way circuit distance including real route length.
cmil is conductor area in circular mils.

Worked examples

3% target on a 240 V branch circuit240 x 0.03 = 7.2 V. The calculator then checks whether the chosen conductor and route length stay within that target.

480 V feeder with a 2% design target480 x 0.02 = 9.6 V. Record one-way route length and expected load current before comparing conductor options.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

The percentage rows are design targets and not a substitute for equipment instructions or owner criteria.
Actual conductor resistance changes with material, temperature, conduit fill, grouping, and operating condition.
Motor starting voltage drop, utility service drop, and internal equipment drop can require a separate review.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Voltage-drop guidance in NEC contexts is commonly treated as design guidance rather than a single pass/fail rule; verify adopted code, equipment requirements, owner specifications, and AHJ expectations.
For workplace installations, equipment suitability, markings, and manufacturer installation instructions still apply after the voltage-drop calculation.

How to use this chart

1Choose the performance targetStart with the owner, equipment, or design voltage-drop target and translate the percent into allowed volts for the circuit voltage.

2Measure the real routeUse the one-way routed path, including vertical travel, offsets, and realistic field routing rather than only straight-line distance.

3Use load current deliberatelyEnter expected operating current or a documented conservative current basis, and keep it separate from breaker rating when they differ.

4Compare conductor optionsUse the calculator to compare material, size, phase, and length options before moving a recommendation into the project record.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Sketch the routeMark panel, load, vertical sections, long offsets, pull points, and route assumptions before entering distance.
Record the load conditionNote whether the current is continuous, intermittent, motor starting, lighting load, control load, or a diversified feeder estimate.
Document the decision triggerWrite whether the calculation is for troubleshooting, design review, value engineering, equipment warranty, or owner performance criteria.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Using panel-to-load straight-line distance when the raceway route is much longer in the field.
Using breaker rating as the load current without labeling it as a conservative assumption.
Treating voltage-drop guidance as final code compliance without checking equipment instructions, adopted code, owner criteria, and AHJ expectations.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Should I use breaker amps or load amps?

Use expected operating load current for the voltage-drop estimate. Breaker rating can be useful as a conservative screen, but it may overstate drop for lightly loaded circuits.

Is one-way distance or round-trip distance used?

Common voltage-drop formulas use one-way distance with a multiplier for the circuit type. Do not double the distance again if the formula already includes the return-path multiplier.

Can voltage drop force a larger wire than ampacity?

Yes. A conductor can pass ampacity review and still be upsized for voltage-drop performance on a long or sensitive run.