Voltage drop formula: Vd = 2 × L × R × I ÷ 1000 (single-phase), Vd = 1.732 × L × R × I ÷ 1000 (three-phase). Resistance values from NEC Table 9 (75°C, in steel conduit). Ampacity from NEC Table 310.15(B)(16), 75°C column. Cells marked — indicate insufficient ampacity for this load. Always verify against current NEC and local amendments.
How to Read This Voltage Drop Table
Select the system voltage, conductor material, and load current using the buttons above. The table shows the calculated voltage drop percentage for each combination of AWG size (rows) and one-way circuit length (columns). Green cells meet the NEC 3% branch circuit recommendation. Amber cells are between 3% and 5% — within the combined feeder + branch circuit NEC limit. Red cells exceed the 5% limit and should be avoided. Cells marked "—" mean the conductor's ampacity is lower than the load current, making that combination non-code-compliant regardless of voltage drop.
NEC Voltage Drop Recommendations
The NEC does not mandate a hard voltage drop limit for most circuits but provides recommendations as informational notes. NEC 210.19(A) Informational Note No. 4 recommends limiting voltage drop on branch circuits to 3%. NEC 215.2(A) Informational Note No. 2 recommends that the combined voltage drop on feeders and branch circuits not exceed 5%.
While these are recommendations rather than requirements, exceeding them causes real problems: motors run hot and draw excess current, sensitive electronics malfunction, and lighting is noticeably dim. Most engineers and inspectors treat the 3%/5% rule as de facto design requirements.
Sizing Wire for Long Runs
On long circuit runs — such as a garage subpanel, outdoor HVAC disconnect, or workshop feeder — voltage drop can force you to use a conductor two or three AWG sizes larger than the minimum ampacity alone would require. This is a common and necessary practice. The wire size calculator on this site can find the minimum AWG that satisfies both constraints (ampacity and voltage drop) for any combination of inputs.
Common Voltage Drop Planning Scenarios
- A 100-foot 120V/15A circuit run to an outbuilding — check if #14 AWG is adequate or if #12 AWG is needed
- A 200-foot 240V/30A feeder from a house panel to a detached garage — often forces #8 AWG or #6 AWG copper
- A 50-foot 240V/50A range circuit — typically fine with #6 AWG copper but worth verifying
- A 300-foot 240V aluminum feeder to a farm building — may require 2/0 AWG or larger
Frequently Asked Questions
The NEC recommends no more than 3% voltage drop on branch circuits (NEC 210.19 informational note) and no more than 5% combined for feeders plus branch circuits (NEC 215.2 informational note). These are recommendations, not hard requirements.
Larger conductors (smaller AWG number or larger kcmil) have lower resistance and therefore lower voltage drop for the same current and length. Doubling the conductor cross-sectional area approximately halves the voltage drop.
Voltage drop increases proportionally with circuit length. A 200-foot run has twice the voltage drop of a 100-foot run at the same load and wire size. Long runs to detached buildings or subpanels often require upsizing the conductor.
Yes. The voltage drop percentage depends on the total supply voltage. A 240V circuit has half the drop percentage of an identical 120V circuit with the same wire, length, and current.
No. Aluminum has higher resistivity than copper, resulting in higher voltage drop for the same AWG size, current, and length. You typically need one to two AWG sizes larger in aluminum to match copper's voltage drop performance.
Use a larger AWG conductor, switch from aluminum to copper, install a subpanel closer to the loads, use 240V instead of 120V where practical, or run a dedicated circuit to high-draw loads to avoid load sharing.