On long wire runs, voltage drops along the way and can starve a circuit. The accepted limit is about 3% for branch circuits. This chart gives the approximate one-way distance before a 120V copper circuit hits 3%.
| Wire (AWG) | Load | Max one-way run |
|---|---|---|
| 14 | 15 A | about 50 ft |
| 12 | 20 A | about 60 ft |
| 10 | 30 A | about 64 ft |
| 8 | 40 A | about 76 ft |
| 6 | 55 A | about 88 ft |
FAQ
How much voltage drop is considered acceptable? Most electrical codes and best practices target no more than a 3 percent voltage drop for branch circuits and 5 percent for the combined feeder and branch circuit, beyond which equipment can run inefficiently or fail to operate correctly.
See the Wire Ampacity Chart and What Wire Size Do I Need.
Why Drop Matters Beyond Code
The 3% limit is a code guideline, but the practical effects show up before you get anywhere near a violation. LED fixtures and drivers are sensitive to under-voltage — dimming, flicker, or a driver refusing to start reliably are common symptoms of a marginal circuit long before a voltmeter shows a code violation. Motors and compressors are even less forgiving, since a sagging start voltage can stall a motor or trip its overload protection on startup.
240V circuits tolerate roughly double the one-way distance of an equivalent 120V circuit for the same percentage drop, because voltage drop as a percentage of a higher base voltage is proportionally smaller for the same current and wire — which is part of why long runs to detached garages or outbuildings are often wired at 240V even for loads that could run on 120V.
This chart’s numbers are for single-phase circuits. Three-phase voltage drop follows a different formula (using √3 instead of the factor of 2 used for a round-trip single-phase circuit), so don’t apply this table directly to three-phase motor or equipment feeders.