Sizing Cable for a Long Run: Managing Voltage Drop
A wire can be perfectly legal from an ampacity standpoint and still deliver noticeably weak power at the far end of a long run. The longer the conductor, the more resistance it accumulates, and that resistance quietly steals voltage before it ever reaches your load. Understanding this distinction is what separates a cable job that merely passes inspection from one that actually performs.
Why Long Runs Are Voltage-Drop Limited, Not Ampacity Limited
Ampacity tables in the NEC tell you how much current a given wire size can carry continuously without overheating. A 12 AWG copper conductor rated at 20 A is safe from a thermal perspective even in a long conduit run, because heat dissipation depends on current, not distance.
Voltage drop is different. Every foot of wire adds resistance. More resistance means more of the supply voltage is consumed by the wire itself rather than the load. A 20 A circuit on 12 AWG for 15 feet barely registers a voltage problem. The same circuit stretched to 150 feet can lose several volts by the time power reaches the outlet, causing motors to run hot, lights to dim, and sensitive electronics to behave erratically.
This is why a subpanel feed to a detached garage or a deep-well pump circuit almost always requires a larger conductor than the load's ampacity alone would suggest. The wire must be upsized to keep resistance low enough that voltage drop stays within acceptable limits.
The 3% and 5% Guidelines
The widely followed rule of thumb, grounded in NEC recommendations (not a hard code violation, but a practical benchmark), is:
- 3% maximum voltage drop on any single branch circuit or feeder
- 5% maximum total from the service entrance to the farthest outlet
On a 120 V circuit, 3% equals 3.6 V. On a 240 V circuit, 3% equals 7.2 V. Those numbers shrink fast across a long run. Always verify your jurisdiction's requirements and consult a licensed electrician for code compliance, since some applications have stricter limits.
For a 240 V, 20 A circuit feeding a garage 120 feet away, you are allowed to drop 7.2 V before you exceed 3%. Whether you actually stay within that budget depends entirely on your conductor size.
The Voltage Drop Formula for Single-Phase Circuits
The standard single-phase voltage drop formula is:
Vd = (2 × K × I × L) / CM
Where:
- Vd = voltage drop (volts)
- K = resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = load current in amps
- L = one-way length of the run in feet
- CM = circular mils of the conductor (cross-sectional area)
The factor of 2 accounts for both the hot and neutral (or both hot legs on a 240 V circuit) because current travels the full round trip through the wire.
Common conductor sizes in circular mils:
- 12 AWG: 6,530 CM
- 10 AWG: 10,380 CM
- 8 AWG: 16,510 CM
- 6 AWG: 26,240 CM
- 4 AWG: 41,740 CM
For more background on how resistance and conductor geometry interact, see voltage drop explained.
Worked Example: 240 V Garage Circuit, 130 Feet
Say you are running a 240 V, 30 A circuit to a detached garage 130 feet from the main panel. The code-minimum wire for 30 A is 10 AWG copper (10,380 CM).
Check 10 AWG:
Vd = (2 × 12.9 × 30 × 130) / 10,380 Vd = (100,620) / 10,380 Vd = 9.69 V
On a 240 V supply, 9.69 V represents 4.04% drop. That exceeds the 3% branch-circuit guideline (7.2 V max). It may still pass a total-system 5% check depending on your service drop, but 4% on the branch alone leaves almost no margin.
Try 8 AWG (16,510 CM):
Vd = (2 × 12.9 × 30 × 130) / 16,510 Vd = 100,620 / 16,510 Vd = 6.09 V (2.54%)
That lands comfortably under 3%. For this run, 8 AWG is the right call even though the load only requires 10 AWG thermally.
See how to size a cable step by step for a complete walkthrough of combining ampacity and voltage drop checks into one selection process.
Distance vs. Recommended Wire Size: Sample Load Reference
The table below shows recommended copper conductor sizes for a 240 V, 20 A single-phase circuit at various one-way distances, targeting less than 3% voltage drop at full load. These are starting points, not a substitute for a full calculation on your specific circuit.
| One-Way Distance | Ampacity Min | Recommended for Vd | Vd at Recommended |
|---|---|---|---|
| 50 ft | 12 AWG | 12 AWG | 2.37% |
| 75 ft | 12 AWG | 10 AWG | 2.23% |
| 100 ft | 12 AWG | 10 AWG | 2.97% |
| 125 ft | 12 AWG | 8 AWG | 2.35% |
| 150 ft | 12 AWG | 8 AWG | 2.81% |
| 200 ft | 12 AWG | 6 AWG | 2.95% |
Beyond 200 feet, you are often looking at 4 AWG or larger, or reconsidering whether a small subpanel fed with appropriately sized conductors makes more practical sense.
Aluminum Wire and Long Runs
Aluminum conductors are common on long feeder runs because they cost significantly less per foot than copper, and the larger sizes needed to offset aluminum's higher resistivity close the gap in conduit fill. The K factor for aluminum is 21.2 versus 12.9 for copper, so aluminum requires roughly one to two wire sizes larger than copper to achieve the same voltage drop performance.
For long pump circuits specifically, sizing wire for a well pump covers the additional consideration of motor startup current, which creates a momentary voltage dip that compounds any steady-state drop already present in the conductor.
Frequently Asked Questions
Does voltage drop affect safety, or just performance?
Mostly performance. Excess voltage drop does not typically create a shock or fire hazard, but it stresses motors, shortens lamp life, and can cause variable-speed drives or electronic controls to malfunction. Some equipment has minimum voltage ratings, and operating below them voids warranties.
Can I use a voltage drop calculator instead of doing the formula by hand?
Absolutely, and most electricians do. The formula is useful for understanding the mechanics, but any reputable calculator gives the same result faster. Just verify it asks for one-way distance, not total circuit length, and that it uses the correct K value for your conductor material.
What if my run is 3-phase instead of single-phase?
Drop the factor of 2 from the formula and replace it with 1.732 (the square root of 3): Vd = (1.732 × K × I × L) / CM
Three-phase distribution is inherently more efficient, which is part of why commercial buildings use it for long panel-to-panel runs.
Is it ever acceptable to exceed 3% on a branch circuit?
The 3% figure is a guideline, not always a hard NEC violation, though some local codes adopt it as mandatory. Exceeding it is sometimes accepted on lighting circuits where the load is resistive and the consequences are mainly cosmetic. On motor circuits, even 3% at full load starts to cause problems during startup when current spikes briefly to six or more times running current. Always check local requirements and confirm with a licensed electrician before deviating.