Why Voltage Drop Matters
Run a load far from the panel and the wire itself becomes a resistor. Some of the supply voltage is "spent" heating the conductor instead of reaching the device. A few volts of drop on a lighting circuit just dims the bulbs; the same drop on a motor can cause overheating, lost torque, and premature failure. Sizing wire for the load current alone is not enough — you must size it for the distance too.
- Voltage drop is wasted voltage lost to wire resistance over distance.
- It gets worse with higher current and longer runs.
- Undersized long runs overheat conductors and starve loads.
The Formulas
Voltage drop is computed from Ohm's law applied to the conductor's resistance. Resistance is given per 1,000 feet of wire, so the run length enters directly.
Single-phase (two wires carry current, so the drop happens on both the outbound and return path): V_drop = 2 × I × R × L ÷ 1000, where I is the load current in amps, R is the conductor resistance in ohms per 1,000 ft, and L is the one-way length in feet.
Three-phase: V_drop = √3 × I × R × L ÷ 1000. The √3 factor reflects the 120° phase separation.
Use the one-way distance L, not the total wire length, in the formula — the "2 ×" in the single-phase equation already accounts for the return conductor.
NEC Recommended Limits
The National Electrical Code does not strictly mandate a maximum voltage drop, but it recommends a maximum of 3% on a branch circuit and 5% combined for the feeder plus branch circuit (NEC Informational Note). Staying within these keeps equipment happy and efficiency high. A 120 V circuit dropping more than about 3.6 V at the outlet is over the branch-circuit guideline.
- Branch circuit: aim for ≤ 3% drop.
- Feeder + branch combined: aim for ≤ 5%.
- These are recommendations, not hard code limits — but inspectors and equipment care.
Copper Resistance by AWG (at ~75°C)
This is the table the calculations use — ohms of resistance per 1,000 feet of copper conductor. Notice how dramatically resistance falls as the wire gets thicker.
Common branch-circuit sizes
14 AWG: 3.07 Ω/1,000 ft
12 AWG: 1.93 Ω/1,000 ft
10 AWG: 1.21 Ω/1,000 ft
Larger feeders
8 AWG: 0.764 Ω/1,000 ft
6 AWG: 0.491 Ω/1,000 ft
4 AWG: 0.308 Ω/1,000 ft
2 AWG: 0.194 Ω/1,000 ft
Going up just one size (e.g., 12 to 10 AWG) can nearly halve the resistance — often the cheapest fix for a drop problem on a long run.
Worked Example: A 120 V, 16 A Run
A 120 V load drawing 16 A sits 100 ft from the panel, wired in 12 AWG copper (R = 1.93 Ω/1,000 ft).
V_drop = 2 × 16 × 1.93 × 100 ÷ 1000 = 6.18 V.
That is 6.18 ÷ 120 = 5.1% drop — over the 3% branch-circuit guideline. Upsize to 10 AWG (R = 1.21): V_drop = 2 × 16 × 1.21 × 100 ÷ 1000 = 3.87 V, or 3.2% — still slightly high. At 8 AWG (R = 0.764) the drop falls to 2.4%, comfortably inside the limit.
The fix is thicker wire, a shorter run, or (where allowed) a higher supply voltage for the long leg.
Practical Sizing Rules
Start with the ampacity the load requires, then check the drop at the actual run length. If the drop exceeds the guideline, bump the conductor one or two sizes before adding repeaters or boosters. For very long runs, consider 240 V loads (half the current for the same power) which inherently halve the drop.
- Size for ampacity first, then verify drop.
- 240 V loads cut current — and drop — in half for the same wattage.
- When in doubt, go one size up on long runs.
Frequently Asked Questions
Is voltage drop a code violation?
The NEC maximums (3% branch, 5% combined) are recommendations in an informational note, not enforceable limits. But excessive drop causes real equipment problems and failed inspections for other reasons, so designers treat them as limits.
Why does the single-phase formula use 2 × but three-phase uses √3?
In single-phase the current flows out and back through two conductors, so both contribute resistance (hence the factor of 2). In three-phase the phases are 120° apart, so the geometry gives a √3 factor instead.
Does aluminum wire change the table?
Yes — aluminum has about 1.6x the resistance of copper for the same size, so drop is larger and you must use the aluminum resistance values, not the copper ones above.
What if my run is already installed and drops too much?
Options are: re-pull with a larger conductor, shorten the effective path, convert the load to a higher voltage, or add a local transformer/booster. Relocating the panel closer to the load is often cheaper than rewiring.