Enter your wire gauge, one-way circuit length, and current to find the voltage drop across the run - useful for checking whether a wire run meets standard 3% guidance for branch circuits.
Voltage Drop Calculator
LiveThe formula
12 AWG copper wire (6,530 circular mils), 50 ft one-way, 15A load, 120V circuit: drop = (2 x 12.9 x 15 x 50) / 6530 ≈ 2.96V, or 2.47% - within the commonly recommended 3% guidance for branch circuits.
Max one-way distance by gauge, before hitting 3%
A common practical question runs the calculation backward: for a given wire gauge and load, how far can the run go before voltage drop becomes a real problem? For a typical 15A, 120V circuit:
| Wire gauge | Max one-way distance for 3% drop (15A, 120V) |
|---|---|
| 14 AWG | 38 ft |
| 12 AWG | 61 ft |
| 10 AWG | 97 ft |
| 8 AWG | 154 ft |
| 6 AWG | 244 ft |
| 4 AWG | 388 ft |
This is specific to a 15A/120V case -- use the calculator above for any other current or voltage, since the relationship isn't linear across different loads.
Where the "3%" figure actually comes from
Many tools and guides present 3% as if it were a hard NEC requirement. It's more accurately described as guidance: the National Electrical Code includes it as an Informational Note under sections like 210.19(A) and 215.2(A) -- recommending, not mandating, that branch circuits keep voltage drop to about 3%, and the combined feeder-plus-branch-circuit total to about 5%, for reasonable operating efficiency. Informational Notes are explanatory, not enforceable Code requirements on their own -- though some local jurisdictions or specific equipment manufacturers do make voltage drop limits a hard requirement in their own amendments or installation instructions, which would then apply. Check your local code and any manufacturer specifications rather than assuming 3% is universally mandatory.
Why voltage drop matters
Excessive voltage drop means equipment at the end of a long circuit run receives less voltage than intended, which can cause dimmer lights, motors that run hotter and less efficiently, or electronics that behave unpredictably. It's a bigger concern on longer runs and higher-current circuits, which is why detached garages, workshops, or long outdoor runs often need a larger wire gauge than the same load would require over a short distance -- the Wire Gauge Calculator can help pick an appropriate starting gauge before checking the drop here.
Common mistakes
Frequently asked questions
What voltage drop percentage is acceptable?
3% is a commonly cited recommendation for a branch circuit alone, and 5% for the combined feeder plus branch circuit -- but this comes from an NEC Informational Note, not a strict universal Code requirement in every jurisdiction. Check your local electrical code and any equipment manufacturer specifications.
Why does aluminum wire need a bigger gauge than copper for the same job?
Aluminum has higher resistance than copper for the same cross-sectional area, so an aluminum wire needs to be larger (more circular mils) to carry the same current with an equivalent voltage drop -- reflected in the higher K value (21.2 vs 12.9) this calculator uses for aluminum.
Does temperature affect voltage drop?
Yes - resistance increases with temperature, so a wire running hot will have somewhat more voltage drop than the same wire at a cooler temperature. This calculator uses the standard 75°C reference figure used in most NEC-based calculations.
Is voltage drop the same as a short circuit or overload?
No -- voltage drop is a normal, gradual effect of a wire's resistance over distance, present in every circuit to some degree. A short circuit or overload is a fault condition, typically involving much higher, sudden current that trips a breaker -- an entirely different concern from the everyday efficiency question this calculator addresses.
Why does a longer wire run need a bigger gauge?
Resistance increases with wire length, so a longer run accumulates more voltage drop for the same current and gauge. A thicker wire (lower gauge number) has more circular mils and less resistance, which offsets that added length -- the reference table above shows roughly how far a given gauge can run before crossing 3% at a standard 15A/120V load.
Does this apply to three-phase circuits?
No -- this calculator uses the single-phase formula, which includes a factor of 2 for the round-trip current path. Three-phase voltage drop calculations use a different formula (typically with a √3 factor instead), so don't use this tool directly for three-phase circuits.
Should I calculate voltage drop for every circuit I install?
It matters most for longer runs, higher-current loads, or sensitive equipment -- a short run to a nearby outlet on standard gauge wire is rarely a practical concern. Detached structures, long outdoor runs, motors, and EV chargers are common cases worth checking specifically.
What's the difference between circular mils and AWG gauge number?
AWG (American Wire Gauge) is a standardized sizing scale where a smaller number means a thicker wire. Circular mils are the actual cross-sectional area measurement used directly in the voltage drop formula -- this calculator converts your selected AWG gauge to its standard circular mil area automatically.
Sources
- Voltage drop formula and K constants. Standard single-phase voltage drop formula and NEC Chapter 9, Table 8 conductor properties (K ≈ 12.9 for copper, 21.2 for aluminum, at 75°C), consistent across electrical engineering and electrical trade references.
- Circular mil (CM) values by AWG gauge. Standard American Wire Gauge circular mil area table -- unchanging physical wire-size data.
- 3%/5% voltage drop guidance. National Electrical Code (NEC) Informational Notes under sections 210.19(A) and 215.2(A) -- explicitly informational/advisory rather than mandatory Code text, though local jurisdictions may adopt stricter binding amendments.
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