Voltage Drop Calculator
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Result
Voltage drop
- As a percentage
- 6.43%
- Against the 3% recommendation
- 6.43% — exceeds the 3% recommendation
- Voltage at the load
- 112.28 V
- Smallest gauge within 3%
- 8 AWG (2.55%)
- Maximum distance at this gauge
- 47 ft one way at 20 A
- Conductor resistance
- 1.93 Ω per 1,000 ft (copper)
- A drop of 7.72 V is 6.43% — well over the 3% recommended for a branch circuit. 8 AWG would bring it to 2.55%. At 20 A this gauge is limited to about 47 ft.
- The distance is doubled because the current travels to the load and back. Using the one-way distance alone halves the answer, and it is the most common error in this calculation.
- The 3% branch-circuit and 5% total figures are informational notes in the NEC, not enforceable requirements — ampacity and overcurrent protection are what the code mandates. Voltage drop is a performance and efficiency matter, and on a long run it is often what determines wire size rather than ampacity.
Every gauge at this load and distance
20 A over 100 ft one way on 120 V copper. NEC Chapter 9, Table 8 resistances.
| Gauge | Ω/1,000 ft | Drop | Percent | 3% check |
|---|---|---|---|---|
| 14 AWG | 3.07 | 12.28 V | 10.23% | Over |
| 12 AWG (yours) | 1.93 | 7.72 V | 6.43% | Over |
| 10 AWG | 1.21 | 4.84 V | 4.03% | Over |
| 8 AWG | 0.764 | 3.06 V | 2.55% | Within 3% |
| 6 AWG | 0.491 | 1.96 V | 1.64% | Within 3% |
| 4 AWG | 0.308 | 1.23 V | 1.03% | Within 3% |
| 2 AWG | 0.194 | 0.78 V | 0.65% | Within 3% |
| 1/0 AWG | 0.122 | 0.49 V | 0.41% | Within 3% |
| 2/0 AWG | 0.0967 | 0.39 V | 0.32% | Within 3% |
| 4/0 AWG | 0.0608 | 0.24 V | 0.2% | Within 3% |
How do you calculate voltage drop?
For single-phase, multiply 2 by the one-way distance, the conductor resistance per 1,000 feet and the current, then divide by 1,000. At 20 A over 100 feet on 12 AWG copper the drop is 7.72 V, which is 6.43% of 120 V.
single-phase: VD = (2 × L × R × I) ÷ 1,000 three-phase: VD = (√3 × L × R × I) ÷ 1,000- L
- One-way distance in feet; the formula doubles it
- R
- Conductor resistance in ohms per 1,000 ft, from NEC Chapter 9 Table 8
- I
- Actual load current in amps, not the breaker rating
- 2 or √3
- Out-and-back path for single-phase; 1.732 for three-phase
Voltage drop is just Ohm's law applied to the length of the conductor, and the factor of 2 is the part people miss: current has to travel to the load and back, so a 100 foot run is 200 feet of copper. Omitting it halves the answer. Resistance comes from the wire's gauge, and the AWG scale runs backwards — 12 AWG is thinner and more resistive than 8 AWG. Aluminium at the same gauge has about 60% more resistance than copper, which is why it generally needs two sizes larger. The figures worth knowing are that the NEC recommends no more than 3% drop on a branch circuit and 5% total from service to load, and that both are informational notes rather than enforceable requirements. What the code actually mandates is ampacity and overcurrent protection. But on a long run, voltage drop is usually what determines the wire size anyway: a 20 A circuit on 12 AWG is perfectly legal on ampacity and still arrives at 6.43% over 100 feet, which means dim lights, slow motors and wasted energy as heat in the wall.
- Find the conductor resistance per 1,000 ft for your gauge.
- Measure the one-way distance from the panel to the load.
- Multiply distance by two for single-phase, or √3 for three-phase.
- Multiply by resistance and by the actual load current.
- Divide by 1,000 to get the drop in volts.
- Divide by system voltage for the percentage.
Worked example: 20 A over 100 ft one way on 12 AWG copper, 120 V single-phase
20 A over 100 ft one way on 12 AWG copper, 120 V single-phase
NEC Chapter 9, Table 8 gives 12 AWG uncoated copper a resistance of 1.93 ohms per 1,000 feet. The current travels out and back, so the conductor length is 2 × 100 = 200 feet. The drop is (2 × 100 × 1.93 × 20) ÷ 1,000 = 7.72 volts, which leaves 112.28 V at the load — a drop of 6.43%, more than double the 3% the NEC recommends for a branch circuit. That matters because 12 AWG is entirely legal for a 20 A circuit on ampacity grounds; it is the distance that fails, not the gauge. Working down the table, 10 AWG gives 4.84 V or 4.03%, still over. 8 AWG gives 3.06 V or 2.55%, which finally comes in under 3%. So this run needs 8 AWG — two sizes up from what ampacity alone would allow. Put differently, 12 AWG at 20 A is limited to about 47 feet one way before it breaches 3%.
What each voltage drop input means
- Conductor material
- Aluminium has about 60% more resistance than copper for the same gauge, so it needs roughly two sizes larger for the same drop.
- Wire size
- American Wire Gauge. Counter-intuitively, a smaller number is a thicker wire with less resistance.
- Load current
- The actual current the circuit will carry, not the breaker rating. Voltage drop depends on what flows, not on what is permitted.
- One-way distance
- From the panel to the load, measured one way. The formula doubles it, because current flows out and back.
- System voltage
- 120 V or 240 V for most residential circuits. The percentage is measured against this.
- Phase
- Single-phase uses a factor of 2 for the out-and-back path; three-phase uses √3, about 1.732.
What this voltage drop does not account for
- Double the one-way distance. Current flows to the load and back, so a 100 ft run is 200 ft of conductor. Forgetting this halves the calculated drop.
- The 3% branch-circuit and 5% total figures are informational notes in the NEC, not enforceable requirements. Ampacity and overcurrent protection are what the code mandates.
- Use the actual load current, not the breaker rating. A 20 A breaker protecting a 7 A load produces far less drop than the rating suggests.
- The AWG scale runs backwards — a smaller number is a thicker, less resistive wire. 8 AWG is heavier than 12 AWG.
- Aluminium has roughly 60% more resistance than copper at the same gauge, so it generally needs two sizes larger, plus listed terminations and anti-oxidant compound.
- Table 8 resistances are DC values. On large conductors at 60 Hz, AC resistance and reactance are slightly higher, and Table 9 should be used instead.
- Resistance rises with temperature, so a conductor running warm in a hot attic or a full conduit drops more than the table figure suggests.
- Motors are the loads most sensitive to voltage drop. Low voltage raises starting current and running temperature, shortening motor life.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% total from service to load. Both are informational notes rather than enforceable requirements, but they are good design practice.
How far can I run 12 AWG wire on a 20 amp circuit?
About 47 feet one way on 120 V before the drop exceeds 3%. Ampacity permits 12 AWG at 20 A regardless of distance, so on longer runs it is voltage drop that sets the wire size.
Why do you multiply the distance by two?
Because current travels out to the load and back through the neutral, so the conductor length is twice the one-way distance. Three-phase uses √3 instead of 2, about 1.732.
Does voltage drop matter on a short run?
Rarely. Drop is proportional to length, so a 15 foot run at normal residential currents is a fraction of a percent. It becomes the governing constraint on long runs to outbuildings, wells and pumps.
Is aluminium wire worse for voltage drop?
Yes, about 60% more resistive than copper at the same gauge, so it generally needs two sizes larger for the same drop. It is still used for larger feeders on cost grounds.
What happens if voltage drop is too high?
Lights dim and flicker, motors run hot and start poorly, and heating elements underperform. The lost voltage becomes heat in the wall, which is wasted energy you still pay for.
Sources and review
- NFPA 70, National Electrical Code — Chapter 9, Table 8, Conductor Properties — National Fire Protection Association
Last reviewed .