Voltage Drop Calculator
Calculate voltage drop across a copper or aluminum branch circuit or feeder. Uses NEC Chapter 9 Table 8 DC resistance at 75 °C and flags whether the run meets the NEC-recommended ≤ 3 % branch-circuit limit.
Voltage drop
3.22%
- Voltage drop
- 3.86
- Voltage at load
- 116.14
- Source voltage
- 120
- Conductor resistance (Ω / 1000 ft)
- 1.93
120 V single-phase. Using NEC Chapter 9 Table 8 DC resistance for copper at 75 °C, voltage drop = 2 × length × current × resistance. Exceeds the NEC-recommended ≤ 3 % branch-circuit limit — consider a larger conductor or shorter run. NEC 210.19(A) Informational Note 4 recommends ≤ 3 % on branch circuits and ≤ 5 % combined feeder + branch.
How to use this calculator
Enter the load current in amps, the one-way run length in feet (panel to load, not the round trip), then pick the conductor size in AWG, the conductor material (copper or aluminum), and the system voltage. The calculator returns the voltage drop in volts, the same drop as a percentage of source voltage, the voltage actually arriving at the load, and the conductor's ohms-per-1000-ft value. It also flags whether the result is within the NEC-recommended ≤ 3 % branch-circuit limit. If the percentage is too high, try a larger conductor (lower AWG number = thicker wire), a shorter run, or — on long runs of low-voltage DC — a higher system voltage.
How the calculation works
For single-phase or DC circuits the drop is 2 × L × I × R, with the factor of 2 accounting for the round trip through the hot and neutral conductors. For three-phase circuits the drop is √3 × L × I × R, reflecting the line-to-line voltage relationship in a balanced three-phase system. L is the one-way run length, I is the load current, and R is the conductor resistance per unit length. The resistance values come from NEC 2017 Chapter 9, Table 8 — DC resistance at 75 °C for uncoated stranded copper and aluminum conductors. DC resistance is the right choice for typical 60 Hz residential and light-commercial runs where inductive reactance is negligible; long, high-current feeders should use the effective impedance from NEC Table 9 instead. The percentage drop is the voltage drop divided by the source voltage. The NEC has no enforceable voltage-drop limit, but the Informational Notes to 210.19(A) and 215.2(A)(1) recommend ≤ 3 % on branch circuits and ≤ 5 % combined feeder + branch for reasonable performance and motor longevity.
Worked example
A 20 A continuous load on a 12 AWG copper branch circuit, 50 ft one-way, on 120 V single-phase. Resistance is 1.93 Ω per 1000 ft (NEC Table 8). Drop = 2 × 50 × 20 × (1.93 / 1000) = 3.86 V. As a percentage of 120 V that is 3.22 % — just above the NEC ≤ 3 % guideline, so the next thicker conductor (10 AWG, 1.21 Ω / kft) would be the safer choice on a continuous-duty run of that length and current. The 10 AWG version computes to 2 × 50 × 20 × (1.21 / 1000) = 2.42 V or 2.02 %, well inside the limit. The same circuit on 240 V single-phase would only see 1.61 % drop with 12 AWG, because the percentage is referenced to a higher source voltage even though the absolute drop in volts is identical.
Frequently asked questions
What is the NEC voltage drop limit?
The NEC does not impose an enforceable hard limit on voltage drop. Informational Note 4 to 210.19(A) recommends ≤ 3 % drop on branch circuits, and a parallel note to 215.2(A)(1) recommends ≤ 5 % combined for feeder plus branch circuit, "for reasonable efficiency of operation." These are guidance figures rather than code requirements, but most inspectors and designers treat them as the working ceiling and most equipment is specified to operate within those tolerances. Some specialty installations — sensitive electronics, motor branch circuits, long PV string runs — pick a tighter target (often 2 %) for performance or longevity reasons.
Why is the formula 2 × L for single-phase but √3 × L for three-phase?
The factor of 2 on single-phase or DC reflects the physical round trip — current flows out on the hot conductor and back on the neutral, so the total resistive path is twice the one-way length. The √3 on three-phase comes from the line-to-line voltage relationship in a balanced three-phase system: instantaneous currents in the three phases sum to zero in a balanced load, so there is no neutral return current, and the relevant geometry gives a √3 ≈ 1.732 multiplier on the one-way drop. Net effect: a balanced three-phase circuit drops only about 86 % of what a single-phase circuit with the same per-conductor resistance and current would drop.
Should I use copper or aluminum?
Copper is the default for branch-circuit wiring in residential and most light-commercial work — better conductivity (about 60 % more conductance per unit volume), no aluminum-specific terminations required, no oxidation concerns at lug connections. Aluminum is widely used on service-entrance, feeder, and high-ampacity utility runs where the cost saving on heavy gauges is significant, and on long runs where the size step-up to compensate for higher resistance is acceptable. For a given ampacity, aluminum is typically two AWG sizes larger than copper. Aluminum requires antioxidant compound on terminations and AL- or CU/AL-rated lugs; the old solid aluminum branch wiring of the 1960s-70s is what caused aluminum's reputation problem, modern stranded AA-8000-series aluminum on properly rated terminations is reliable.
Does temperature affect resistance?
Yes. The values used here are NEC Table 8 DC resistance at 75 °C, which is the standard reference for sizing branch circuits and feeders sized to the 75 °C ampacity column. At 25 °C the resistance is roughly 16 % lower (copper temperature coefficient ≈ 0.393 % per °C); at 90 °C it is roughly 6 % higher. For most practical drop calculations the 75 °C figure is close enough — the conductor warms toward 75 °C under sustained load anyway. If you are doing a precision calculation on a circuit that genuinely runs cold (cold-room circuits, outdoor in winter) or hot (motor branch in a 50 °C ambient), apply the temperature correction explicitly.
Should I use DC resistance or effective Z (Table 9)?
DC resistance from Table 8 is appropriate for typical 60 Hz branch circuits and short feeders where inductive reactance is small compared to resistance — essentially anything 1 AWG and smaller, or shorter runs at larger sizes. For long high-current feeder runs at 2/0 AWG and larger, the conductor reactance becomes a noticeable contributor and the NEC Table 9 effective Z values (which include reactance and account for conduit material) give a more accurate answer. The difference is in the few-percent range and matters most for utility-style feeders and motor branch circuits at long distances; for residential branch circuits Table 8 is the standard choice and is what almost every published voltage-drop calculator (Southwire, IDEAL, Mike Holt) uses by default.
How do I read AWG sizes?
American Wire Gauge is inverse — smaller numbers mean thicker wire. Common residential branch sizes are 14 AWG (rated 15 A under the 60 °C column), 12 AWG (20 A), 10 AWG (30 A), and 8 AWG (40-50 A depending on insulation). Once you go thicker than 1 AWG the sizing notation switches to "aught" — 1/0 (one aught), 2/0, 3/0, 4/0 — and above 4/0 it switches again to thousands of circular mils (250 kcmil, 350 kcmil, 500 kcmil, etc.). This calculator covers 14 AWG through 4/0, which spans almost all residential and light-commercial work; for parallel feeder runs in the kcmil range you would typically be using NEC Table 9 effective Z and a dedicated feeder-sizing tool.