Calculator
Electrical Voltage Drop Calculator
Verify voltage drop before you pull wire — enter load amps, system voltage, one-way run length, and conductor size to see drop in volts and percent against NEC 3% branch and 5% combined limits. Free, no signup.
Free
Circuit preset
Advanced options
Parallel runs divide effective impedance. Power factor and raceway only change the result for AC circuits below unity power factor, where conductor reactance matters.
General branch — 20 A120 V · Single-phase ACPVC / nonmetallic
Voltage drop
CopperOver 3%
6.6%
7.92 V drop
Upsize to 8 AWG
AluminumOver 3%
10.8%
13.00 V drop
Upsize to 6 AWG
20 A · 100 ft one-way
Calculation breakdown
Design current
20.0 ALoad current used
NEC limit
3.0% (3.6 V)NEC 210.19(A) Informational Note 4 — branch circuits ≤ 3%
Copper — 12 AWG
7.92 V (6.6%)R 1.9800 Ω/1000 ft @ unity PF (Table 8)
Aluminum — 12 AWG
13.00 V (10.8%)R 3.2500 Ω/1000 ft @ unity PF (Table 8)
Max run — copper
45 ft12 AWG within 3.0%
Max run — aluminum
27 ft12 AWG within 3.0%
Wire size comparison
| Size | Copper | Aluminum |
|---|---|---|
| 14 AWG | 10.5% | — |
| 12 AWG | 6.6% | 10.8% |
| 10 AWG | 4.1% | 6.8% |
| 8 AWG | 2.6% | 4.3% |
| 6 AWG | 1.6% | 2.7% |
Green meets the 3.0% limit · red exceeds it.
Notes
- One-way run length in feet — phase factor (2 or √3) accounts for current path.
- Small-gauge aluminum branch circuits are uncommon — verify termination compatibility and local practice.
What you'll get
Choose your calculation mode
Check voltage drop on a selected wire size, find the minimum conductor that meets your limit, or calculate the maximum one-way run length for a given gauge.
Enter circuit parameters
Set load current, system voltage, phase, one-way run length in feet, and NEC limit — 3% branch, 3% feeder, or 5% combined feeder plus branch. Power factor and raceway are optional for inductive loads.
Compare copper and aluminum
Every result is shown for copper and aluminum side by side — voltage drop in volts and percent, the minimum size that meets your limit, or the maximum run length, plus a wire size comparison table.
How it works
- 1
Choose your calculation mode
Check voltage drop on a selected wire size, find the minimum conductor that meets your limit, or calculate the maximum one-way run length for a given gauge.
- 2
Enter circuit parameters
Set load current, system voltage, phase, one-way run length in feet, and NEC limit — 3% branch, 3% feeder, or 5% combined feeder plus branch. Power factor and raceway are optional for inductive loads.
- 3
Compare copper and aluminum
Every result is shown for copper and aluminum side by side — voltage drop in volts and percent, the minimum size that meets your limit, or the maximum run length, plus a wire size comparison table.
Why voltage drop matters on every electrical run
Voltage drop is the reduction in voltage between the source and the load caused by conductor resistance over distance. Even when a wire meets ampacity, excessive drop can cause motors to overheat, lights to dim, and sensitive equipment to fault. NEC Informational Notes recommend limiting branch-circuit drop to 3% and combined feeder plus branch drop to 5% — limits most jurisdictions enforce as standard practice even though they are not mandatory Code rules.
This calculator uses the job-site method from NEC Chapter 9 Table 8 resistance values with single-phase (×2) and three-phase (√3) factors, and adds Table 9 conductor reactance so power factor changes the result the way it does on real inductive feeders — effective impedance Zₑ = R·cosθ + X·sinθ. It supports the continuous-load 125% factor, parallel conductor runs, PVC/aluminum/steel raceways, and three operating modes used by industry tools: drop check, minimum wire size, and maximum circuit distance. Copper and aluminum from 14 AWG through 1000 kcmil are sized together, so you compare both conductors at a glance. Pair it with the wire size calculator to verify both ampacity and voltage drop before you order material.
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View statesVoltage Drop Calculator FAQs
What is the NEC voltage drop formula?
Single-phase: VD = (2 × K × I × L) ÷ CM. Three-phase: VD = (√3 × K × I × L) ÷ CM. K is 12.9 for copper and 21.2 for aluminum at 75°C. I is current in amps, L is one-way length in feet, and CM is conductor area in circular mils from NEC Chapter 9 Table 8. For inductive loads the calculator uses the Table 9 effective impedance Zₑ = R·cosθ + X·sinθ in place of resistance, so the power factor and raceway type change the drop; at unity power factor this reduces to the resistance-only result.
What are the NEC voltage drop limits?
NEC 210.19(A) Informational Note 4 recommends branch-circuit voltage drop not exceed 3%. NEC 215.2(A) Informational Note 2 recommends feeder drop not exceed 3% and combined feeder plus branch drop not exceed 5%. These are recommendations, not enforceable rules — but most inspectors and engineers treat them as standard.
Is run length one-way or round-trip?
Enter one-way distance in feet. The formula factor of 2 (single-phase) or √3 (three-phase) already accounts for the current path through the conductors — do not double the length.
When should I upsize wire for voltage drop?
On long runs at lower voltage — especially 120 V branch circuits over 75–100 feet, EV charger circuits, detached garage feeders, and rooftop equipment — voltage drop often requires a larger conductor than ampacity alone. If drop exceeds 3% on a branch or 5% combined, upsize the conductor or add parallel runs.
