Voltage Drop Calculator | NEC Wire Sizing & Circuit Compliance
Calculate voltage drop percentage for single-phase and three-phase AC circuits. Checks compliance with NEC 3% branch-circuit and 5% total-circuit limits, recommends the minimum wire gauge (AWG) to meet code, and supports both copper and aluminum conductors.
Panel to load. Total conductor = 2× (single-phase).
What Is the Voltage Drop Calculator | NEC Wire Sizing & Circuit Compliance?
Voltage drop is the reduction in electrical potential along a conductor carrying current. Every wire has resistance, and per Ohm's Law, current flowing through resistance generates a voltage loss between the source and the load. Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction or fail. The NEC sets maximum limits to protect equipment and ensure safe operation.
- ›NEC limits are recommendations, not hard code requirements. NEC 210.19(A) Informational Note 4 recommends 3% for branch circuits and 5% total (feeder + branch). AHJs may enforce these as mandatory — check local codes. For sensitive equipment, many engineers target 2%.
- ›Resistance data from NEC Table 9. This calculator uses NEC Table 9 alternating-current resistance values for copper and aluminum conductors in steel conduit — the most commonly applicable conditions. PVC conduit reduces resistance by approximately 5%.
- ›Conductor material matters significantly. Aluminum wire has roughly 1.6× the resistance of copper at the same gauge. Aluminum is common in service entrance and feeder runs where larger conductors offset the difference in resistivity.
- ›Ampacity is a separate constraint. A conductor must satisfy both voltage drop and ampacity (current-carrying capacity) requirements. The calculator flags when your selected wire is undersized for the stated load current.
Formula
Single-Phase Voltage Drop
VD = 2 × R × L × I / 1000
R = conductor resistance (Ω/1000 ft, NEC Table 9), L = one-way length (ft)
I = load current (A), factor 2 = round trip (hot + neutral/return)
Three-Phase Voltage Drop
VD = √3 × R × L × I / 1000
√3 ≈ 1.732 accounts for phase relationship in 3-phase systems
NEC Compliance Thresholds
Branch circuit: VD% ≤ 3% | Feeder + branch total: VD% ≤ 5%
VD% = (VD / System Voltage) × 100
How to Use
- 1
Select system voltage (120, 240, 208, 277, 480V, or custom).
- 2
Choose single-phase or three-phase configuration.
- 3
Select conductor material (copper or aluminum) and wire gauge.
- 4
Enter one-way run length in feet and load current in amps.
- 5
Click Calculate Voltage Drop to see drop voltage, percent, and NEC compliance.
- 6
Review the minimum compliant gauge recommendation if your selection fails the 3% limit.
- 1Select system voltage: Choose 120V, 240V, 208V, 277V, 480V, or enter a custom voltage. The system voltage determines percent voltage drop.
- 2Choose phase configuration: Single-phase uses the factor 2 (round trip); three-phase uses √3 (1.732). Most residential circuits are single-phase.
- 3Select conductor material: Copper or aluminum. Copper is standard for branch circuits; aluminum is common for feeders and service entrance.
- 4Choose wire gauge: Select AWG or MCM size. Sizes range from 14 AWG through 500 MCM. The NEC Table 9 resistance for that size is applied automatically.
- 5Enter run length and load: Enter the one-way distance in feet and the load current in amps. The calculator computes round-trip drop automatically.
- 6Review compliance and minimum gauge: See voltage drop percent, NEC status, and the minimum gauge that achieves code compliance for your run length and load.
Example Calculation
Example: 240V circuit, 30A load, 150 ft one-way, copper #10 AWG
NEC Table 9: #10 AWG copper resistance = 1.24 Ω/1000 ft
VD = 2 × 1.24 × 150 × 30 / 1000
VD = 2 × 1.24 × 4500 / 1000 = 11,160 / 1000 = 11.16 V
VD% = 11.16 / 240 × 100 = 4.65% — exceeds 3% NEC limit
Upgrade to #8 AWG: R = 0.778 → VD = 7.00 V → VD% = 2.92% ✓
Minimum compliant gauge: #8 AWG copper
#10 AWG ampacity (30A) is sufficient for load — voltage drop is the binding constraint
Understanding Voltage Drop | NEC Wire Sizing & Circuit Compliance
When Voltage Drop Becomes a Real Problem
The 3% NEC limit is a practical engineering target, not a hard physical boundary. A circuit at 4% voltage drop will still function — but motors run at reduced torque, incandescent and halogen fixtures dim noticeably, and variable-frequency drives may fault on low-voltage alarms. Where voltage drop matters most: long runs to outbuildings, EV charger circuits (long garage runs are very common), irrigation pump systems, workshop circuits with large motor loads, and commercial LED drivers with tight input voltage tolerances.
Practical Wire Sizing Strategy
- ›Size for ampacity first, then check voltage drop. The minimum code-compliant wire must satisfy the ampacity table. Voltage drop then determines whether you need to upsize beyond that minimum.
- ›Upsizing is cheap insurance. On a 150-foot run, upgrading from #10 to #8 AWG copper adds perhaps $30–50 in material cost and eliminates potential equipment issues for the life of the installation. Labor cost is identical.
- ›Paralleling conductors is an option for very long runs. Two #8 AWG conductors in parallel are equivalent to one #5 AWG (which does not exist as a standard size). Parallel runs must be the same length, material, gauge, and conduit type to share current evenly.
- ›Three-phase systems inherently have lower voltage drop. For the same load, a three-phase circuit carries less current per conductor, reducing drop. Three-phase motors typically specify ≤2% voltage drop at rated load.
Frequently Asked Questions
Why does the NEC have separate limits for branch circuits and total circuits?
The 5% total cap accounts for cumulative drop across feeder and branch circuits. Budget the drop across both segments — if the feeder uses 2%, only 3% remains for the branch circuit.
Does this calculator account for temperature corrections?
NEC Table 9 values assume 75°C conductor temperature. High-ambient runs (attics, direct sun) have higher resistance — apply temperature correction factors for critical applications.
When should I use aluminum wire instead of copper?
Use aluminum for feeders #4 AWG and larger to save cost. Always use anti-oxidant compound and AL-rated terminations. Avoid aluminum for #12 AWG and smaller branch circuits.
What is the difference between AWG and MCM (kcmil)?
AWG counts down (smaller number = larger wire). Above 4/0, conductors switch to MCM (circular mils × 1000) which measures area directly. 250 MCM is larger than 4/0 AWG.
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