🔌Voltage Drop Calculator
Calculate voltage drop and percentage voltage drop for AC single-phase, AC three-phase, and DC circuits. Uses NEC Table 9 data or resistivity-based estimation.
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Voltage Drop (V)
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Voltage Drop Calculator
Voltage drop is the reduction in electrical potential along a conductor as current flows through it. Excessive voltage drop causes equipment to run below its rated voltage — shortening life, reducing efficiency, and causing motors and sensitive electronics to malfunction. This calculator uses NEC Table 9 data, resistivity-based estimation, or custom impedance values to compute voltage drop for single-phase AC, three-phase AC, and DC circuits.
NEC recommendation: Keep voltage drop at or below 3% for branch circuits and 5% combined (feeder + branch).
Every conductor — copper, aluminum, or otherwise — has resistance. When current flows through that resistance, energy is lost as heat and voltage falls between the source and the load. The longer the conductor run and the higher the current, the greater the voltage drop. Understanding and controlling voltage drop is essential for any electrical installation, from residential wiring to industrial power distribution.
Calculation Methods Explained
This calculator offers three methods to suit different levels of available data:
- NEC Data (Table 9): Uses the resistance and reactance values published in NEC Table 9, measured at 75 °C conductor temperature — the standard design temperature for US circuit calculations. Values account for the skin effect and proximity effect in stranded conductors, and vary by conduit material (steel, aluminum, or PVC).
- Estimated Resistance: Computes conductor resistance from first principles using the wire's cross-sectional area (in mm²) and the resistivity of copper or aluminum at 75 °C. Useful for metric wire sizes or non-AWG conductors not covered by NEC Table 9.
- Custom Impedance: Allows direct entry of resistance and reactance in mΩ/m. Use this when working from IEC or BS wire standards, or when a manufacturer's data sheet provides impedance values directly.
NEC Voltage Drop Limits
| Circuit Type | NEC Recommendation | Notes |
|---|---|---|
| Branch circuit | ≤ 3% | Panel to outlet/device |
| Feeder | ≤ 3% | Service entrance to panel |
| Combined feeder + branch | ≤ 5% | End-to-end maximum |
| Sensitive equipment | ≤ 2% | Computers, medical devices, motors |
Why Three-Phase Circuits Have Lower Voltage Drop
The three-phase formula uses a multiplier of √3 (≈ 1.732) rather than 2. In a balanced three-phase circuit, the return current is distributed across three conductors rather than a single return wire, which effectively reduces the total conductor length in the calculation. This means a three-phase circuit has roughly 13% less voltage drop than a single-phase circuit of the same wire size, current, and length — one of the key efficiency advantages of three-phase power distribution.
Strategies to Reduce Voltage Drop
- Upsize the conductor: Going from 12 AWG to 10 AWG nearly halves the resistance per foot.
- Use copper instead of aluminum: Copper has about 60% of the resistivity of aluminum at the same temperature.
- Increase supply voltage: Using 240 V instead of 120 V halves the current for the same load, cutting voltage drop by 75%.
- Run parallel conductors: Two conductors in parallel halve the effective resistance.
- Shorten the run: Voltage drop is directly proportional to length — reducing run length is always effective when feasible.
When Voltage Drop Is the Controlling Factor
For most short branch circuits, conductor ampacity is the design constraint. But for long runs — to outbuildings, motors, sub-panels, or EV chargers — voltage drop often requires a larger conductor than ampacity alone would dictate. A 100-amp subpanel run 200 feet away may need 2/0 AWG copper not because 2/0 is needed for the current capacity, but because anything smaller would produce unacceptable voltage drop at full load.
Frequently Asked Questions
What is voltage drop and why does it matter?
Voltage drop is the loss of electrical potential as current flows through a conductor's resistance. If the supply is 120 V but the device receives only 108 V due to a 10% drop, motors run hotter and less efficiently, fluorescent lights flicker, and sensitive electronics may malfunction or fail prematurely. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% combined to ensure equipment operates within its rated voltage range.
What is the difference between single-phase and three-phase voltage drop formulas?
For single-phase and DC circuits, the formula multiplies by 2 to account for the outgoing and return conductors. For three-phase circuits, the multiplier is √3 (≈ 1.732) because the current returns through three conductors simultaneously in a balanced system. This means three-phase voltage drop is about 13% lower than single-phase for the same wire size, length, and current.
Should I use NEC Table 9 or the estimated resistance method?
NEC Table 9 is the preferred method for US electrical work because it uses empirically measured values that account for real-world factors like the skin effect, proximity effect, and stranding in standard conductors at the NEC design temperature of 75 °C. The estimated resistance method is better suited to metric wire sizes, non-standard conductors, or situations where the wire is not listed in NEC Table 9.
How does conduit material affect voltage drop in AC circuits?
Steel conduit is magnetic, which increases the inductive reactance of conductors inside it compared to non-magnetic aluminum or PVC conduit. This effect is included in NEC Table 9 through separate reactance values for each conduit type. For DC circuits or very low-frequency AC, reactance is negligible and conduit type has almost no effect on voltage drop.
What wire size should I use for a 100-foot run at 20 amps on 120 V?
For a 100-foot one-way run (200 feet total conductor length) at 20 A on a 120 V single-phase circuit with 12 AWG copper in PVC conduit, the NEC method yields a voltage drop of approximately 3.8 V, or about 3.2% — just above the 3% recommendation. Upgrading to 10 AWG reduces the drop to about 2.4%, comfortably within the limit. Always run this calculation for any circuit over 50 feet.
Does voltage drop waste energy?
Yes. The energy lost to voltage drop is dissipated as heat in the conductor — this is resistive loss, sometimes called I²R loss. On a heavily loaded long circuit, conductor heating can be significant. This is one reason why high-efficiency industrial facilities and data centers invest in larger-than-minimum wire sizes and why transformer proximity to loads matters in power distribution design.
Can I run conductors in parallel to reduce voltage drop?
Yes. Running two conductors in parallel per phase effectively doubles the cross-sectional area and halves the resistance, reducing voltage drop by approximately 50%. The NEC requires that parallel conductors of the same phase be identical in material, size, length, and type. This approach is common for large feeders and motor circuits where a single conductor of the required size would be impractically large.