Voltage Drop Calculator
Calculate voltage drop, load voltage, and the minimum wire size needed to stay under 3% loss.
Suggested Minimum Wire Size (for ≤ 3% drop)
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Based on standard NEC Chapter 9, Table 8 DC resistance values for uncoated conductors. This is an estimate for planning purposes — always verify against the current NEC and local code, or consult a licensed electrician, before sizing a real installation.
About this tool
What this voltage drop calculator does
Run a wire far enough and the resistance in the conductor itself eats into your voltage before it ever reaches the load — that's voltage drop. This calculator takes your supply voltage, load current, one-way distance, wire material (copper or aluminum), and AWG/kcmil wire size, and works out how many volts and what percentage you're losing over that run for DC, single-phase AC, or three-phase AC circuits. It also scans through standard wire sizes and tells you the smallest one that keeps the drop at or under 3%, which is the threshold most commonly cited for branch circuits.
How to use it
- Choose the circuit type: DC, AC Single-Phase, or AC Three-Phase.
- Enter the supply voltage and the load current in amps.
- Enter the one-way distance from source to load, in feet or meters.
- Pick the wire material (copper or aluminum) and the wire gauge.
- Read the voltage drop, drop percentage, voltage actually reaching the load, and a pass/fail-style status against the common 3%/5% guidelines.
- Check the suggested minimum wire size section for the smallest gauge that keeps you under 3% drop at these exact conditions.
How the math works
Voltage drop is calculated as 2 × I × R × L for DC and single-phase AC (the factor of 2 accounts for the out-and-back path through the circuit), or √3 × I × R × L for three-phase AC, where I is the current in amps, R is the conductor's resistance per foot, and L is the one-way distance in feet. Resistance values come from standard NEC Chapter 9, Table 8 DC resistance figures for uncoated copper and aluminum conductors.
Common use cases
- Sizing wire for a long run to a detached garage, workshop, barn, or shed.
- Checking whether an existing circuit's wire gauge is adequate for its length before adding a new load.
- Solar, RV, marine, or 12V/24V DC system wiring where long low-voltage runs make drop a bigger concern.
- Comparing copper vs. aluminum wire for a given run to see the practical difference in performance.
Frequently asked questions
What's an acceptable voltage drop percentage?
The commonly cited guideline (based on NEC recommendations, not a hard code requirement in most jurisdictions) is 3% or less for a branch circuit and 5% or less total for the combined feeder and branch circuit. This calculator flags results against both thresholds, but always check your local electrical code for anything that's actually being inspected.
Why does three-phase use a different formula than single-phase?
Single-phase and DC circuits need a complete out-and-back current path, so the resistance effectively doubles over the round trip. Three-phase circuits have a different phase relationship between conductors, which works out mathematically to a √3 (about 1.732) multiplier instead of a factor of 2.
Is this calculator accurate enough for a real installation?
It's a solid planning estimate based on standard resistance tables, but it doesn't account for temperature derating, conduit fill, or every code nuance. For anything safety-critical, verify against the current NEC (or your local equivalent) and have a licensed electrician confirm the final wire size.
Why isn't every wire gauge available for aluminum?
Aluminum conductors smaller than 12 AWG aren't standard in most electrical applications due to strength and connection-reliability concerns, so this tool's aluminum table starts at 12 AWG, matching common industry practice.