Voltage Drop and Wire Size Calculator
Estimate circuit voltage drop or compare listed copper and aluminum conductor sizes using load current, circuit length, and a user-set voltage-drop target.
Circuit details
Enter the one-way distance from the source to the load. The calculation includes the return path where applicable.
Estimated voltage drop
Reference AC resistance for 12 AWG copper at 75°C.
About this calculator
Enter the circuit voltage, load current, one-way length, conductor material, and phase. Calculate voltage drop for one size or compare the listed sizes against a voltage-drop target you choose.
Use the result as an initial screening check. It does not select a final conductor size. Verify ampacity, overcurrent protection, terminal ratings, installation conditions, equipment requirements, and locally adopted electrical rules separately.
Method and assumptions
Single-phase
Balanced three-phase
In these formulas, I is load current in amperes, L is one-way length in feet, and R is the listed AC resistance in ohms per 1,000 feet at 75°C. Single-conductor mode uses the resistance value for the selected conductor. Comparison mode checks each listed conductor against the target you enter and reports the smallest listed size that meets it.
The formula uses resistance only. It does not include inductive reactance or power factor, and the tabulated resistance is not adjusted for actual conductor temperature, parallel sets, or connection resistance.
Interpret the result
Use the voltage drop and load-end voltage to compare circuit options. A user-entered target is a design criterion, not a code limit. The comparison does not determine whether a conductor, breaker, or wiring method is permitted. Check ampacity, overcurrent protection, equipment instructions, installation conditions, and locally adopted requirements separately.
FAQs
Should I enter one-way or round-trip length?
Enter the one-way distance from the source to the load. For a single-phase circuit, the formula includes both the outgoing and return conductors. For a balanced three-phase circuit, it uses the standard three-phase relationship.
Does this tell me which wire size the NEC requires?
No. It estimates voltage drop only. It does not check conductor ampacity, overcurrent protection, wiring method, terminal temperature, or local code adoption and amendments.
Why might measured voltage drop differ from this result?
Measured voltage drop can differ because of conductor temperature, AC reactance, power factor, connection resistance, parallel conductors, conductor construction, and load variation. This calculator uses a simplified resistance-only model.
Sources
- Southwire cable specifications: AC conductor resistance at 75°C. Copper values use 14–10 AWG SIMpull CoilPAK data and 8 AWG–1000 kcmil THHN/THWN-2 data; aluminum values use 8 AWG–1000 kcmil XHHW-2 data.
- Southwire SIMpull CoilPAK copper specifications (14–10 AWG)
- Southwire copper THHN/THWN-2 specifications (8 AWG–1000 kcmil)
- Southwire aluminum XHHW-2 specifications (8 AWG–1000 kcmil)
- Calculation method: resistance-based voltage-drop equations for single-phase and balanced three-phase circuits.
- Code requirements: this calculator does not apply NEC conductor-selection rules. Verify the requirements adopted by the local authority.