Voltage Drop Calculator

This calculator screens branch-circuit and feeder runs using the entered voltage, load current, circuit type, conductor material, selected wire size, one-way run length, and target voltage drop limit. The result estimates voltage at the load, percent drop, and whether the selected conductor stays within the chosen screening threshold.

When the selected conductor is above the target, the upsizing ladder compares larger conductor sizes so a field user can see how each step changes the voltage drop result. This helps avoid guessing whether the next size is enough or whether the job needs a larger jump before material is ordered or pulled.

The calculator also flags conditions that should not be treated as a simple resistance-only answer. NEC 250.122(B) EGC upsizing review, parallel conductor limits, motor or VFD loads, magnetic raceways, and large AC feeder conditions can all require additional field or engineering review.

The calculator uses a resistance-based field formula with circular-mil conductor area. It is intended for fast screening of common branch-circuit and feeder runs, not as a full AC impedance model for every installation.

In those formulas, K is the conductor resistance constant, I is load current in amps, L is one-way conductor length in feet, and CM is conductor circular-mil area. Enter one-way length only; the single-phase equation already accounts for the return path.

Large AC feeders, motor loads, VFDs, parallel feeders, and magnetic raceways may need NEC Chapter 9 Table 9 effective impedance review or engineering software before the result is used for final approval.

For a long 120V branch-circuit run, the calculation result can show why a conductor that is acceptable for ampacity may still deliver too little voltage at the load. In that case, the output is used to compare wire upsizing options, not to approve the entire circuit by itself.

For a detached garage or subpanel feeder, the same process helps compare voltage available at the load before the wire is pulled. A 60A, 240V feeder over a long one-way distance may need a larger conductor for performance even after the ampacity path has been checked.

The examples are screening workflows. Final conductor size still depends on ampacity, temperature correction, conductor-count adjustment, terminal rating, grounding, raceway fill, equipment instructions, and AHJ review.

The upsizing ladder shows larger conductor options and the estimated result for each size. This is useful when a foreman or estimator needs to decide whether the next conductor size is enough or whether the improvement is too small to justify the material change.

Treat the ladder as a voltage-performance comparison. It does not replace conductor ampacity review, small-conductor rules, terminal temperature limits, raceway fill, grounding, or equipment requirements.

If the selected conductor is increased because of voltage conditions, remember that the equipment grounding conductor may also require review. That is why the tool keeps the EGC reminder near usable results instead of hiding it below the page.

A common field miss happens when ungrounded conductors are increased to reduce voltage loss, but the equipment grounding conductor is left at the original size. NEC 250.122(B) can require proportional EGC upsizing when the ungrounded conductors are increased beyond the minimum size for reasons such as voltage performance.

This calculator does not know every project baseline: original OCPD, minimum ungrounded conductor, wiring method, equipment grounding path, and final raceway arrangement may not all be entered here. For that reason, the EGC warning is a review prompt rather than a final grounding-conductor size.

After using the ladder, confirm grounding and raceway conditions before installation. Larger conductors may change raceway fill, box fill, bend limits, pulling conditions, lug compatibility, and equipment termination requirements.

A favorable calculation result does not approve the conductor, circuit, or installation. It only means the entered run screened within the selected target using the assumptions shown in the result.

• Conductor ampacity, ambient-temperature correction, conductor-count adjustment, and terminal-temperature limits.
• OCPD selection, equipment grounding conductor sizing, bonding path, and fault-current conditions.
• Raceway fill, box fill, pulling conditions, lug compatibility, equipment instructions, and AHJ acceptance.
• Detailed AC impedance, harmonic, motor-starting, VFD, power-factor, or engineered study requirements.

Start with load and circuit basis. For dwelling service or panel-level checks, use the Residential Load Calculator or the Breaker Size Calculator before treating a run as ready for conductor selection.

Next, confirm conductor size and derating. Use the Wire Size Calculator for baseline conductor sizing, then verify ambient temperature, conductor count, and terminal-temperature basis with the Ampacity & Derating Calculator.

Use this page after the conductor path is known, especially for long branch-circuit or feeder runs. After any conductor upsizing, verify raceway fill with the Conduit Fill Calculator.

For EV charging circuits, screen continuous-load sizing first with the EV Charger Circuit Sizing Calculator, then use this calculation tool to check long-run performance. For source scope and code-basis notes, review the Voltage Drop Calculator code citation record.

TradeHub.tools provides field screening references for trained trade professionals. This page helps evaluate voltage at load and conductor upsizing options, but it does not replace project drawings, manufacturer instructions, engineered review, local amendments, inspection requirements, or AHJ decisions.

Before installation, confirm the complete circuit path: load basis, breaker or OCPD, conductor ampacity, derating, terminal temperature, grounding, raceway fill, equipment limitations, and site conditions.