A voltage-drop report is a design record that explains why a conductor size is acceptable for delivered voltage. It is not just a calculator screenshot. The useful report tells the reviewer what load current was used, how the route length was measured, which conductor data controlled the calculation, how much voltage is lost in the feeder and branch circuit, and whether the result leaves margin for real equipment operation. When those assumptions are missing, a 2.8% result can be harder to trust than a 3.4% result with clear inputs.

This guide is written for electricians preparing permit packages, engineers reviewing feeder schedules, and DIYers who want the calculator output to survive a serious plan-check conversation. It uses the familiar NEC 210.19(A)(1) and NEC 215.2(A)(1) informational-note targets of about 3% on branch circuits and feeders, and about 5% total to the farthest outlet, while also explaining how IEC 60364-5-52 projects document design current, installation method, cable data, and permissible voltage drop. The goal is not to turn every small branch circuit into a formal study. The goal is to make long, expensive, or sensitive circuits auditable before wire is bought.

In a 2026 review of a small warehouse tenant improvement, the original submittal listed only “2 AWG copper, 480V, 3 phase, voltage drop under 3%.” The route actually had a 210 ft feeder to a machine panel plus a 70 ft branch to the farthest motor controller. When the feeder and branch were separated, the feeder used about 2.2% and the branch added about 1.1%, leaving a combined 3.3%. That was acceptable for the equipment, but the clearer report changed the reviewer response from “revise calculation” to “approved with assumptions noted.”

TL;DR

A good report states the load current, not only the breaker size.
Show feeder, branch, and combined voltage drop so the 3% and 5% design targets are visible.
List conductor material, size, temperature basis, one-way length, phase, voltage, and power factor if used.
Reference NEC informational notes or IEC 60364 project limits without claiming a universal mandatory NEC voltage-drop rule.
Attach enough calculator inputs that an inspector or engineer can reproduce the result in minutes.

The design baseline in this article is anchored to the National Electrical Code , the International Electrotechnical Commission , Ohm’s law . Those references matter because code language, conductor physics, and equipment behavior usually fail in the same place: a circuit that was technically legal on paper but poorly optimized for the distance, load, or operating temperature in the field.

"The first thing I look for in a voltage-drop report is the current basis. A 60A breaker, a 48A continuous EVSE load, and a 37A motor MCA are three different design stories even before the wire size appears."
— Hommer Zhao, Technical Director

What a Permit-Ready Voltage Drop Report Must Define

A permit-ready calculation starts with entity definitions. Design current is the current used for the circuit sizing decision, such as calculated load, continuous-load current, motor MCA, EVSE rating, or a documented demand value. Voltage drop is the voltage lost in conductors as current flows through resistance and, on AC systems, impedance. Combined voltage drop is the total loss from the source through feeder and branch conductors to the farthest relevant load. Those definitions prevent three common mistakes: using breaker size without explanation, checking only one segment, and mixing ampacity approval with voltage performance.

For NEC work, be precise about language. The NEC commonly cited voltage-drop values appear as informational notes around branch-circuit and feeder sizing, not as one universal mandatory rule for every installation. NEC 210.19(A)(1) Informational Note No. 4 and NEC 215.2(A)(1) Informational Note No. 2 are often used as design targets: about 3% on a branch circuit or feeder and about 5% combined for reasonable efficiency of operation. Ampacity, terminal temperature, continuous loads, motors, and equipment instructions still come from mandatory code sections such as NEC 110.3(B), 110.14(C), 210.20(A), 215.3, 310.16, 430, 440, and 625 depending on the load.

For IEC-style projects, the report should name the design current Ib, protective device current In, cable current-carrying capacity Iz, installation method, correction factors, and the voltage-drop limit from IEC 60364-5-52 or the national implementation. The structure is different from NEC practice, but the documentation habit is the same: prove the conductor is thermally acceptable and prove the load receives acceptable voltage. A report that only says “cable sized per code” has not answered the delivered-voltage question.

The most useful reports also state the calculation temperature or resistance basis. Many quick calculations use published conductor resistance at a reference condition, while field conductors may operate hotter. If the circuit is in a rooftop raceway, hot attic, industrial area, or crowded conduit, mention whether resistance was adjusted or whether the result includes margin for elevated operating temperature. This is especially important when the result is close to 3% on a feeder or close to 5% total.

Load current basis: State whether the report uses connected load, calculated demand, 125% continuous-load sizing, motor MCA, EVSE output, or measured current.
Circuit segment: Identify feeder, branch circuit, transformer secondary, control circuit, or complete path. Percent drop without a segment label is ambiguous.
Conductor data: List copper or aluminum, AWG/kcmil or mm2, insulation/terminal temperature basis, resistance source, and parallel conductor count if applicable.
Route length: Use one-way installed path length, including risers, sweeps, offsets, service loops, and equipment location changes.

Comparison Table: Weak vs Permit-Ready Voltage Drop Submittals

The difference between a weak report and a useful report is usually not more math. It is clearer assumptions. Use this table before sending a calculator result to a reviewer, owner, or field lead.

Report item	Weak submittal	Permit-ready submittal	Why it matters
Current basis	60A circuit	48A EVSE continuous load on 60A branch; checked at 48A and 60A design basis	Shows NEC 625 and continuous-load assumptions
Route length	150 ft	184 ft one-way installed route including trench offsets and risers	Voltage drop follows installed conductor length
Voltage-drop budget	2.7% drop	1.6% feeder + 1.1% branch = 2.7% combined	Reviewer sees the full path to the load
Conductor description	No. 2 wire	2 AWG copper THHN, 75C terminal basis, resistance source noted	Avoids material and temperature ambiguity
AC load assumptions	480V three phase	480V 3-phase, 96A, 0.88 power factor, impedance method used	Power factor and reactance can affect long feeders
Code reference	Per NEC	NEC 210.19(A)(1) IN No. 4, 215.2(A)(1) IN No. 2; ampacity checked separately under 310.16	Separates design guidance from mandatory ampacity checks
IEC project notes	IEC cable okay	Ib 72A, In 80A, Iz after correction 94A, voltage drop 3.4% against 4% project limit	Matches IEC 60364-5-52 review logic

"A reviewer should not have to guess whether 2.9% means feeder only, branch only, or the full path. I like reports that show 1.8% feeder, 1.1% branch, and 2.9% combined on the same line."
— Hommer Zhao, Technical Director

Example 1: EV Charger Branch Circuit Report

Assume a 240V Level 2 EV charger draws 48A continuously on a 60A branch circuit. The panel is 138 ft one way from the charger location, and the design compares 6 AWG copper with 4 AWG copper. A weak report says only “6 AWG is under 3%.” A useful report says: load is 48A continuous EVSE; NEC 625 and 210.20(A) establish a 60A branch-circuit sizing basis; voltage drop checked at the sustained 48A load; one-way route is 138 ft; conductor is copper; target is about 3% branch and 5% combined unless the project requires tighter.

If 6 AWG copper is near 3.2% at 48A and 4 AWG is near 2.0%, the report should show both options. That lets the owner decide whether the lower voltage drop justifies the material cost. If the upstream feeder already uses 1.5% of the total budget, the 4 AWG branch becomes easier to defend because the total path stays closer to 3.5% instead of approaching 4.7%.

Example 2: 480V Machine Feeder with Branch Extension

A machine panel is served by a 480V three-phase feeder carrying 96A at an estimated 0.88 power factor. The feeder route is 210 ft one way, then a branch inside the machine area adds 70 ft to the farthest controller. The report should not hide those as one vague distance. Show the feeder drop, branch drop, and combined drop so the reviewer can see where the voltage budget is being spent.

For long AC feeders, include the method: resistance-only screening or impedance calculation with power factor and conductor reactance. If the feeder is about 2.2% and the branch is about 1.1%, the combined 3.3% result is generally easier to approve than a report that merely says “less than 5%.” The equipment may also specify a voltage tolerance, such as plus or minus 10%, and that should be noted separately from NEC informational-note targets.

Example 3: IEC 230V Final Circuit Documentation

For a 230V final circuit feeding a 3.2 kW fixed appliance, the design current Ib is about 13.9A. If the protective device is 16A and the corrected cable current-carrying capacity Iz is 20A, the thermal coordination may be acceptable. The voltage-drop report still needs cable length, conductor size, installation method, and the applicable project limit, such as 3% for lighting or 5% for other loads where permitted by the local specification.

A clear IEC report might read: Ib 13.9A, In 16A, Iz 20A after correction, 35 m one-way copper cable, calculated drop 2.6%, project limit 4%. That single line tells the reviewer much more than “2.5 mm2 cable accepted.” It also gives the installer a basis for checking substitutions if the cable type or route changes.

Example 4: Field Change After Route Length Increases

During construction, a feeder route may move around structure, ductwork, fire-rated walls, or underground conflicts. If a 160 ft feeder becomes 205 ft, update the voltage-drop report before pulling wire. At 80A, that 45 ft increase can add enough resistance to move a circuit from about 2.6% to about 3.3%, depending on conductor size and material.

The report should document the revised route and conductor decision. If upsizing is selected, note whether ampacity changed, whether terminals accept the larger conductor, and whether conduit fill or pulling tension must be reviewed. Voltage drop is one calculation, but field constructability still controls whether the correction is practical.

Mistakes That Trigger Plan-Review Questions

Reporting only a percentage.

A percentage without voltage, current, length, conductor size, material, and segment tells the reviewer almost nothing. Always include the inputs that produced the result.

Calling NEC informational notes mandatory limits.

The common 3% and 5% values are important design guidance, but the report should not misstate them as a universal enforceable NEC rule for every circuit.

Using breaker size without explanation.

Breaker size can be conservative, misleading, or required depending on the load. State why 20A, 32A, 48A, 60A, or MCA current was used.

Ignoring the upstream feeder budget.

A branch circuit at 3% may be fine by itself, but not if the feeder already consumes another 3%. Combined drop is what the farthest load experiences.

Forgetting equipment voltage tolerance.

A circuit can meet a general design target while a specific controller, drive, charger, or motor starter still needs tighter delivered voltage.

A Practical Checklist Before You Submit

Use this checklist after running the calculator and before attaching the result to a permit set, engineering memo, or field change request.

1. Name the load and current basis. Write the connected load, calculated load, continuous-load factor, motor MCA, EVSE rating, or measured current. Do not make the reviewer infer it from breaker size.
2. Separate feeder and branch results. Show each segment and the total. A report line such as 1.7% feeder + 1.2% branch = 2.9% total is much clearer than one isolated value.
3. List conductor and route inputs. Include material, size, parallel sets, one-way route length, voltage, phase, power factor when relevant, and whether resistance was temperature-adjusted.
4. Reference the correct standards. For NEC, cite the informational notes and the separate ampacity or equipment articles used. For IEC, cite IEC 60364-5-52 and the project voltage-drop limit.
5. Add a substitution rule. State that route length, conductor material, conductor size, load current, or installation method changes require recalculation before installation.

FAQ

What should a voltage drop report include for permit review?

Include voltage, phase, load current basis, one-way route length, conductor material and size, resistance or impedance method, feeder drop, branch drop, combined drop, and code references such as NEC 210.19(A)(1), NEC 215.2(A)(1), or IEC 60364-5-52.

Is 3% voltage drop required by the NEC?

The common 3% branch or feeder and 5% total values appear as NEC informational-note design guidance, not as one universal mandatory rule. They are still widely used for permit review, owner standards, and engineering quality control.

Should a report use breaker amps or actual load amps?

Use the defensible design current. For example, a 48A EVSE on a 60A circuit should document the 48A sustained load and the 60A continuous-load circuit basis. A motor should use nameplate MCA or applicable NEC Article 430 values, not only the breaker.

How do I show feeder and branch voltage drop together?

Calculate each segment from its own current, conductor, and length, then add the voltage losses or percentages on the same voltage base. For example, 2.0% feeder plus 1.4% branch equals about 3.4% total to the farthest load.

What IEC details belong in the voltage drop report?

For IEC work, list Ib design current, In protective device current, Iz corrected cable capacity, installation method, cable length, conductor size, material, calculated drop, and the IEC 60364-5-52 or national project limit such as 3%, 4%, or 5%.

Do I need a voltage drop report for every small circuit?

Usually no. Reports are most useful for long runs, feeders, EV chargers, motors, transformer secondaries, control circuits, voltage-sensitive loads, or any circuit near 3% branch or 5% total drop.

Which site tools help create the report?

Start with the voltage drop calculator, compare conductor choices with the wire size calculator, and use the three-phase calculator for 208V, 400V, or 480V feeders with known power factor.

Need a Cleaner Voltage Drop Submittal?

Use the calculator, wire size calculator, and NEC requirements guide to build the first pass, then contact the engineering team if a feeder, EV charger, motor circuit, or IEC project is close enough that the assumptions need review before permit submission.

Contact the Engineering Team Review More Guides

Voltage Drop Report for Permit Review: NEC and IEC Documentation Checklist