Feszultsegeses ellenorzolista tervellenorzeshez
A voltage-drop submittal is a short calculation record that shows the circuit source, load, conductor material, one-way route length, selected wire size, and resulting load voltage. It helps an inspector, engineer, or owner see that the design is not only ampacity-compliant but also usable at the equipment terminals.
Roviden
- Show the actual load current, not only the breaker rating.
- Keep branch-circuit and feeder drop separate before checking the combined total.
- Cite NEC informational notes and IEC voltage-drop limits as design targets, not hidden assumptions.
- Re-check terminal temperature, conduit fill, and lug range when the conductor is upsized.
What the reviewer expects to see
A plan reviewer is usually checking three separate issues. Ampacity is the thermal suitability of the conductor for the load and installation conditions. Voltage drop is the loss of electrical potential caused by conductor impedance. A design target is a project or code-reference limit, such as 3% for a branch circuit or 5% total in common NEC practice.
For NEC work, cite the voltage-drop informational notes associated with 210.19(A)(1) and 215.2(A)(1), terminal temperature requirements in 110.14(C), conductor resistance values from Chapter 9 Table 8, and adjustment or correction checks from 310.15. For IEC work, cite IEC 60364 and cable selection under IEC 60364-5-52. Background references for the National Electrical Code and the International Electrotechnical Commission are useful for owner-facing documentation.
"When I review a long 120V branch circuit, I want one line that says 16A, 185 ft one-way, 8 AWG copper, 2.9% drop. That is easier to approve than three pages of unsourced formulas." — Hommer Zhao, electrical calculator editor
Submittal checklist
Circuit identity
Panel, circuit number, voltage, phase, load name, and whether the load is continuous for 3 hours or more.
Load basis
Actual design current, calculated load, motor FLA, EVSE current, nameplate current, or demand value used in the calculator.
Route length
One-way conductor length from source to load. Do not enter round-trip length in a calculator that already applies the circuit factor.
Conductor data
Copper or aluminum, AWG/kcmil or mm2 size, insulation temperature, raceway/cable type, and parallel sets if used.
Code checks
NEC 110.14(C), 210.19(A)(1), 215.2(A)(1), 310.15, Chapter 9 Table 8, and IEC 60364-5-52 where applicable.
Result and decision
Voltage drop in volts and percent, load-end voltage, target limit, and whether the conductor was upsized.
Example calculation log
| Circuit | Calculator input | Code reference | Review note |
|---|---|---|---|
| Garage EVSE | 240V, 48A, 150 ft, 6 AWG copper: about 2.95%; 4 AWG: about 1.85% | NEC Article 100, 210.19(A)(1), 210.20(A) | 60A circuit basis is from 48A x 125%; voltage drop still uses 48A actual load. |
| Outdoor sign | 120V, 16A, 180 ft, 12 AWG copper: about 7.6%; 8 AWG: about 3.0% | NEC 210.19(A)(1), 310.15 | Breaker size passes, but voltage performance controls conductor size. |
| Workshop feeder | 208V 3-phase, 70A, 210 ft, 1 AWG copper: about 2.4% | NEC 215.2(A)(1), Chapter 9 Table 8 | Document feeder drop separately from final branch circuits. |
| IEC machine | 400V 3-phase, 32A, 70 m, 10 mm2 copper: about 2.2%; 16 mm2: about 1.4% | IEC 60364-5-52 | Include installation method and grouping factor in the same submittal. |
| Detached garage | 120/240V feeder, 60A, 120 ft, 4 AWG aluminum: near 3%; 2 AWG aluminum: near 2% | NEC 215.2(A)(1), 310.12 if applicable | Show whether load is balanced line-to-line or mostly 120V line-to-neutral. |
Worked examples to include in notes
120V branch circuit, 185 ft to last receptacle
A workshop receptacle row has a calculated 16A load at the farthest outlet. 12 AWG copper at 185 ft can exceed 7% drop, so the submittal uses 8 AWG copper at roughly 3.1%. The note should say the 20A overcurrent device remains; only the conductor was upsized for voltage drop, and box fill under NEC 314.16 must be checked for the larger conductor.
480V motor feeder, 40 hp, 260 ft
A 40 hp motor at 480V may have a running current near 52A depending on table and equipment data. At 260 ft, 6 AWG copper may be acceptable thermally but can be close to a 3% running-drop target. The reviewer should also see a separate motor-starting check because inrush can be 5 to 7 times running current.
400V IEC packaging machine, 32A, 70 m
For a 32A three-phase machine, a 10 mm2 copper cable may land near 2.2% drop before grouping corrections. If the route shares tray space with six loaded circuits, IEC 60364-5-52 grouping can push the design toward 16 mm2 even when voltage drop alone looks acceptable.
"A voltage-drop report should not hide the weak assumption. If the route length is estimated from drawings, label it as 150 ft scaled length plus 10 ft vertical allowance." — Hommer Zhao, electrical calculator editor
Common rejection points
- Using breaker rating instead of actual load current without explaining why.
- Entering round-trip length when the formula or calculator expects one-way distance.
- Quoting 3% and 5% as mandatory NEC rules instead of informational-note design recommendations.
- Upsizing conductors but not checking terminal temperature limits under NEC 110.14(C).
- Ignoring conduit fill, box fill, bending space, or equipment lug maximum size after upsizing.
- Mixing NEC AWG resistance values and IEC metric cable assumptions in one calculation without notes.
FAQ
Is voltage drop required by NEC for every permit?
NEC commonly treats voltage drop as an informational-note design recommendation near 210.19(A)(1) and 215.2(A)(1), but many engineers, utilities, and local authorities require a calculation for long runs or sensitive loads.
Should the report use 3% or 5%?
A common target is 3% for a branch circuit or feeder segment and 5% total. Some industrial, medical, data, or owner specifications require 2% or less.
Do I calculate voltage drop before or after conductor derating?
Do both checks separately. Derating confirms thermal ampacity under NEC 310.15 or IEC 60364-5-52; voltage drop confirms delivered voltage at the load current.
What length should a DIYer enter for a detached garage feeder?
Enter the one-way route length from the source panel to the garage panel, including vertical rises. For a 120 ft trench plus 8 ft up each wall, use about 136 ft.
Can I submit calculator screenshots?
Screenshots help, but a clean table is better. Include voltage, phase, current, length, material, wire size, drop volts, drop percent, and load-end voltage.
What happens if upsized wire will not fit the breaker lug?
Use a listed splice, terminal kit, gutter, or equipment approved for that conductor range. Do not trim strands or force a conductor into a lug outside its listing.
"The best submittals show a decision trail: 12 AWG failed at 7.6%, 10 AWG was still high, 8 AWG met the 3% target and was checked for box fill." — Hommer Zhao, electrical calculator editor
Build the calculation package
Run the circuit in the voltage-drop calculator, then cross-check related constraints with the wire-size, conduit-fill, and derating tools before sending the result to a reviewer.
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