Should voltage drop be calculated using breaker size or load current?

Use load current when the load is known, and use breaker size only when the circuit is intended to deliver that current. For a 12A dedicated load on a 20A breaker, 12A describes operation; for a general 20A receptacle, 16A continuous or 20A full-capacity checks may be better.

Do NEC voltage-drop notes require a 3% maximum?

NEC 210.19(A) and 215.2(A) voltage-drop language is commonly in informational notes, not a universal mandatory rule. The 3% branch or feeder and 5% combined targets are still widely used for efficient design and plan documentation.

What current should I use for a 32A EV charger?

For a 32A continuous EVSE on a 40A branch circuit, calculate voltage drop at 32A for expected charging operation and at 40A for the 125% continuous-load design basis under NEC EVSE and branch-circuit sizing rules.

For motors, should I use FLA, MCA, or breaker size?

Use the equipment nameplate, MCA, or NEC Article 430 table current for conductor and operating voltage-drop checks. Do not use a 60A motor breaker as the running current unless the motor load can actually draw 60A continuously.

How does IEC 60364 handle this differently?

IEC 60364 design normally separates design current Ib, protective-device current In, and cable capacity Iz. Voltage drop is usually checked from Ib, while In and Iz are used to verify protection and cable coordination.

Should I calculate at 80% of breaker size for continuous loads?

For many NEC circuits, continuous load is limited to 80% of a standard breaker unless equipment is rated otherwise, which is equivalent to sizing at 125%. A 20A circuit often supports 16A continuous load, and a 40A circuit often supports 32A EVSE load.

電壓降要用設計負載還是斷路器容量？NEC 與 IEC 範例

斷路器把導線保護起來，但不代表負載一定會吃到斷路器額定電流。電壓降先看負載模型。

摘要

設計負載是依銘牌、連續負載規則或負載計算得到的電流；斷路器容量則是過電流保護裝置的額定值。
120 V、115 ft、12 AWG 銅線、12 A 充電器，運轉壓降約 4.38 V，也就是 3.65%；按 16 A 連續負載則約 4.87%。
馬達與 HVAC 應優先看 MCA、NEC 430/440 或設備說明，不要把 60 A MOCP 當成長時間運轉電流。
IEC 60364-5-52 會分開處理 Ib、In 與 Iz；壓降通常從設計電流 Ib 檢查。

NEC and IEC references are useful only when the current basis is written clearly beside the calculation.