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.

电压降应按设计负荷还是断路器容量计算？NEC 与 IEC 实例

在电压降计算中，断路器容量不是自动等于运行电流。先确认负荷模型，再选择导线规格，结果才有工程意义。

摘要

设计负荷是按设备铭牌、连续负荷规则或负荷计算得到的电流。断路器容量是过电流保护装置的额定值，主要用于保护导体。
例如 120 V、115 ft、12 AWG 铜线的 12 A 充电器，运行压降约 4.38 V 或 3.65%；若按 16 A 连续负荷检查，则约 4.87%。
对于电机和 HVAC，优先使用铭牌 MCA、NEC 430/440 表值或设备说明，而不是把 60 A MOCP 当作持续运行电流。
IEC 60364-5-52 通常从设计电流 Ib、保护电器电流 In 和电缆载流量 Iz 分开校核，电压降应从实际设计电流出发。

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