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.

電圧降下は設計負荷かブレーカー容量か: NEC・IECの実例

ブレーカー定格は運転電流そのものではありません。電圧降下は、まず機器が実際に流す電流を決めてから計算します。

要約

設計負荷は銘板、連続負荷、MCA、負荷計算で決まる電流です。ブレーカー容量は主に導体保護のための定格です。
120 V、片道115 ft、12 AWG銅、12 A充電器では約4.38 V、3.65%の低下です。16 A連続負荷で見ると約4.87%になります。
モーターや空調機では、60 AのMOCPではなくMCA、NEC 430/440、銘板値で定常電圧降下を確認します。
IEC 60364-5-52ではIb、In、Izを分けて扱います。電圧降下は通常、設計電流Ibから確認します。

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