Do I calculate voltage drop at 100% or 125% for a continuous load?

Use the design current that the circuit is expected to carry. For many NEC continuous-load branch circuits, that means 125% sizing under NEC 210.20(A) and 215.3, so a 32A continuous EV load is commonly checked as a 40A circuit while voltage drop is evaluated against the real sustained current and feeder budget.

Is an EV charger a continuous load under the NEC?

Yes. NEC 625 treats EV charging loads as continuous for branch-circuit and feeder sizing. A 32A Level 2 EVSE normally needs a 40A circuit, and a 48A EVSE normally needs a 60A circuit before voltage drop is checked.

What voltage-drop target should I use for continuous lighting?

A common design target is 3% on the branch circuit and 5% total feeder plus branch, matching NEC 210.19(A)(1) and NEC 215.2(A)(1) informational guidance. Long LED rows often perform better when the branch stays closer to 2% to 3%.

Does 125% continuous-load sizing automatically solve voltage drop?

No. The 125% factor helps conductor and overcurrent sizing for sustained heat, but distance still creates resistance. A 60A EV circuit at 48A over 180 ft can still need larger copper or aluminum conductors for performance.

How does IEC 60364 handle continuous-load voltage drop?

IEC 60364-5-52 coordinates design current, installation method, grouping, ambient correction, conductor size, and allowable voltage drop. The terminology differs from NEC, but the practical sequence is the same: thermal legality first, voltage performance second.

When should I upsize wire beyond the continuous-load minimum?

Upsize when the route is long, source voltage is weak, the equipment runs for 3 hours or more, or the feeder has already used 2% or more of the total voltage-drop budget. Compare at least two conductor sizes before releasing material.

連續負載壓降：125%電流、線徑與NEC/IEC設計

連續負載會讓電流和導線溫度維持在現場的真實狀態。48 A電動車充電器、18 A照明回路或36 A加熱負載，不能只看斷路器大小就結案。

摘要

3 hours or more makes the load continuous in NEC practice.
125% sizing and voltage-drop calculation answer different questions.
Use branch, feeder, and total voltage-drop budgets together.
EVSE, lighting, heaters, and ventilation loads need sustained-current checks.

連續負載是最大電流預期持續3小時或更久的負載。壓降是電流流經導體阻抗造成的負載端電壓損失。載流量是導體在指定條件下可安全承載的電流。

先依NEC 210.20(A)、215.3、625或設備規則完成125%尺寸與熱檢查，再用實際持續電流、路徑長度和導體材質計算壓降。 NEC and IEC references keep the thermal and performance checks separate.

The 125% continuous-load rule is a heat and protection rule; it does not erase the resistance of a 150-foot branch circuit.
— Hommer Zhao, Technical Director

Continuous-load design checks

48 A、240 V EVSE通常按60 A回路處理。6 AWG銅線單程165 ft時，支路約降6.3 V，也就是2.6%；加上1.8%饋線後已接近4.4%。

Load	Sustained current	NEC basis	Route	Design action
Level 2 EVSE	32 A at 240 V	40 A circuit	130 ft	Compare 6 AWG Cu
High-output EVSE	48 A at 240 V	60 A circuit	165 ft	Check feeder plus branch
LED lighting row	18 A at 277 V	125% review	240 ft	Split row or upsize
Process heater	36 A at 208 V	45 A design current	110 ft	Check 6 AWG and source
Exterior lighting	12 A at 120 V	15 A basis	95 ft	Use 10 AWG or split

For a 48-amp EV charger, document the 60-amp circuit sizing and check voltage drop at the sustained charging current.
— Hommer Zhao, Technical Director

Practical warning

125%規則不是壓降捷徑。它處理長時間發熱與保護，長距離導線仍需要獨立計算。

Use the wire size calculator, the NEC requirements article, and the voltage drop vs ampacity guide together before material is released.

Continuous loads turn small voltage-drop misses into daily operating losses because the circuit stays loaded for hours.
— Hommer Zhao, Technical Director

FAQ

壓降要用100%還是125%？

用設備實際持續電流計算壓降，同時保留125%作為NEC回路尺寸依據。

EVSE是連續負載嗎？

是。NEC 625要求按連續負載思考，32 A常見為40 A回路，48 A常見為60 A回路。

照明回路目標是多少？

常用支路3%、總路徑5%；長排LED建議更保守。

加大線徑何時值得？

路徑長、電源偏弱或饋線已用掉2%左右預算時，通常值得比較。

IEC怎麼看？

IEC 60364-5-52會把設計電流、敷設方式、環境和允許壓降一起檢查。

施工前先用計算器比較最小線徑與加大一號後的壓降。

Voltage Drop Calculator Contact

連續負載壓降：125%電流、線徑與NEC/IEC設計

摘要

Continuous-load design checks

Practical warning

FAQ

壓降要用100%還是125%？

EVSE是連續負載嗎？

照明回路目標是多少？

加大線徑何時值得？

IEC怎麼看？

开始计算

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