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AmpSize

不平衡负荷的中性线电流

不平衡三相负荷的中性线电流。

AS/NZS 3000

Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.

尝试一个工程 4

每项工作都会预填示例数值。请替换为您自己的数值。

输入

使用默认输入值的示例计算。

线电压
400 V
A相电流
32 A
B相电流
20 A
C相电流
10 A
A相功率因数
1
B相功率因数
1
C相功率因数
1

中性线电流

19.08A

中性线占最大相电流的比例
59.6%
最大相电流
32A
三相有功功率
14.32kW

此结果仅供设计参考。请对照现行标准进行核验。

计算步骤

  1. 第 A 相电流，相对 A 相电压相位 0°32A
  2. 第 B 相电流，相对 A 相电压相位 −120°20A
  3. 第 C 相电流，相对 A 相电压相位 120°10A
  4. 公式：单一功率因数，I_N = √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)19.08 A
  5. 中性线电流结果19.08A

    AS/NZS 3000:2018 Cl 3.5.2(b)(ii)

  6. 中性线电流相对 A 相的角度−27
  7. 中性线占最大相电流的比例59.6%
  8. 三相有功功率14.32kW

关于此计算器

How this is calculated

In a star load each phase current returns through the neutral. The three phase voltages are 120° apart, so the three currents are too. When the currents are equal and at one power factor, they cancel and the neutral carries nothing. When they are not equal, the neutral carries what is left over.

The tool works that left-over current as a phasor sum. It sets phase A at 0°, phase B at −120° and phase C at +120°. It then turns each current back by its own power factor angle, because a lagging load draws its current after its voltage. The three currents are added as vectors, and the size of the sum is the neutral current. The tool also gives the angle of the sum from the phase A voltage.

When all three power factors are the same, the angles cancel out of the sum. The tool then shows the short formula: the square root of the sum of the three squares, less the three products of pairs.

The neutral can carry more than the largest phase. That happens when the power factors differ a lot, so two currents line up. The tool warns when it does.

The answer is the current at the supply frequency only. Third harmonic currents from electronic loads add in the neutral, not cancel. The harmonics tool works those.

StepTable or clauseWhat it decides
Phasor of each phaseThe phase angle less the power factor angleWhere each current points
Neutral currentThe phasor sum of the threeThe current the neutral carries
One power factorI_N = √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)The short form of the sum
Neutral sizeCl 3.5.2(b)(ii)A neutral at least the size of the largest active
A smaller neutralCl 3.5.2 Exception 3Allowed for a mostly multiphase load that still carries the out-of-balance current
HarmonicsCl 3.5.2(b)(i)Third harmonic currents add to the out-of-balance current

Worked examples

Limits

The tool takes three phase currents and their power factors, all lagging. It does not take a leading load, such as a capacitor bank, or a load connected between two phases.

It gives the current, not the conductor size. Take the larger of the neutral current and the largest phase current to the cable size tool.

See also: Three phase power, Max demand by phase and Cable size.

问题

结果仅供设计参考。请依据现行标准核对。

如何求出三相负荷的中性线电流？

将三相电流作为相量相加，相位相差 120°，每相按其功率因数角旋转。相量和的大小即为中性线电流。若各相功率因数相同，则为 √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)。

为何中性线电流不是各相之差？

各相电流相隔 120°，因此不能简单地直线相加或相减。32 A、20 A 和 10 A 得出中性线电流为 19.08 A，而不是 22 A 或 12 A。

中性线能否承载比相线更大的电流？

是的。当各相功率因数相差很大时，相量可能同向排列，使中性线电流超过最大相电流。电子负荷产生的三次谐波电流同样会在中性线中叠加。当合计值超过最大相电流时，工具会发出警告。

三相回路的中性线须多大截面？

AS/NZS 3000 Cl 3.5.2(b)(ii) 要求中性线截面不小于最大的相线。例外 3 允许在大部分负荷为多相且中性线仍需承载含谐波的不平衡电流时采用较小截面。

平衡负荷是否需要中性线？

三相电流大小相等、功率因数相同时相互抵消，因此中性线在基波下无电流。若星形负荷含单相设备，仍需中性线，因为负荷切除时三相平衡会发生变化。

本工具读取的条款和表格

所有条款和表格，及各自的核验方式

表格最后核对于 2 Oct 2026。

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