Neutral current ng unbalanced load
Ang neutral current ng unbalanced three‑phase load.
Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Subukan ang isang job 4
Nilalagyan ng halimbawang value ang bawat job. Palitan ang mga ito ng sarili mong value.
- Find the neutral current of a shop boardThe neutral current of a board with 45, 30 and 25 A on its phases.
- Neutral current with heaters and a motorHeaters on one phase and a motor on another: the neutral with two power factors.
- Neutral current of a farm on one phaseMost of the load on one phase: how close the neutral comes to that phase.
- Check the neutral of a mostly three‑phase loadThe out-of-balance current a reduced neutral must still carry.
Mga input
Worked example gamit ang mga default na input.
- Line voltage
- 400 V
- Current ng phase A
- 32 A
- Current ng phase B
- 20 A
- Current ng phase C
- 10 A
- Power factor ng phase A
- 1
- Power factor ng phase B
- 1
- Power factor ng phase C
- 1
Neutral current
19.08A
- Neutral bilang bahagi ng pinakamalaking phase
- 59.6%
- Pinakamalaking phase current
- 32A
- Real power, tatlong phase
- 14.32kW
Design aid lang ang resultang ito. I-verify ito laban sa kasalukuyang standard.
Mga hakbang ng kalkulasyon
- Current ng phase A, sa 0° mula sa phase A volts32A
- Current ng phase B, sa −120° mula sa phase A volts20A
- Current ng phase C, sa 120° mula sa phase A volts10A
- Formula: isa ang power factor, kaya
I_N = √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)
19.08 A - Neutral currentResulta19.08A
AS/NZS 3000:2018 Cl 3.5.2(b)(ii)
- Anggulo ng neutral current mula sa phase A−27
- Neutral bilang bahagi ng pinakamalaking phase59.6%
- Real power, tatlong phase14.32kW
Tungkol sa calculator na ito
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.
| Step | Table or clause | What it decides |
|---|---|---|
| Phasor of each phase | The phase angle less the power factor angle | Where each current points |
| Neutral current | The phasor sum of the three | The current the neutral carries |
| One power factor | I_N = √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia) | The short form of the sum |
| Neutral size | Cl 3.5.2(b)(ii) | A neutral at least the size of the largest active |
| A smaller neutral | Cl 3.5.2 Exception 3 | Allowed for a mostly multiphase load that still carries the out-of-balance current |
| Harmonics | Cl 3.5.2(b)(i) | Third harmonic currents add to the out-of-balance current |
Worked examples
- A shop board with 45, 30 and 25 A on its phases has 18.03 A in the neutral, 40.1 % of the largest phase.
- A farm supply with 40 A on one phase and 5 A on the others has 35 A in the neutral, 87.5 % of the largest phase. Nearly all of the load comes back on it.
- Heaters on phase A and a motor on phase B at 0.8 power factor give 19.69 A in the neutral, 65.6 % of the largest phase.
- A mostly three‑phase load of 60, 55 and 50 A at 0.85 power factor leaves 8.66 A in the neutral, 14.4 % of the largest phase.
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.
Mga tanong
Design aid ang resulta. I-verify ito laban sa kasalukuyang standard.
Paano kinakalkula ang neutral current ng three‑phase load?
Idagdag ang tatlong phase current bilang phasors, 120° ang pagitan, bawat isa ay pinaikot ayon sa power factor angle nito. Ang laki ng kabuuan ay ang neutral current. Kapag iisang power factor sa lahat ng phase, ito ay √(Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia).
Bakit hindi ang pagkakaiba ng mga phase ang neutral current?
120° ang layo ng mga phase current, kaya hindi sila nagdaragdag o nagbabawasan nang diretso. Ang 32 A, 20 A at 10 A ay nagbibigay ng 19.08 A sa neutral, hindi 22 A o 12 A.
Kayang dalhin ba ng neutral ang higit sa isang phase?
Oo. Kapag magkakaiba nang husto ang power factor, puwedeng magkahanay ang phasors, at puwedeng dalhin ng neutral ang higit sa pinakamalaking phase. Nadaragdagan din sa neutral ang third harmonic current mula sa electronic loads. Binabalaan ng tool kapag lumampas ang kabuuan sa pinakamalaking phase.
Gaano kalaki dapat ang neutral ng three‑phase circuit?
Hinihingi ng AS/NZS 3000 Cl 3.5.2(b)(ii) ang neutral na kahit papaano ay kasing laki ng pinakamalaking active. Pinapayagan ng Exception 3 ang mas maliit kapag karamihan ng load ay multiphase at dala pa rin ng neutral ang out-of-balance current kasama ang harmonics nito.
Kailangan ba ng neutral ng balanced load?
Nagkakansel ang tatlong pantay na current sa iisang power factor, kaya walang current ang neutral sa fundamental. Kailangan pa rin ito ng star load na may single‑phase na item, dahil nagbabago ang balance kapag may load na nag-switch off.
Mga clause at table na binabasa ng tool na ito
- AS/NZS 3000:2018 Cl 3.5.2(b)Neutral of a multiphase circuit
- AS/NZS 3000:2018 Cl 3.5.2Neutral conductor of a circuit
Bawat clause at table, kasama kung paano ito na-verify
Huling na-check ang mga table noong 2 Oct 2026.
Mga kaugnay na tool
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Voltage unbalance ng supplyVoltage or current unbalance from three line readings.Walang standard
- Harmonics, neutral current at K-factorTHD, neutral current and K-factor from a spectrum.AS/NZS 3008.1AS/NZS 3000
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