Neutral Cable Sizing 1

[Histor]

Hi,
Please help me to determine the number of neutral cables for my design.

It is a 3phase 4 wire system.
The requirement is that I need to use single core cables.

For MCC, I've designed the incomer cables of 4Runs of Brown, Black, Grey and Blue. Do I really need 4 numbers of Blue neutral cables or 1 neutral is enough?

Thanks

 

[che12345]

Dear Mr. Histor (Electrical)(OP)14 Aug 22 09:07
"....to determine the number of neutral cables..... It is a 3phase 4 wire system.... The requirement is that I need to use single core cables....For MCC, I've designed the incomer cables of 4Runs of Brown, Black, Grey and Blue. Do I really need 4 numbers of Blue neutral cables or 1 neutral is enough? "
1. Based on the cable colour code, I take it to be in the IEC world.
2. Refer to IEC 60364-series for installation <1kV based on TN-S system, the neutral and the PE are separated. The neutral conductor can be < than phase conductor if a) there is no high harmonic distortion of the load, b) if there is high harmonic distortion (where the harmonic content > 10%); the neutral shall be = phase conductor, c) when phase conductor > 16 mm sq Cu, minimum neutral =/>16mm sq Cu.
2.1. Info: motor loads are considered as low harmonic load.
3. Note: You should also run a PE conductor from the power source i.e. 3-phases + neutral + PE separately to the MCC. Attention: Do NOT connect the neutral to the PE at the MCC.
4. Caution: there are numerous single-core cables, pay particular attention to the phasing and layout formation.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Normally, Neutral current rating can be half that of Phases. The exception is when you need to allow excessive unbalance in distribution of loads or when the connected loads present excessive harmonics (Electronic loads like VFDs).
How is it in MCC - is the neutral busbar same size as that of phases? This can be a quick check as you size the cables, in general.
Incomer Circuit breaker protection modules also allow setting 1/2 or Full Neutral so that appropriate neutral protection is ensured.

 

[cranky108]

As RR said, it depends. Half rated neutrals work most of the time. The exception I have seen is when serving casinos, where lots of slot machines (using switching power supplies), can cause three times the current on the neutral.

 

[JezNZ]

Interesting to hear how the rest of the world does it.

Here, in my part of the world, nobody runs reduced size neutrals in LV world anymore. It is technically still allowable, you simply need to show your calculation of the neutral current, but most, if not all, consider it not worth it, given the issues that have arisen in the recent past, particularly related to introduced harmonic content into systems.

 

[VTer]

If you only have 3phase 3-wire loads, typical of many MCCs, you may not need the neutral. It really depends on your load.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[dpc]

You'll have to follow your local code. In the US, the NEC allows reduced neutrals. For MCCs, we generally did not allow L-N loads in our designs, so no neutrals at all.

 

[jghrist]

If you have a grounded source with no neutral feeding a non-linear load that produces triplen harmonics, where do the triplen harmonic currents go? Do they return through the equipment grounding conductor or are they suppressed by having no neutral return?