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patyrick avatar image
patyrick asked

3 Phase ESS 3 Multiplus II/ phase with predominantly single phase loads Neutral conductor size

Good day 29-03-24

3 phase installation, 9 MultiPlus II 48/500’s, 3/phase, neutral wire question, as most of the load on the system will be 220V single phase with some 380V 3 phase machinery (motors mainly)

Each of the 3 per phase +v’s are connected to a individual terminal on the 4P isolation breaker rated to suit the inverter @25 amps?? 6mm² house wire the outputs thereof are then bridged to form a common i.e. “L1 Red” of the three phases 10 or 16mm² to the main DB.

The neutrals, 6mm² house wire, are all grouped and go through the Neutral of the four-pole isolator again to one 10 or 16mm² conductor on the opposite switch terminal.

My understanding is that I have got three times 25 amps of power going through the neutral if it is used in a single-phase load condition i.e. the breaker is underrated by 50 amps it and the wiring or conduct a size should be equally able to the 75 amps.


Thus a 100A 4P breaker and the Neutral conductor as well as the positive conductor as stated above, should be equated to the 75A SANS “in South Africa that would be 16mm² house wire”


Please advise/correct my single-phase thinking

Yours

Patrick

Multiplus-II
2 comments
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Alexandra avatar image Alexandra ♦ commented ·

@Patyrick

MP 500s? Or 5000?

Can we assume 5kva?

1000033569.jpg


The neutrals only ever carry a single phase at a time because the neutrals are 'out of phase with each other'. So at any one time will be carrying 2 x 5kva. So you can match the gauge of the phase current.

Usually it not have to be the sum of all three phases. Unless there is alot of harmonics from the single phase items? Have you power logged?

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patyrick avatar image patyrick Alexandra ♦ commented ·
Thanks Alexandra,

no power logged as its in planning at present.

your answer makes sense and provides somewhat relief


thanks

Patrick

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1 Answer
sharpener avatar image
sharpener answered ·

Because the relationship between the 3 phases is 120 deg phase shift, the current in the neutral is the vector sum of the three individual loads.

It is easy to show that it cannot be higher than the highest phase (live) current and the other two actually reduce the current flowing in the neutral. If they are equal it is reduced to zero.

So in a typical installation the neutral and the 3 live conductors are all the same size.

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