Measuring harmonic currents in the neutral 1

[bdn2004]

We have an existing UPS system that we are relocating. The mfg recommends a 200% neutral. There is not one now. There doesn't seem to be any issues. How can I prove that it is not necessary? Do the harmonic currents show up on a standard ammeter?

 

[rbulsara]

What is not there now? No neutral at all or there is a 100% rated neutral.

If there is no neutral required ( all 3 wire loads) then of course you don't need one. If there is one, then measure the true rms current through there.

 

[bdn2004]

Yes there is a 100% rated neutral now, it's a 208Y/120V input and output to the UPS.

When we measure the neutral current with an ammeter are the harmonic currents part of the measured value?

 

[rbulsara]

Depends on the type of ammeter. Conventional clamp on type ammeters will not measure the effect of harmonics. You need to use true harmonic measuring instruments.

Also more important is the magnitude of the true rms current in the neutral. You can have 100% harmonics in neutral current but if the actual true rms current is well within the rating of the cable, you need not oversize the neutral. You of course need to review this in context of how much UPS capacity of the UPS is used and future changes in the type of loads.

And then there is practical view, if it is not a big deal to install 200% neutral at new location, just do it and call it a day. Not all issues are worth fighting for.

 

[sibeen]

I’m making the assumption that the UPS is to be used to power computers.
A normal ammeter will measure the magnitude of the neutral current. The requirement for an over-sized neutral was due to the fact that computer power supplies were a fairly dirty switched mode power supply which drew a substantial amount of harmonics. In the worst case you could get approximately 1.7 times the line current flowing in the neutral.

Fortunately, computer power supplies have improved considerably over the last ten years or so and generally don’t draw excessive harmonics, so the need for an over-sized neutral is very rarely required.
Measure the neutral current with the existing load and if it isn’t excessive I wouldn’t bother with the over-sized neutral.