×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Measuring harmonic currents in the neutral

Measuring harmonic currents in the neutral

Measuring harmonic currents in the neutral

(OP)
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?

RE: Measuring harmonic currents in the neutral

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.

 

RE: Measuring harmonic currents in the neutral

(OP)
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?

 

RE: Measuring harmonic currents in the neutral

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.

 

RE: Measuring harmonic currents in the neutral

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.
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources