Dedicated or Oversized Neutrals?

[charlie tuna]

as i remember harmonics was a new word in our industry in the early 90's and that their main creater were power supplies. as soon as the problem began apparent, they came up with a cure through electronic circuitry to near limit or reduce harmonic currents. but there were already millions of the harmonic causing power supplies in use and also many in manufacturing etc. at that point. so what the industry was looking at was a slow increase in harmonic problems and then a slow reduction. and from what i have seen in the infrared scanning business, this has occurred. i would say around 1997 might have been the peak for hot or overloaded neutral connections. i don't believe it is much of a problem today. also most of the new construction and engineering design has asked for dedicated circuits in data areas, so there is little exposure to the use of common neutrals which is where harmonics used to be a problem...................

 

[cpal]

Bob
does your ammeter respond to freq other than 60Hz???


Charlie

 

[ramsy]

Without MikeHolt.com having a keyword search that works, Trevor's (infinity) excellent link on this subject can't be found.

It was a document by APC titled "White Paper #26" "Hazards of Harmonics and Neutral Overloads," (c)2003 by American Power Conversion.

This doc. showed test data that harmonic currents on shared neutrals or neutral-feeders from computer PC's and/or data centers rarely exceeded phase current, or justified a supersized neutral, much less approached a theoretical maximum of 1.7 times. See Pg. 6.

The same page of this document claims undersized neutrals are a fire hazard in office environments, since PC loads do generate phase level triplen currents on shared neutrals, neutral feeders, and drive K-factors up to 11 on transformers loaded near capacity. See Pg. 6.

According to the data, equipment samples, & test methods referenced in this document, matching neutral size to phase size was critical in office, PC labs, or other computer lab / data center environments, but supersizing was not neccessary for harmonics alone.

There are other factors that may demand a supersized neutral, such as combining harmonic loads with the unbalanced arrangement described in 310.15(B)4b, or finding a full boat (4-wire ckt) fed with the same phase at the panel (since neutral current would add rather than cancel). There may be other factors?

One more I can think of is, industrial harmonics from variable speed drives, SCR's, lighting balasts, etc., were not sampled in the referenced study above, so the need to supersize a neutral, due to harmonics alone, could still exist there.

 

[ramsy]

The MikeHolt.com keyword search has become completely useless, and for quite some time now, all previous links to important documents I've needed are also broke on google as well.

For those of us that can't remember exactly which forum and date those critical links went to, there's currently no reliably way to retrieve them or share them with firends, colleagues, or the public domain.

Since, trying to find a previous critical document link on mikeholt.com has finally deteriorated from impractical to impossible, I advise everyone to keep their own document copies, and stop relying on links.

Organizing my PC with a few new subdirectories and changing the file names to better desribe the contents is the only way I'm finding critical-document references anymore.

Shareing the doc Title, Date, Publisher, etc., may provide the most reliable reference for external search engines, since URL links, domains, and database lookup methods tend to vaporize frequently, breaking the search engines as they go.

 

[iwire]

Bob
does your ammeter respond to freq other than 60Hz???


Charlie

Yes, I believe it does.

It is a True RMS Fluke 36.

Here are the AC Current specs for it


[FONT=Verdana, Arial, Helvetica]2% ? 5 counts (10-100 Hz)[/FONT]

[FONT=Verdana, Arial, Helvetica]6% ? 5 counts(100-400 Hz)[/FONT]

 

[cpal]

Bob
If the meter is rated only to 400 H z!!, it may not see current flow beyond the 3ed harmonic



just a thought
Charlie

 

[iwire]

Bob
If the meter is rated only to 400 H z!!, it may not see current flow beyond the 3ed harmonic
just a thought
Charlie

It is certainly possible.

It is also possible that in an otherwise well designed electrical system harmonic currents in branch circuits may not the problem they are inflated to be by those with a direct interest in selling copper.

It really makes no difference I am just a plan follower, if the plans show 4 AWG run to 15 amp receptacles I will do it and the company will make money.

 

[dlhoule]

This thread just reminded me of something I experienced a few years back. Following another electrician on a job: was told machine not functioning properly and fuse is getting hot. They were not kidding. I put my ammeter on wire from fuse. Drawing 7.87 amps on 15 amp fuse rated at 250 Volts. I decided to see what would happen if we maintained power to the fuse. Guess what happened. The paper wrapping giving the fuse rating and brand burst into flame. This was in a circuit involving high frequency and after talking to the manufacturer, it was decided to try a few things. After changing just 2 componets problem was solved.

At the time I couldn't understand how that fuse could get that hot without blowing.