Harmonic effects on breaker selection. 2

[edryder]

Does any one have a rule of thumb or some insite for sizing the main breaker for a panelboard that is going to be used to power computer equipment. Since the load will be 100% non-linear, should the breaker be oversized to makeup for the harmonic currents. The loads will be 208V and 120V. I believe the power conductors should be sized at 125% and the neutral should be sized at 200% of the calculates load.
Thank You,
Ed

 

[rbulsara]

I am not aware of any such rule. But I can share my experience with numerous data center designs, which is our core business.

If you are really concerned, you can use true rms sensing breakers which are widely available (in the USA again) for 225A and above.

On the other hand our experience has shown that we seldom see a main breaker loaded more than its 50-60% capacity as we always run out of physical space of the branch circuits much before we utilize the rated capacitiy of a panelboard or MCB.

In last 15 years of experiecne not once anyone has complained about harmonic related tripping or issues in any data centers that I have dealt with. It may be because most of such facilites are properly or adeqautely designed.

However, I have recently heard of 200A breakers tripping at 170 or 180 amps feeding some temporary facilities with signinficant harmonic loads (no measured data avaialble, yet), if this gives you any ideas. Bear in mind these reported breakers were operatinng under less than ideal conditions such are high ambient etc.

I am sure someone with more research information will post further answers.

 

[advidana]

I have design tons computer centers, breakers and fuses are not the problem with harmonic. The neutrals grounds, UPS and sometimes the transformer are the main harmonic issues.

 

[edryder]

Thank you fbulsara and advidana for your post.
I have a better idea on how to handle my design.
edryder

 

[DanDel]

Just a note, any molded case C/B with thermal/magnetic trip unit is inherently true-rms.

 

[rbulsara]

Yes, while thermal magnetic breaker may sense heat generated by actual harmonics (true rms sensing), they do not have any display. Many solid state Trms sensign trip unis come with a built in display that will indicate trms current.

Insatalltion without true rms sensing metering, may not realize that they are approching trip because of harmonics.

 

[rbulsara]

The solid state trip units are also not affected by normal ambient teperature withing their operating range.

 

[peebee]

Re: "I believe the power conductors should be sized at 125%" -- Please note that you're required to size feeders at 125% of the continuous load regardless of harmonics on any panelboard. So you're really not doing any oversizing here. You could be powering 100% incandescent light bulbs and you'd still need your phases sized at 125% of connected load.

I agree with all above posts. There's no particular unusual sizing requirements for phase conductors/protection associated with harmonics.

Triplen harmonics can add constructively on the neutral to about 173% of phase current, that's why 173% or 200% neutral conductors are often specified. There's no similiar issue involved with the phase conductors, though.

 

[HCBFlash]

The best info I've come across regarding harmonics, power factors etc has been from ---- the manufacturers of circuit breakers! --- I particularly like Square D for tech info, but Cutler-Hammer and Siemens have been good too, at least a couple times in the past.

I wouldn't bother with that finance company that makes jet engines tho. They're not even "Good Enough" to me anymore.

 

[peebee]

I hear you, HCBFlash. They just don't seem to have any interest in anything electrical anymore.

 

[upsengineer]

dont agree with HCBFlash on this one they have taken a role in electronics now.