Question on 3 phase Closed Delta Service to Office Building

[Andy Sherrill]

I am working on adding an 8750 sq ft office area to an existing strip mall type building. The existing service is 240 V, 3 phase, closed delta (high leg). I plan to feed all of my three phase loads from one panel. All of the office lighting and receptacles will be fed from single phase panels. I am trying to decide if I need to size the neutral for 200% of the phase conductors?

Even though these are single phase panels, this is a three phase service, so I assume the harmonic currents will still add? Also, with this type of service, there will be significant "normal" current on the neutral due to the inherent imbalance. The load for the single phase panels will be almost all non-linear. All lighting is fluorescent. The receptacle load will be PCs, copiers, printers, etc. There is also a server that will be fed from one of these panels. I am sure the owner has a UPS for the server. These panels will probably not be heavily loaded. They will be 200 A panels. My calculated load is around 135 A. The load will probably be musch less under normal circumstances due to the large number of receptacles.

Another question I have is if the neutral conductor needs to be sized for 200%, doesn't the panel neutral also need to be sized for 200%?

Thanks for your help!

 

[ptonsparky]

The neutral will not need to be oversized. The transformer feeding your single phase load is just that, a single phase transformer. They could remove the other two transfomrers and not affect your single phase service. Around here we usually have open deltas with high leg.

 

[mivey]

I am trying to decide if I need to size the neutral for 200% of the phase conductors?

No.

My calculated load is around 135 A. The load will probably be mush less under normal circumstances due to the large number of receptacles.

And the fact that the NEC calcs are way conservative.

 

[roger]

As Tom and Mivey said, there is no need to make any special provisions for this, just treat it as you would a single phase installation.

Roger

 

[Andy Sherrill]

I thought that since this was a 3 phase system with the single phase panels fed from Phase A & C, the currents would be 120 degrees out of phase and the harmonic currents would be "in-phase" and add together. Maybe I don't understand what is required for these harmonic currents to be additive?

 

[mcclary's electrical]

I thought that since this was a 3 phase system with the single phase panels fed from Phase A & C, the currents would be 120 degrees out of phase and the harmonic currents would be "in-phase" and add together. Maybe I don't understand what is required for these harmonic currents to be additive?

This thread covers this pretty thoroughly:


http://forums.mikeholt.com/showthread.php?t=127369&page=2

 

[IslandSparky]

Triplen harmonics are addative on the neutral when connected in a three phase wye. Your three phase loads don't have a neutral.

 

[roger]

I thought that since this was a 3 phase system with the single phase panels fed from Phase A & C, the currents would be 120 degrees out of phase and the harmonic currents would be "in-phase" and add together. Maybe I don't understand what is required for these harmonic currents to be additive?

In a center tapped "wye" you would be correct however, in your "delta" you are dealing with a center tapping single winding.

Roger

 

[steve66]

As Roger stated, the currents for standard loads will cancel out. The harmonic currents will add on the neutral, but I think most of us believe this problem is way exaggerated by manufacturers trying to sell more copper.

I also think most office equipment (including lights and computers) has gotten better power supplies and ballasts than they had 10 or 15 years ago. So I think you will find only a small part of the load will be harmonics.

I would be interested in hearing what others have seen as a typical harmonic load on a typical office building. I would be supprised if it is more than 10% of the total load.

Steve