13.8kV - 120/240V Trailer Single Phase Service

[porf]

I need to power 16 double-wide trailers, each at 120/240V and each double-wide having 2-100A single phase panels. I have a 13.8kV supply. I am thinking of a 3-phase 13.8kV-480/277V dry type Xfmr and then split up phases to feed three single phase 480-120/240V Xfmrs. However, I believe that if system is not well balanced, I run the risk of dumping the main 480V Xfmr. Practically speaking, am I going against convention or do you think there may be a better way? Appreciate your comments.

 

[dpc]

Imbalance on the 480V transformers is not really a problem, as long as it isn't overloaded on any one phase.

But I'm curious as to why you want to drop down to 480V first? You can easily go from 13.8 kV directly to 120/240V using some small pad-mounted transformers.

 

[porf]

dpc you are correct in that I could use three 13.8kV-120/240V Xfmr's. I was thinking that I cold simplify the drops from the pole to just one set of conductors coming down the pole whereas with three single phase Xfmr's I would have three drops and three connections to the grid lines. I suppose three individual Xfmr's, however, could eliminate an imbalance situation, plus eliminate one transformer. Thank you for your feedback.

 

[busbar]

Depending on secondary-circuit distances, going 13.8kV – 208Y/120 may be an option. One gotcha is that trailer ‘wall-pack’ air conditioning be intended for 208 and not only 240V service.

 

[dpc]

I was thinking more about multiple transformer locations, close to the mobiles, running the 13.8 kV underground.

How are you going to want to meter this? By individual space?

 

[porf]

The trailer complex is only going to be up for 6-9 months. Consequently, we are also looking at using heavy G-GC cords to supply the trailers. I am already supplying about four other trailers from a pole mounted 13.8-480V 3-phase Xfmr because these trailers do have 120/208V 3-phase service panels. Rental trailers, however, are being supplied from a trailer rental company with specifications of 120/240V - 2-100 amp panels. I will confirm this so we don't get stuck with, as you mention, A/C equipment that could possibly be rated at 208V. Thank you for your questions.

 

[jbartos]

Suggestion to porf (Electrical) Jan 31, 2004 marked ///\\I need to power 16 double-wide trailers, each at 120/240V and each double-wide having 2-100A single phase panels. I have a 13.8kV supply. I am thinking of a 3-phase 13.8kV-480/277V dry type Xfmr
///Have you estimated kVA size of this transformer? What about a type, a pad mounted or mobile?\\ and then split up phases
///In large junction box? Or daisy chain the single phase transformers in three chains? This will depend on a location of the 13.8kV-480/277V dry type Xfmr.\\ to feed three single phase 480-120/240V Xfmrs. However, I believe that if system is not well balanced, I run the risk of dumping the main 480V Xfmr.
///Considering 16 trailers, 100A services, the 480V secondary transformer can reasonably be balanced. Even, if some imbalance exists, it will be unavoidable or tolerable considering the kVA size of the 480V transformer. The electrical contractor check the load of each of 16 trailers and balance the 480V power distribution.\\ Practically speaking, am I going against convention or do you think there may be a better way?
///There may be some marginal savings, depending on locations of the trailers. Also, two 13.8kV-480V transformers may be considered to provide redundancy of power supply to 16 trailers, with essentially 8 trailers on each transformer and provisions to power 8 trailers from the other transformer.\\

 

[porf]

Thank you, jbartos-Response: Jan 31, 2004 marked ///\\1. Have you estimated kVA size of this transformer? What about a type, a pad mounted or mobile? ///Initially, I had envisioned a 3-phase, 1500 kVA, 13.8 kV – 480/277V dry type pad mounted-Y having 1804 amps on secondary. This would then feed three 1-phase 250kVA, 480-120/240 with 521A on primary each to yield 1042 amps on secondary of each. This would in turn feed, roughly, 5-double wides (5x200A=1000A), as each double wide would need 200 amps (2-100A panels.)\\2. In large junction box? Or daisy chain the single phase transformers in three chains? This will depend on a location of the 13.8kV-480/277V dry type Xfmr. ///13.8 kV would feed down the pole to the 3-phase Xfmr, to JB to the 3-1Phase Xfmrs all in close proximity. All trailers would also be right close to each other in a cluster, not too far from power source\\3. Considering 16 trailers, 100A services, the 480V secondary transformer can reasonably be balanced. Even, if some imbalance exists, it will be unavoidable or tolerable considering the kVA size of the 480V transformer. The electrical contractor check the load of each of 16 trailers and balance the 480V power distribution. ///Correct, adjustments would be made before final service.\\4. There may be some marginal savings, depending on locations of the trailers. Also, two 13.8kV-480V transformers may be considered to provide redundancy of power supply to 16 trailers, with essentially 8 trailers on each transformer and provisions to power 8 trailers from the other transformer.///Thank you, redundancy is now a consideration.\\\

 

[jbartos]

Comment on the previous posting: You are welcome.
The 1500kVA transformer for 16 trailers seems to have a big kVA margin. Are there big motor loads in the trailers? Or are you anticipating a load growth (additional trailers)?
1500kVA/(16 trailers x .240V)=391A/trailer versus 200A panel/trailer.

 

[jghrist]

Why dry type transformers? If you are serving from overhead 13.8 kV (you mentioned a pole) you should consider four separate single phase 8kV-120/240 volt oil filled pole type distribution transformers. Each serving four trailers and placed centrally to each group of four. You need to get a better idea of the load on each trailer, but a 25 to 50 kVA transformer would probably be adequate for four typical double wide trailers. You can't go by the panel size alone; panels are normally way oversized for the actual load and you can count on some diversity among trailer loads.

 

[jbartos]

Suggestion: The pole mounted transformers as indicated in the previous posting would lead to the simplest power distribution. However, if a redundancy and power supply availability are required the better transformer and power distribution solution is necessary.
Also, the pole mounted transformers are more convenient for permanent installations. The mobile transformer solution is more convenient for a temporary power distribution.

 

[FACB25]

Regarding busbar's comment about 208/220, I have a vague memory of section 550 prohibiting that feed. My copy of the NEC is at work, but maybe someone else can check it.