Load Calculation

[Alwayslearningelec]

Hello.
I have the (5) worker trailers that I need to feed with temp. (4) have 60MCB and (1) has 100A MCB.
I walked the site today to try and determine where I could pull power from.
There is only 277/480 available from a panel( see pic). Our plan is to run a feeder from this panel( use a transformer to get 120/208v) to a temp panel by the trailers and then feed the trailers from the temp panel.

A few questions.
1. How do I determine what size temp panel i would need for the trailer. Is it just adding up 60Ax4 and 100A. ...340A?
2. I guess once I determine the temp panel size then I can determine if the existing panel(277/480v) has enough power available to feed the temp panel.

A 75kva xfrm would give me 208amps on the step down side and a 112.5 would give me 312amps on the step down side. Again I need to know what size temp panel(main breaker) I'd need to feed those trailer.
Also with either of those size xfrmrs I'd need either a 125A or 175A OCPD in the existing panel( in the pics). Not sure If I have that power available. Can one tell just by looking at the breakers and panel schedule?

 

[augie47]

There is a whole cornucopia things to consider:
What is the actual (or calculated) load on each trailer (any HVAC, large loads, etc) ?
What size feeder supplies the 480v panel ? .the panel is 250 amps but the actual feed may be smaller.
(It appears you have only 1 breaker larger than 20 amp so the panel will likely accommodate the added load)
You are ta;ling about a 208 secondary... is that comparable with the loads ?
You have a 3 phase supply and single phase loads... be best to do your load calculations using kw.

 

[infinity]

Are the trailer panels single phase or three phase?

 

[Alwayslearningelec]

Are the trailer panels single phase or three phase?

Single phase.

 

[Alwayslearningelec]

There is a whole cornucopia things to consider:
What is the actual (or calculated) load on each trailer (any HVAC, large loads, etc) ? Dont know
What size feeder supplies the 480v panel ? .the panel is 250 amps but the actual feed may be smaller. Don't know, Why does that matter? Because if the feeder is smaller whatever the feeder ampacity is that's what's avaiable?
(It appears you have only 1 breaker larger than 20 amp so the panel will likely accommodate the added load)
You are ta;ling about a 208 secondary... is that comparable with the loads ? Yes
You have a 3 phase supply and single phase loads... be best to do your load calculations using kw.

 

[infinity]

Single phase.

And how do you plan to distribute this to the trailer panels? Panelboard after the transformer? I would run a 480 feeder to the 75 kva transformer next to the trailers and then into a 225 amp 208Y/120 volt panel with 5-1Ø circuit breakers one for each trailer.

 

[Alwayslearningelec]

And how do you plan to distribute this to the trailer panels? Panelboard after the transformer? I would run a 480 feeder to the 75 kva transformer next to the trailers and then into a 225 amp 208Y/120 volt panel with 5-1Ø circuit breakers one for each trailer.

Yes makes sense to distribute 480v.
But would a 75kva xfrm put out enough current for the temp panel? How would I determine the size of the temp panel? I only know the trailer MCB's and not the actual loads,

 

[Alwayslearningelec]

This is actually one of the trailers panel schedule. Actually has 150A main breaker...not 100.

 

[infinity]

But would a 75kva xfrm put out enough current for the temp panel? How would I determine the size of the temp panel?

With a 3Ø output of 208 amp it would probably be large enough to run all five 1Ø trailers (maybe an engineer can join in with a calculation). You would need to know the actual load on the trailers to make an accurate. Figure that each trailer will have electric heaters running all winter long 24/7.

Typically the power allotted for these things is too little and there are issues with tripping. We had a job where all for the trailers were fed from a temp panel and the main would trip frequently during extreme cold weather. A few times when we came into work on Monday the water cooler was frozen solid. One electrical contractor has a portable generator that they used to make their own power because the temp power available wasn't enough for the load.