3-phase XFMR sizing confirmation

[Richardh247]

Hi all,

It has been a long time since I have had to do any real engineering type stuff: as a foreman, we had prints and engineers that determined for us much of what I have been asked to calculate for this job. I'm confident, but I have always thought that running things through others is what sets apart those that just do a job from those that care about the job they are doing.

A little background:

I began in commercial electric back in 1993 and had a nasty fall in 2001 that put me out of work for a long time. After twelve years of basically doing little to nothing I got sober and re-entered the electrical trade as a small contractor several years ago. My company has grown exponentially over the last many months and was recently asked to do a large commercial/industrial project that I feel like we are ready for. The issue is that, typical of small-town sole-owner thinking, the owner of the plant has no prints or calculations drawn up to bid on - those have also fallen in my lap and, while I know we can do the work itself, I've been a bit vexed on the gear and would appreciate someone taking a look at what I have derived for the service.

The job calls for a 600A, 480V main (3-phase) service that will then feed a wye-wye xfmr to juice up EITHER a 240V single phase service or a 208V 3-phase service. The owner doesn't know which one he wants, so I have planned for the bigger demand of the 3-phase 208V. He states, and mind you that this is a direct quote, "I want room for expansion but I don't know how far I will take that." Sigh. The only certainties I currently have are 10 wall packs, 13 T-5 5-lamp Hi-Bays and a few receptacles. That's not much to go on, especially for a xfmr.

Given the ambiguity of the job, I decided on a 200A 3-phase low volt service. Lights are all 277V, so it seems really silly to have ten receps on a 200A 208V panel.

My plan is to run parallel 300's with a #2 bond. 75-degree lugs should then give me 285(2) = 570, then next size up to 600A. The run to the primary is 125'.

Not having any idea what the end load on the xfmr is going to be, or even if he will want single- or 3-phase low volt, I went with 3-phase and came up with a 75 KVA dry-type xfmr. That's a lot of amperage on the supply side, but with no known demand except for the ten or so receptacles I was afraid of a 50 being insufficient if he decided on the 3-phase.

I dunno... this looks both right and way over-sized to me just thinking back on my experience.

Anyone want to opine? Maybe give me something I haven't thought of to go read on and research?

 

[augie47]

Somewhat a "no win" situation is it not.
As soon as you install a 208/120 transformer they will want 240 volt.
If you put in a 240/120 single phase they will want 3 phase.
I think the 75kav fits into what I often see.
450.3 will limit the primary protection to 225 amps so it will not be protected by a 600 amp feeder.
What I see most often is a 600 amp 480/227 panelboard with a 100-125 amp feeder to a 75 kva.


EDIT TO CORRECT TYPO

 

[Richardh247]

Somewhat a "no win" situation is it not.
As soon as you install a 208/120 transformer they will want 240 volt.
If you put in a 240/120 single phase they will want 3 phase.
I think the 75kav fits into what I often see.
450.3 will limit the primary protection to 225 amps so it will not be protected by a 600 amp feeder.
What I see most often is a 600 amp 480/227 panelboard with a 225 amp feeder to a 75 kva.

Exactly. I was hoping that I wasn't the first electrician to be told make something happen with so many variables surrounded by so much ambiguity. But, hey - I may not be able to plan for every contingency, but I can have a contingency for every plan.

I wasn't planning anything like 600A of overcurrent protection for the secondary. More like 150, which is what I have the suppliers bidding on for me. I'll jump that up if I have to, but given that the equipment schedule, which doesn't yet exist, will be mostly 480 3-phase (by his words, at any rate) I really can't see him at 125% of 150A. Regardless, I have sized the feeders at 2/0 just in case.

Anyway, the only way I could possibly think to protect myself is to put a clause in my bid proposal that reads something like, "Provide for and install a 3-phase 208V, 200A low voltage panel in the initial installation; the customer will be responsible for replacement fees associated with any service other than specified on a time and material basis." He's a businessman so I am sure that he knows that he can't arbitrarily change his mind without some financial consequence.

I appreciate the reply. Thanks much. Props to my wife for making me swallow my pride and ask. "Men," she says, "are silly. Just ask!" Shrug.

 

[hbendillo]

You say the job calls for a 600 amp main, three-phase service. I am assuming that is a main panel and not the feed for the transformer. A 75 KVA transformer requires no more than a 125-amp primary circuit for three-phase 480-volt. Secondary can be up to 250 -amp (1.25 factor applied). The most common size panelboard the load side of a 75 KVA transformer in a 120/208-volt side is 225-amp.

What kind of building is this facility? I am assuming that there will be HVAC and other loads. If you have the square footage and building type you could get a better idea of the load.

 

[Richardh247]

You say the job calls for a 600 amp main, three-phase service. I am assuming that is a main panel and not the feed for the transformer.

Correct. The HV panel will service the lights and equipment as well as the XFMR.

A 75 KVA transformer requires no more than a 125-amp primary circuit for three-phase 480-volt. Secondary can be up to 250 -amp (1.25 factor applied). The most common size panelboard the load side of a 75 KVA transformer in a 120/208-volt side is 225-amp.

What kind of building is this facility? I am assuming that there will be HVAC and other loads. If you have the square footage and building type you could get a better idea of the load.

The building is 82x126, so a little over 10kft^2.

The company makes and ships products like the decorative wood chips (among other things) that you put around trees. Normally, the operation takes about 15 guys, and this new building will house automated equipment to take the workload down to only two guys. There isn't much going in it, really: a shredder, a chipper and some other components like a bagger that Phase II will have us back for.

The owner was very specific that he only wants the 3 quadraplex receptacle stations at the center posts. There is a bathroom/break room area that's about 20x20, so I have set a few switches and a GFI as well as some dedicated receps for coke and snack machines. Those receps and two outside GFIs are all that is on the load side of the 208 XFMR. What he did say was that eventually there will be more, but he isn't sure what, how much or when. He's a helpful fella', that one.



It's a minuscule demand factor for the XFMR. I currently have all three quadraplex stations on dedicated circuits, the bathroom GFI on its own circuit, the outside GFIs on a circuit, the convenience receps in the breakroom on another circuit and, just because I can, I gave both snack machines a dedicated circuit. That totals to 12 receps (15 - the 3 that are dedicated) at 180VA ea = 2,160VA.

As of this moment, that's it on the secondary side.

There will be no HVAC load... yet. If he adds that later then I have to assume it will be blower assemblies rather than any actual conditioned air producer except for possibly a heat pump. None of the other production buildings I looked at while I was there to try and get a feel of what the end product would resemble had anything for conditioned air.

 

[kingpb]

Honestly, I would only put in a xfmr large enough to supply the load you know of plus maybe 25% spare capacity, i.e. house load.

The 480V will handle the larger loads, and an additional Xfmr can be installed later, as needed.