The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Locked rotor, no burnout
by andey
Today at 04:48 AM
Temporarily feeding a panel...
by Potseal
Yesterday at 11:05 PM
Massive power outage South Australia
by Meadow
Yesterday at 06:46 AM
short circuit??
by Meadow
Yesterday at 06:43 AM
Norwegian power?
by Meadow
Yesterday at 06:36 AM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
HotLine1 14
sparky 10
gfretwell 8
sparky66wv 8
Potseal 7
Who's Online
1 registered (jraef), 292 Guests and 5 Spiders online.
Key: Admin, Global Mod, Mod
Page 1 of 2 1 2 >
Topic Options
Rate This Topic
#211708 - 11/12/13 08:58 AM Small transformer better than over sized?
cgw Offline
Member

Registered: 07/29/04
Posts: 133
Loc: Rochester NY
The load calculated for a (480X208) transformer for multifamily using the optional method (220.84)is significantly smaller than the load calculated using the regular method. It is close but it would allow a smaller transformer (750 vs. 1000).
Are there any disadvantages going with the smaller size?

The advantages are smaller physical size and heat output.

Top
Work Gear for Electricians and the Trades

Work Gear for Electricians and the Trades
Work Gear for Electricians and the Trades
Arc Flash Clothing, Gloves, KneePads, Tool Belts, Pouches, Tool Carriers, etc. etc....

#211710 - 11/12/13 12:51 PM Re: Small transformer better than over sized? [Re: cgw]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6786
Loc: Brick, NJ USA
This is a design issue. Are you refering to the POCO xfr?? Our utility co's here routinely install 'small' transformers (13.2 to 480/13.2 to 120/208) on their supply side.

That said, do you have any capacity for future expansion, if you opt for the smaller unit that you reference as 'close'?

IMHO, a good design would allow for future capacity, unless this resi is being built with no possibility for any significant future electrical requirements. (Such as resi apts)
_________________________
John

Top
#211720 - 11/12/13 06:59 PM Re: Small transformer better than over sized? [Re: cgw]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
I assumed he was talking about a customer owned transformer.
This may be a Scott question.
At say 600 KVA, is a 1000 KVA more efficient than a 750 (less heat)
I also guess there are higher efficiency transformers for a few extra bucks.
I don't "spec" them I only know how to inspect them wink
_________________________
Greg Fretwell

Top
#211722 - 11/13/13 06:03 AM Re: Small transformer better than over sized? [Re: cgw]
ghost307 Offline
Member

Registered: 05/09/05
Posts: 877
Loc: Chicago Illinois USA
The downside that I could see would be if the actual load turned out to be the higher value and you have to go back and replace everything (on your nickel).
If push came to shove, all that the customer's attorney would have to do is point out that the value determined in the Code was indeed greater than the one that you used and claim that you 'cherry picked' another calculation method that let you use the cheapest transformer.

On the other side of the coin, transformers will handle a larger load than they are nameplated at for brief periods of time and you might be okay based on the load profile of the building...but if everyone starts cooking and turns up their thermostats on the same cold winter day you might be up the proverbial creek.
_________________________
Ghost307

Top
#211725 - 11/13/13 07:48 AM Re: Small transformer better than over sized? [Re: ghost307]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8530
Loc: SI,New Zealand
CGW,
I would never under-spec something like a transformer, especially where it has multiple loads on it, as you state.
You always need a certain amount of operating capacity in a transformer, over and above what your maximum demand calcs tell you, to make sure you don't cause it to overheat or indeed over-saturate.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

Top
#211727 - 11/13/13 08:32 AM Re: Small transformer better than over sized? [Re: cgw]
cgw Offline
Member

Registered: 07/29/04
Posts: 133
Loc: Rochester NY
Thanks for the input.
The question concerns a dry type transformer in the building (the service is 480V and the transformer is for a feeder that serves all the apartments and the 120/208 house loads).
One of the main questions is - do we trust the 220.84 calculation when it is significantly higher than the 220.40 calculation?
Including house load:
220.40 - 945 KVA
220.84 - 664 KVA
750KVA transformer or 1000KVA transformer?

Top
#211728 - 11/13/13 12:40 PM Re: Small transformer better than over sized? [Re: cgw]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6786
Loc: Brick, NJ USA
IMHO, 220.40 would be my choice.

1000KVA transformer.

(Keep in mind that it will all come back to the design professional, in the event of any issues)


Edited by HotLine1 (11/13/13 12:43 PM)
_________________________
John

Top
#211729 - 11/13/13 12:43 PM Re: Small transformer better than over sized? [Re: cgw]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Several years ago the US government passed an 'Energy Act'. One of the most visible outcomes was the 'killing off' of incandescent lamps, another was the requirement that small power transformers (e.g. non-utility installations) be designed with 'ideal' efficiency at 35% loading.

So a 1000kVA transformer would be most efficient at 350kVA, but it may still be less efficient than a 350kVA unit loaded to 100%.

Have you done a loss (cost of ownership) analysis yet?

Top
#211740 - 11/13/13 10:49 PM Re: Small transformer better than over sized? [Re: cgw]
twh Offline
Member

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
Smaller transformers require smaller wire and possibly equipment.

Top
#211801 - 11/16/13 01:43 PM Re: Small transformer better than over sized? [Re: cgw]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
The step-wise introduction of LEDs and less consumptive PCs...now replaced by laptops... means that you would go small.

You're burning kW even at idle. The rare heavy loads can be sustained for hours on end for such transformers.

Be certain that ventilation is good and pure. If it is not so then you have to go large.

You would not believe how hot some e-rooms get because the design disregarded heat-rejection from a dry-type transformer.



Edited by Tesla (11/16/13 01:44 PM)
_________________________
Tesla

Top
Page 1 of 2 1 2 >



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals