Transformer sizing help

Status
Not open for further replies.

Scoop789

Member
Location
Los Angeles
I'm installing 4 pieces of equipment, each piece is labeled 7.2 Kw. They operate at 208V or 240V 1phase.
I'm looking at installing a new transformer fed from a 480/277V panel to supply a new 208V sub-panel. The manufacturer of the equipment are telling me it's fine to supply the 4 pieces with a 30 Kva transformer as that only totals 28.8Kw.
Now, when you divide 7200/208 I you get 34.6Amps x 4 = 138.4A which would mean a 75 Kva.
What am I missing here?
Thanks.
 

Scoop789

Member
Location
Los Angeles
Transformer sizing help

I'm installing 4 pieces of equipment rated at 7.2Kw each and operate at 208 or 240 single phase. I have to install a new transformer fed from the 480/277 panel to supply a new sub-panel to feed the equipment.
The manufacturer is telling me that a 30Kva transformer is good as the total is only 28.8Kw but when you divide 7200 by 208 you get 34.6Amps x 4 = 138.4 Amps. Looking at my transformer calculator this would call for a 75Kva transformer.
Any help on what I'm missing is appreciated.
Thanks.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
You have 4 pieces of single phase equipment going on a three phase transformer.

One phase will have two pieces of equipment on it, while the other two phases will only have one piece of equipment.

Hope that helps.

Steve
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110707-1020 EDT

Scoop789:

138.4 * 208 = 28,800

You are missing --- providing enough information.

480/277 implies a 3 phase Y source.

What is the 208 subpanel. I have to assume it will be a 208/120 three phase Y.

If this is the case, then spread the 4 loads over the three phases. Thus, you have two 7200 loads and one 14,400. Now do your calculation. But find out if the load is really 7200 at 208.

If the loads are basically resistors (heaters) and there is no way to adjust the resistance, then your power dissipation is much less at 208 compared to 240.

Power = V^2 * R. So power might be 75% of the 7200. If there is electronic control of power, then average power could be constant independent of voltage over some substantial range.

One more point. If you operate the devices at 208, then is the performance the same as if operated at 240? If heaters, then heating time will be slower at 208 compared to 240.

.
 
Last edited:

broadgage

Senior Member
Location
London, England
If your sub-panel is to be single phase, then each load will use about 34 amps, and the four loads in total will be about 135 amps.
135 amps at 208 volts is about 28 KVA so a 30KVA transformer should be fine.

It would be well to check what the current draw of the load is at different voltages, and if 208 volts or 240 volts is prefered.

If the sub-panel is to be three phase, then a larger transformer will be required since the loading of 4 equal single phase loads from a 3 phase supply is inherently unbalanced, and a three phase transformer would be sized according the most loaded phase, not the average.
 

Besoeker

Senior Member
Location
UK
I'm installing 4 pieces of equipment rated at 7.2Kw each and operate at 208 or 240 single phase. I have to install a new transformer fed from the 480/277 panel to supply a new sub-panel to feed the equipment.
The manufacturer is telling me that a 30Kva transformer is good as the total is only 28.8Kw but when you divide 7200 by 208 you get 34.6Amps x 4 = 138.4 Amps. Looking at my transformer calculator this would call for a 75Kva transformer.
Any help on what I'm missing is appreciated.
Thanks.
You're missing power factor from your calculation. If it isn't unity, the the current could be significantly higher than your calculated 138A. Note that transformer is rated in kVA, not kW.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
In addition to the details that Besoeker mentions, I see what I think is a "basic flaw" with the 30 kva. For conversations sake, I don't think it is incorrect to look at a 30 kva 3 phase transformer as (3) 10 kva single phase units. One of those units would be loaded (or overloaded) at 14.2 kw. Without taking power factor, etc into consideration, I would think you would be looking at 45 kva minimum.
 

Besoeker

Senior Member
Location
UK
In addition to the details that Besoeker mentions, I see what I think is a "basic flaw" with the 30 kva. For conversations sake, I don't think it is incorrect to look at a 30 kva 3 phase transformer as (3) 10 kva single phase units. One of those units would be loaded (or overloaded) at 14.2 kw. Without taking power factor, etc into consideration, I would think you would be looking at 45 kva minimum.
Yes. I went back to add a point about the unequal loading but I got interrupted and the edit window timed out.
So, thank you for adding that.
 
Status
Not open for further replies.
Top