Transformer Size vs. Service Size

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elecshop

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What are the disadvantages of having a transformer much larger (providing ~900A) than the main service disconnect (~300A) for a 208V service?
I was planning to size the underground feeders incoming from the transformer to the main service disconnect per the maximum size, to allow for future expansions, but the feeders between the disconnect and main panel/downstream per the connected load. I do not see any code violations in that, please advise.
Thank you!
 
Your AIC rating of your breakers / switchgear may need to increase to allow for the higher available fault current. That will increase equipment costs, but planning for future expansion is always a good idea. It'll be cheaper to buy higher rated equipment now, rather than rip it all out and replace it later when you need to upgrade.


SceneryDriver
 
The only problem with oversized transformers is the losses in its core. I like the idea of designing for realistic expansion, but I don't view electrical equipment as an investment.
 
elecshop said:
What are the disadvantages of having a transformer much larger (providing ~900A) than the main service disconnect (~300A) for a 208V service?
I was planning to size the underground feeders incoming from the transformer to the main service disconnect per the maximum size, to allow for future expansions, but the feeders between the disconnect and main panel/downstream per the connected load. I do not see any code violations in that, please advise.
Thank you!
Click to expand...
Some clarification is needed. Is this a utility transformer? IF so they will size it. Have they told you they will be using a 300KVA based on the loads you have provided them? You call the conductors from the transformer "feeders". Or are they service conductors?
 
as pointed out by above posters

A larger transformer means more energy available if there is a fault. This depends on a multitude of factors, most notably the transformer capacity and impedance. a 750 kVA transformer with impedance of 4.5%, will contribute about 20,000 Amps. By contrast, a 300 kVA with similar impedance will contribute 8,000 Amps. This is a gross oversimplication.

A larger transformer also means more core losses even at 0% load. Core losses are constant, the larger the transformer, the more core losses.

At 900 amps (about 750 kVA, assuming 480V, 3-phase) probably looking at around 2 kW at no load. Compare that with 300 amps (250 kVA 480V 3-phase), around 700 watts. Difference is 1300 watts all day every day.

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FWIW, I have seen a commercial 480/277V 400A service powered by a 225kVA utility transformer (full load current = 270A). I called the utility to ask about it and the rep I talked to just said, "It happens."
 
@wwhitney @electrofelon Right, the transformer is before the service disconnect and it would be utility provided.
We are expecting a future load increase so I have the tendency to be okay with an oversized transformer, also the utility doesn't look like they have much flexibility since they currently have a 150kVA which they want to replace with a 300kVA due to several derating factors. The thing is, the lead times are much shorter with the smaller disconnect that we have so I am planning to :
- keep service feeders sized for the max current
- size the disconnect per the current load, without future expansion, which is now available and ready to be installed (300A),
- have the feeders downstream the disconnect to the main MLO panel (whose rating is higher than the disconnect, and capable of handling the future load) be sized to carry the max current, since they are already protected by the 300A disconnect upstream
that way, when we expand the load in the future, only the disconnect would need replacement and the feeders+panels would stay the same.

Any problem with this installation?
 
PSE (and I assume most poco) are VERY sensitive to core losses.
Worked on a habitat 6 unit project with 6 ea 200A services and had a 25 kVA transformer (yes, twentyfive) installed by the poco contractor Potelco. Gas heat and WH but all elect kitchens.
One of the habitat EE volunteers questioned the 'undersized' xfmr, poco response was 'no problem'. AFAIK, no problems.
 
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