# Transformers and switch gear.



## DiegoXJ (Jul 29, 2010)

Ok, so there is a customer HOA, that is looking to add a few meters to their existing service in order to set up car charging stations. 

I have taken a look at the switch gear (1600 amp), and the disconnect (800 amp) feeding the section of meters stacks we want to add to.

This is all 208v 3 phase service, and 100 amp single phase to the units.

Talking with the utility company has gleamed this much info. The building is powered by a 75Kva transformer, and it's peak usage appears to be 127Kva

My questions are, would the utility really undersize a transformer and let it run overloaded at times? 

My calculations say 127Kva is about 350 amps per phase, seems kinda low for a 4 story building with 35ish units... 

Is there any reasoning that i am missing with why the gear is rated so high compared to the usage, other than, with that many units it's just what would fit... 

Thanks for the info.


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## Black Dog (Oct 16, 2011)

DiegoXJ said:


> Ok, so there is a customer HOA, that is looking to add a few meters to their existing service in order to set up car charging stations.
> 
> I have taken a look at the switch gear (1600 amp), and the disconnect (800 amp) feeding the section of meters stacks we want to add to.
> 
> ...


Yes they would to save money.


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## RIVETER (Sep 26, 2009)

DiegoXJ said:


> Ok, so there is a customer HOA, that is looking to add a few meters to their existing service in order to set up car charging stations.
> 
> I have taken a look at the switch gear (1600 amp), and the disconnect (800 amp) feeding the section of meters stacks we want to add to.
> 
> ...


Yes...


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## micromind (Aug 11, 2007)

Oil-filled transformers can take a pretty substantial overload for a fairly long time. How much and for how long depends mostly on the ambient temperature. 

Most POCO engineers will tell you 'if you don't burn up a transformer once in a while, you've got too much iron out there wasting money'.


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## Meadow (Jan 14, 2011)

micromind said:


> Oil-filled transformers can take a pretty substantial overload for a fairly long time. How much and for how long depends mostly on the ambient temperature.
> 
> Most POCO engineers will tell you 'if you don't burn up a transformer once in a while, you've got too much iron out there wasting money'.




Did they do this to save up front? Id think the reduced efficiency would eventually off set that?


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## Meadow (Jan 14, 2011)

DiegoXJ said:


> Ok, so there is a customer HOA, that is looking to add a few meters to their existing service in order to set up car charging stations.
> 
> I have taken a look at the switch gear (1600 amp), and the disconnect (800 amp) feeding the section of meters stacks we want to add to.
> 
> ...


NEC is a grossly oversized code. Most services can easily be 1/2 to 1/5 the size.

POCO transformers are oil filled and have thermal properties very different from dry type units.

The oil has thermal inertia, which means when the oil has cooled it can absorb heat generated by an overload. When load goes back down that oil cools. 

A pole pig can actually take a 200% overload for an hour if the load was light enough before that to let it cool. 

Further, cooling fins, oil circulation and fans increase heat loss. An classic example that you will find in nearly every substation is a 30MVA coil in an oil tank with fins, fan and pump. The fans increase the capacity to 40MVA and kick an oil pump on and it goes to 50MVA. Not only does this save on capitol costs but it lowers the short circuit current values because less iron equal less through fault current. 

http://electrical4u.com/transformer-cooling-system-and-methods/


Loss of life vs investment costs can further play a role. If the unit is determined to only need 40 years of service before failing they will deliberately save money and install a smaller unit in such a way the overheating patterns stress the unit enough to fail after 40 or how many ever years.


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## micromind (Aug 11, 2007)

meadow said:


> Did they do this to save up front? Id think the reduced efficiency would eventually off set that?


Savings up front is part of it, the other part is that lightly loaded transformers are not as efficient as heavily loaded ones.


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## Meadow (Jan 14, 2011)

micromind said:


> Savings up front is part of it, the other part is that lightly loaded transformers are not as efficient as heavily loaded ones.


How much less are losses in overloads over Idle losses?


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## mwbc4ever (Nov 24, 2013)

meadow said:


> How much less are losses in overloads over Idle losses?


A building like an apartment complex will generally have an extremely low load factor. This means the ratio of average load to peak demand is very low. You are correct that there will be more losses during overload but the utility probably knows that the typical load is very low and decided they were better off minimizing no load losses.


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