Correct Sizing for Utility transformers

[JakeandElwood]

Hi,
I was looking online for an answer and found your fourm. Hopefully you can help me.

We recently moved into a building that is supposed to have 200amps of 3wire 3phase 240V service to one of the meters. During last weekends storms we lost power. Upon inspection one of the pole mounted transformers had triped its safety. Not a big deal Utility people came reconnected it. but while I was waiting for them I noticed that the pole mounted transformers were two 10 Kva's and one 15Kva. By my calculation (35Kva x 1000 / 415.68V) those transformers would only provide 84.199 amps not the 200 amps we are supposed to be rated for. As we expand and add more equip. should I be concerned that we are really not capable of utilizing the full 200 amps as the utility company states in our service?

Thanks in advance for your help.

 

[bob]

You are correct. The capacity of the bank is 84 amps. The utility does not provide transformer capacity based on your main panel size. It has a method to estimate what the actual demand will be. If you add more load, You should inform the utility so they can check to see if the capacity is adequate.

 

[JakeandElwood]

Thank you Bob,

When I called the Utility service and asked them what amperage our service was rated at they told me 200amps so I guess I should call them back and ask for an Engineer to come have a look at our pole and explain to me how we can get 200 amps from the 84 amp bank. Since our average machine uses 24.6 amps it wont take very long to reach 84.

 

[charlie b]

. . . I guess I should call them back and ask for an Engineer to come have a look at our pole and explain to me how we can get 200 amps from the 84 amp bank.

You can't, and you don't need to, and they don't need to provide the ability to. The process should start with a load calculation. Give them a calculation that proves you need more than 84 amps, and they might let you pay them to install a larger transformer bank. But the size is their call, not yours. Typically, a utility will provide a service transformer that is rated for 40$ to 50% of the calculated load. That is based on their experience with the actual load a building typically sees, in comparison to the typical calculated load.

 

[iwire]

First off I agree 100% with everything Bob said.

I will add that just because the transformer is rated 84 amps does not mean that it will not deliver more than 84 amps for a long time. Could you actually drag a full 200 amps out of them and if yes how long would they last I do not know. I do know that utilities commonly run the transformers beyond the nameplate limits with great success.

 

[iwire]

Give them a calculation that proves you need more than 84 amps, and they might let you pay them to install a larger transformer bank.

Well put.

Typically, a utility will provide a service transformer that is rated for 40% to 50% of the calculated load.

I have heard that as well, the NEC load calculations are very conservative.

 

[LarryFine]

Blues Brothers, welcome to the forum!

We recently moved into a building that is supposed to have 200amps of 3wire 3phase 240V service to one of the meters.

... while I was waiting for them I noticed that the pole mounted transformers were two 10 Kva's and one 15Kva.

I'll bet you have a high-leg, 4-wire service, with the larger unit having the grounded neutral.

Your max calculated supply would be 10Kva 3-ph and 5Kva 1-ph. How does it work with those numbers?

I agree that you tell the POCO your loads, and they install whatever they decide is adequate.

 

[JakeandElwood]

So Charlie your saying that a utility company wont upgrade their transformers to provide the service that they are stating that they are delivering? My main concern now based on the amp calculation is when we add equip. Based on our machines operations it is quite feasable that in the course of normal running mutiple machines could use their peak at the same time several times a day. This could posssibly cause trasformer failure, lost production time, and property damage if the transformer failure is catostrophic based on their location. Why would a utility company want to take that kind of risk?

 

[JakeandElwood]

Larry,

You would be correct. Counting the ground. With a center neutral. How ever this service is strictly 3ph. We have a seperate service for our lighting, HVAC and 110 power.

 

[bob]

Thank you Bob,

When I called the Utility service and asked them what amperage our service was rated at they told me 200amps so I guess I should call them back and ask for an Engineer to come have a look at our pole and explain to me how we can get 200 amps from the 84 amp bank. Since our average machine uses 24.6 amps it wont take very long to reach 84.

J
When the utility person said you have a 200 amp service, he may have been referring to the actual service conductors and not the transformers. Check and you will likely see a 1/0 or 4/0 AL triplex installed.

 

[JakeandElwood]

Thank you for the help guys.

I'm going to call the POCO and tell them what I think our amp usage will be and I guess they will either put in larger transformers or wait untill they have too.

 

[rcwilson]

Like others have already said, those transformers out on the pole can deliver a lot of amps before they burn up, especially if the load is up and down. ANSI/IEEE transformer loading calculations look at 24 hour averages, or shorter time periods depending on the transformer size. It takes a while for the transformer oil and windings to heat up to maximum levels. Even then, there is an emergency overload rating that could be used before the protection trips.

One reason they can get away with it is the transformer is up on the pole where cooling air is not a problem compared to an overheated utility closet.

 

[charlie b]

So Charlie your saying that a utility company won?t upgrade their transformers to provide the service that they are stating that they are delivering?

What is it that you think they are stating, with regard to their service? They will deliver what their own internal processes have determined is the minimum required capacity for the facility.

This could possibly cause transformer failure, lost production time, and property damage if the transformer failure is catastrophic based on their location. Why would a utility company want to take that kind of risk?

They will take the risk because the economics work out in their favor. If 100,000 customers each tell them they have calculated their load at 200 amps, and if they provide each customer with a transformer that is rated for 80 amps, then they are likely to experience only a handful of transformer failures. The cost of replacing (or even upgrading) those few failures is much lower than the cost of giving everybody a 200 amp rated transformer.

As Bob said earlier, the NEC?s load calculation process is very conservative. A building will seldom draw anywhere near the amount of load that the calculation gives as its answer. So installing transformers that are much bigger than will ever be needed is also a waste of natural resources (i.e., copper and other materials of construction).