80% Continuous

Status
Not open for further replies.
W

whisperinghill

Member
I know this type of question has been answered many times, but I still don't get it.

If I have a 75 KVA 3ph 480-120/208 transformer. And it is fed with 1/0 AWG wire (150A) for the primary and 250 kcmil (255a) for the secondary with a 150A feeder breaker, can I use the full 75 kva or is it also need a derate to 80% / 60kva?
 
I don't know of any requirement to "derate" a transformer. But in order to properly select a transformer size that will satisfy the connected load, you need to perform a load calculation. That process includes steps to add an extra 25% of the continuous load.

So if you add up 100% of the transformer's non-continuous load and 125% of the transformer's continuous load, and if the result is less than or equal to 75KVA, then you can use a 75KVA transformer to serve that load.
 
End result is the transformer rating is 80% if it's all continuous load.

So if the continuous load is 58 kva and no non-continuous load then basically this is fully loaded (72.5 kva). With the installed wire and breakers, I could then upgrade the transformer to 112.5 kva with 100kva availability.
Correct?

Thanks for the quick post

Bud
 
Last edited:
whisperinghill said:
End result is the transformer rating is 80% if it's all continuous load.
Click to expand...
No. The end result is that the transformer rating is 75KVA, and you can use every bit of that 75 KVA. The word "derating" should not be used in this context.

You begin by finding out what the load is. That process involves a 125% factor for continuous load. But when you are done with that process, what you have is "the load." You can use a transformer to serve 100% of that load.

As an example, if there is 60KVA of connected load, and if all of it is continuous, then the "calculated load" is 75KVA. That happens to be 100% of the rating of a 75KVA transformer, and you can indeed use a 75KVA transformer for that application.
 
As an example, if there is 60KVA of connected load, and if all of it is continuous, then the "calculated load" is 75KVA. That happens to be 100% of the rating of a 75KVA transformer, and you can indeed use a 75KVA transformer for that application.
Click to expand...

Is this spelled out in the code? (connecting art.220 calc's to xfmr size)

What about duty cycle? is that continuous load 3hrs, 8hrs, 24hrs/day?

Required transformer size is purely a design issue? they can be overloaded, some have fans even...
 
charlie b said:

No. The end result is that the transformer rating is 75KVA, and you can use every bit of that 75 KVA.
Click to expand...

Charlie, I stand corrected. I kinda figured that one out when I calculated replacing the 75kva transformer using the existing wires and came up with 100 kva availability with the supply wire but only 73 kva with the secondary wire using 80% of each size's capability.

Thanks for the better understanding.

Can you let me know If I got this right?
 
OOPS I messed that up.

Since the secondary wires are rated at 255a and that comes to 92 kva and derate that by 80% which allows us 73 kva and that is the transformer size.

So no matter if I need to upsize the transformer, I still need to upsize the wiring.
 
wasasparky said:
Is this spelled out in the code? (connecting art.220 calc's to xfmr size)What about duty cycle? is that continuous load 3hrs, 8hrs, 24hrs/day?
Click to expand...

wasasparky, The load is continuous 24/7 IT Loads. I think I finally got it straight in my head, see my last post.
 
whisperinghill said:
Charlie, I stand corrected. I kinda figured that one out when I calculated replacing the 75kva transformer using the existing wires and came up with 100 kva availability with the supply wire but only 73 kva with the secondary wire using 80% of each size's capability.

Thanks for the better understanding.

Can you let me know If I got this right?
Click to expand...
A transformer capacity is not required to be derated with a continuous load.
If you have a continuous load of 75 kva you can connect it to the transformer. The problem is what size wire and breaker is required to make this connection. You will need a conductor and breaker that will carry the 75 kva x 1.25. The ampacity is determined by the voltage you are using..
 
In many of the code sections that require sizing of '125% of the continuous load plus 100% of the non-continuous load', there is an exception permitting the use of 100% of the continuous load when the OCPD is suitably rated.

Presuming that I use 100% rated OCPD, could a 75 KVA transformer be loaded with a 75KVA _continuous_ load, and still be part of a code compliant installation? Given the example in the original post, can a 208A continuous load be placed on this transformer, presuming suitable OCPD and conductors?

(To be clear, I am asking, not telling, because I don't know the answer. Code book is not in the lab:)

Thanks
Jon
 
winnie said:
In many of the code sections that require sizing of '125% of the continuous load plus 100% of the non-continuous load', there is an exception permitting the use of 100% of the continuous load when the OCPD is suitably rated.

Presuming that I use 100% rated OCPD, could a 75 KVA transformer be loaded with a 75KVA _continuous_ load, and still be part of a code compliant installation? Given the example in the original post, can a 208A continuous load be placed on this transformer, presuming suitable OCPD and conductors?
Click to expand...

Yes, the adjusment is for conductors and overcurrent protective devices. A transformer does not need te adjustment any more than a motor does. There are many valid reasons to not use them at 100%, but these are design choices not NEC requirements.
 
wasasparky said:
If the load calculation ends up at 80kVA, can you use a 75kVA transformer?
Click to expand...

I cannot find an article that requires the transformer be sized for the load, am I blind? Is there not one?
 
wasasparky said:
If the load calculation ends up at 80kVA, can you use a 75kVA transformer?
Click to expand...
wasasparky said:
I cannot find an article that requires the transformer be sized for the load, am I blind? Is there not one?
Click to expand...
The best I can do is 240.3, but I don?t think it really does the trick.
 
There are actually several things in play here, that are related, but not necessarily the same.

1. The size of the transformer. This is calculated as others have mentioned. This tells you the minimum size the transformer itself has to be.

2. The size of the secondary OCPD. There is a maximum OCPD size based on the transformer size and the primary OCPD. In a few cases it is not even required. The secondary OCPD may or may not also protect the secondary conductors.

3. The size of the secondary conductors. The ampacity of the secondary conductors is calculated based on the load the conductors will see, not the size of the transformer.

4. The secondary conductor OCPD.

5. The primary conductor size.

6. The primary OCPD size.

Normally one selects a primary OCPD and conductors so the same OCPD can protect the transformer and the conductors at the same time. But this is not actually required.

Normally one would do the same thing on the secondary side, but again it is not required.

There is also no requirement at all that the OCPD or the conductors selected on either the primary or secondary side allow you to use the full capacity of the transformer.
 
whisperinghill said:
I know this type of question has been answered many times, but I still don't get it.

If I have a 75 KVA 3ph 480-120/208 transformer. And it is fed with 1/0 AWG wire (150A) for the primary and 250 kcmil (255a) for the secondary with a 150A feeder breaker, can I use the full 75 kva or is it also need a derate to 80% / 60kva?
Click to expand...

I think that the 80% came from other, somewhat related requirements but it is NOT applicable to the transformers.

- Lighting circuits in panelboards supposed to be loaded to no greater than 80% of their continuous rating.
- Large case circuit breakers shall be designed for 80% load, if you wish to use their full ampere rating you must specify 100% rated breakers. The former is the standard stock item, the latter is less common.

Having said the above transformers have various designs and ratings. As it was mentioned earlier the transformers life determined by the load(temperature) and time. Utility companies are routinely and knowingly overload their transformers. It's economics.

Dry type transformers of 30kVA and greater have a 15-30% emergency overload capability, that means about 8 hours of continuous. IEEE-141 also comments about on that depending on the temperature rating of the transformer, they have different life expectancy. Ex. 115C rise transformer will last 10 times longer than the 150C rise when subjected to the same load.
 
Status
Not open for further replies.
Top