Load calculations for panel fed from a transformer

[jglavin427]

When performing a load calculation for a panel, I always limit the total demand load to less than the transformer rating. I'm reviewing a set of drawings where they are proving the panel based on the demand being less than the main breaker (but more than the transformer rating).

The job has a number of transformer/panel calculations, but just an example of one where I saw this: Load summary shows existing load at 138.5kVA, New load of 16.7kVA, for a total of 155.2kVA on a 150kVA transformer. They are asserting that with this 431A of load, the 500A MCB panel still has 69A of spare capacity.

What are your thoughts?

 

[charlie b]

We are allowed to protect the transformer at 125% of its rating. For a 150 KVA transformer with a secondary voltage of 208 volts, that equates to 520 amps. So I don't see a code violation in that installation.

 

[Dennis Alwon]

Charlie I am having trouble understanding how a 150kva trany can have 155kva load and be compliant. I understand the ocpd can be 125% but are saying the load can be more than 150kva

 

[jglavin427]

I agree the installation is code compliant but for the purposes of adding load to an existing installation, I am having trouble justifying to myself how we can propose more than the transformer's rating.

Edit: I tend to think code-wise, its a listing issue, as transformer manufacturers publish listed FLA for all their transformers corresponding to the kVA rating. If the engineer can't prove they are under that listed amperage, their proposed installation doesn't meet 110.4(B), no?

 

[david luchini]

Edit: I tend to think code-wise, its a listing issue, as transformer manufacturers publish listed FLA for all their transformers corresponding to the kVA rating. If the engineer can't prove they are under that listed amperage, their proposed installation doesn't meet 110.4(B), no?

I don't think there are any instructions included with a transformer's listing that says it cannot be loaded above its rated KVA. I don't think 110.3(B) would be an issue.

I agree with Charlie, I don't see any Code violation.

 

[charlie b]

Dennis, I don't have a strong feeling about this one. But I can point to an article that says not to load a panel beyond its rating, and to an article that says not to load a wire beyond its ampacity. I just don't seen an article that says the same thing about transformers.

The way I would do a design would include (not necessarily in this order) selecting the 150 KVA transformer, calculating its rated current at 125%, selecting a 500 amp breaker for the downstream panel, and using the panel schedule (Excel file) to keep track of the load on that panel. As I add various types of load to the schedule, the Excel file knows how to calculate certain items at 125%, and to take available demand factors on other loads. At the end of the day, if the total demand load is under 500 amps, I would call it code-compliant. That said, I would probably consider some changes if the total demand load were that high. I like to leave at least 20% spare capacity for future load growth. What I am seeing in this discussion is that the future has arrived, and that the load growth is happening now.

 

[topgone]

Dennis, I don't have a strong feeling about this one. But I can point to an article that says not to load a panel beyond its rating, and to an article that says not to load a wire beyond its ampacity. I just don't seen an article that says the same thing about transformers.

The way I would do a design would include (not necessarily in this order) selecting the 150 KVA transformer, calculating its rated current at 125%, selecting a 500 amp breaker for the downstream panel, and using the panel schedule (Excel file) to keep track of the load on that panel. As I add various types of load to the schedule, the Excel file knows how to calculate certain items at 125%, and to take available demand factors on other loads. At the end of the day, if the total demand load is under 500 amps, I would call it code-compliant. That said, I would probably consider some changes if the total demand load were that high. I like to leave at least 20% spare capacity for future load growth. What I am seeing in this discussion is that the future has arrived, and that the load growth is happening now.

I agree. For as long as the transformer temp doesn't rise abruptly, any loading beyond rating is permitted --> for period of time. The penalty will be shortening the service life of the transformer. Utilities overload their distribution transformers. They compare foregone revenue if they can't serve a feeder vs. the replacement cost of transformers by overloading them to destruction.

Also, if it's an oil-immersed transformer, you can load your transformer beyond it's nameplate rating provided you monitor the temperature of the transformer oil and you implement cooling interventions to prevent runaway temperatures. For guidance, please get a copy of IEEE Std C57-91-2011 or even earlier publications.

 

[kwired]

What I am seeing in this discussion is that the future has arrived, and that the load growth is happening now.

Now you have a new future though - so take that into consideration when making design decisions.

Isn't necessarily anything wrong with adding an additional transformer for new loads vs. increasing the size of an existing one either.

As mentioned you may get away with overloading an oil cooled transformer easier then air cooled one as well.

 

[jglavin427]

Thanks for the replies. From the perspective of an owner's rep evaluating a consultant's design, I would prefer not to consider capacity above the transformer's nameplate rating as spare capacity, just for the sake of justifying new equipment to management in the future. Beyond that, it sounds like we're good to go with the load calcs as is. :happyyes:

 

[charlie b]

Allow me to put it this way: I don't see a code violation, but I do see a poor design. I would not want to hand this over to the owner, without at least documenting my concerns in a memorandum or letter to the owner.

 

[Smart $]

I agree with the compliant assessments by others. I've pointed out this issue in some other threads. What I see not explicitly stated in this discussion is consideration of continuous and noncontinuous load. First, one must understand that a transformer's kVA rating is based on a continuous load under nominal operating conditions (ambient temperature and adequate ventilation mostly)..

Code doesn't explicitly state it, but one can infer from the maximum 125% OCPD requirement that we can compliantly load a transformer to 125% of its rating with noncontinuous load... or a combination of noncontinuous load and continuous load factor at 125%. The OP did not mention how much load, if any, was continuous. I realize many include 125% continuous load factoring in what they say is the calculated load, but some do not, and technically Article 220 calculated load does not include 125% for continuous loads.

 

[jglavin427]

I agree with the compliant assessments by others. I've pointed out this issue in some other threads. What I see not explicitly stated in this discussion is consideration of continuous and noncontinuous load. First, one must understand that a transformer's kVA rating is based on a continuous load under nominal operating conditions (ambient temperature and adequate ventilation mostly)..

Interesting take. So, essentially a standard NEC load calculation will prove load on an OCPD but is too conservative for a transformer. I suppose I can get on board with that...

Code doesn't explicitly state it, but one can infer from the maximum 125% OCPD requirement that we can compliantly load a transformer to 125% of its rating with noncontinuous load... or a combination of noncontinuous load and continuous load factor at 125%. The OP did not mention how much load, if any, was continuous. I realize many include 125% continuous load factoring in what they say is the calculated load, but some do not, and technically Article 220 calculated load does not include 125% for continuous loads.

As an engineer this is the kind of thing I'm looking for - a way to arrive at a definitive (and reasonably accurate) "yes" or "no" as to whether a particular piece of equipment is overloaded.

So, when you say "combination" - if I were going to create a calc to show transformer loading, it sounds like I'd want to take the total noncontinuous load multiplied by 80% and add to the total continuous load. From that I could derive spare capacity or loading percentage based on the assumption that typically a transformer is good for up to 125% noncontinuous load, or 100% continuous.

What I'm getting at is some kind of reasonable quantification I can use to justify buying new equipment to management folks

I wonder if I could use the same concept on a utility transformer, by changing allowable load factors to, say, 125% continuous and 200% noncontinuous? Explaining to a customer why they had to pay for that fancy 400A service when the utility installed a 75kVA transformer is always difficult.

 

[Smart $]

Interesting take. So, essentially a standard NEC load calculation will prove load on an OCPD but is too conservative for a transformer. I suppose I can get on board with that...

Well not exactly. It's a matter of how the equipment is rated. Transformers are rated for continuous loading. OCPD devices are rated, per se, for noncontinuous loading. Ordinary inverse-time breakers are only listed to support continuous loading to 80% of their rating. The NEC compliments the listing by requiring the continuous load be factored 125% and compared to the rating... the inverse of comparing 80% of rating to continuous load.

As an engineer this is the kind of thing I'm looking for - a way to arrive at a definitive (and reasonably accurate) "yes" or "no" as to whether a particular piece of equipment is overloaded.

Following NEC requirements will not permit any installation's noncontinuous plus 125% continuous load to exceed 125% of transformer rating per line... because of the transformer protection maximum 125% OCPD rating. Where there may be doubt is where the OCPD is upsized under Note 1 of Table 450.3(B)

So, when you say "combination" - if I were going to create a calc to show transformer loading, it sounds like I'd want to take the total noncontinuous load multiplied by 80% and add to the total continuous load. From that I could derive spare capacity or loading percentage based on the assumption that typically a transformer is good for up to 125% noncontinuous load, or 100% continuous.

What I'm getting at is some kind of reasonable quantification I can use to justify buying new equipment to management folks

It's a perhaps oversimplification, but yes... and only in the sense of NEC compliance. As Charlie mentioned, compliance is no assurance of good design.

I wonder if I could use the same concept on a utility transformer, by changing allowable load factors to, say, 125% continuous and 200% noncontinuous? Explaining to a customer why they had to pay for that fancy 400A service when the utility installed a 75kVA transformer is always difficult.

Also add in that utility transformers are typically located where they have little to no ventilation restrictions, away from combustible materials... but even more so, utilities are not required to calculate or determine the effective load per the NEC.