Exceeding Transformer Capacity

[iwire]

While the NEC absolutely contains some design guidelines,

Guide lines?

You mean rules that tell where and how many?

It should come as no surprise, that there are many design questions that cannot be answered solely by use of the NEC.

I am not shocked, as a matter of fact that is my position.

At the same time, the AHJ does not have to agree to engineers view of a proper design as there is no specific criteria spelled out for it.

 

[iwire]

I would point out the the inspector is not the AHJ and has no authority whatsoever on his own to make up requirements not found in whatever code requirements are adopted.

Absolutely true

And I am not suggesting making up any rules.

I think Smart and I agree that 110.2 and 110.(3) require that we don't overload the transformer. The NEC does not spell out how the load is determined that is going to be up to the AHJ. They could ask for the calculated load, or ask for the connected load, or just ask for an explanation of how the transformer size was determined or perhaps something different.

i would also point out the 90.4 explicitly says the AHJ has this power, not a mere inspector. I am not aware of any AHJ that has delegated any code making powers to any inspector.

Again true but no rules are being made.

 

[Smart $]

IIRC, the 25% thing only applies to ampacity of conductors and not directly to sizing OCPD.

The 25% "thing" doesn't apply to conductor ampacity... it applies to minimum conductor size... and it does apply directly to OCPD sizing where not 100%-rated equipment.

 

[Smart $]

...
I think Smart and I agree that 110.2 and 110.(3) require that we don't overload the transformer. The NEC does not spell out how the load is determined that is going to be up to the AHJ. They could ask for the calculated load, or ask for the connected load, or just ask for an explanation of how the transformer size was determined or perhaps something different.
...

I can agree with that... :angel:

 

[Smart $]

I should have checked back on this thread quicker after posting it.

Some background on this discussion; We are doing a very small renovation in a commercial office space with a existing 30 kVA transformer in an existing electrical room. When the existing load is added to the additional loads, the new total load is approximately 38 kVA.

When I opened section 450, I kind of expected to see something similar to what is says for generators, (ie. The transformer shall be sized sufficiently to handle the loads calculated in accordance with 220).

This appears to be more of a discretionary issue with room for interpretation. However, if we were talking about sizing a panel, feeder, or MOCP, there would be much less grey area here.

This is a great example. Under Code maximum OCPD rating for the transformer protection is 30kVA ? 125% ? V, or 37.5kVA ? V, and permitted to be rounded up to the next higher standard rating.

Now if the 38kVA load is all noncontinuous, the minimum OCPD rating is required to be 38kVA ? V, and permitted to be rounded up to the next higher standard rating.

As you can see, everything could be compliant depending on interpretation. Would it be deemed a borderline or unsafe installation???

 

[Npstewart]

This Now if the 38kVA load is all noncontinuous

The 38 kVA is the total demand load meaning this is after all demand factors have been applied. There isnt any continuous loads cause all the lighting is 277, before the xfrmr.

 

[Smart $]

The 38 kVA is the total demand load meaning this is after all demand factors have been applied. There isnt any continuous loads cause all the lighting is 277, before the xfrmr.

So with the lighting at 277V before the transformer, I'll assume the secondary is 208/120 and get some real numbers...

Maximum Transformer OCPD:
30kVA ? "360" ? 125% = 104A
Rounded up to next standard size: 110A

Minimum Load OCPD:
38kVA ? "360" = 106A
Rounded up to next standard size: 110A

So is the transformer compliant or not?

 

[Besoeker]

From experience:

On two occasions following Tx failures a single transformer has taken the full load of a board normally fed by two units with the bus-coupler open.
One occasion the remaining unit being loaded to 130% FLC. The other occasion the remaining unit took 125% FLC.

We fastened 15? fans and baffles to the cooling fins so instead of being ONAN they became ONAF. Like all of our transformers they had oil over temperature trips.
Both ran like that for several months while repairs were carried out on the failed units. That was over a decade ago, as far as I know the two abused units are still in service.

As an aside both the transformers we overloaded were made in the early 60?s, I wonder if a modern unit would take the abuse?:?

Hi compatriot! Absconding from that other place?
How's the weather bomb affecting you?
Yes, I've seen the ONAF fudge in a few places.
Given the life expectancy of transformers, I think replacing the transformer with a correctly rated ONAN unit and amortising over a 20 or 30 year period (or longer) would be a more prudent option.

My main dealings with transformers is unit transformers for variable speed drives and high current rectifiers. My usual design practice is to allow a 20% margin.

 

[steve66]

So with the lighting at 277V before the transformer, I'll assume the secondary is 208/120 and get some real numbers...

Maximum Transformer OCPD:
30kVA ? "360" ? 125% = 104A
Rounded up to next standard size: 110A

Minimum Load OCPD:
38kVA ? "360" = 106A
Rounded up to next standard size: 110A

So is the transformer compliant or not?

O

Edit: Opps, just realized the load is non-continuous, not continuous.

I would say it doesn't comply.

 

[Smart $]

So the real load is 38 * .8 = 30.4 KVA. Only 400VA above the transformers rating...

I'm going by Npstewart implying the load is noncontinuous. Your 38 * .8 would be indicative of 30.4kVA continuous load.

Being a noncontinuous load, there is no way to determine what loads will be conducting and how much at any point in time. What really determines whether the transformer is overloaded is the actual demand (as seen by POCO, not Code calculated demand).

 

[Smart $]

...

I would say it doesn't comply.

And what is the basis for that conclusion?

 

[steve66]

And what is the basis for that conclusion?

Not complying with the listing and labeling. It is labeled a 30KVA transformer.

If you can find some other load calc. that justifies the 30KVA transformer will cover the load, I'm all ears.

 

[templdl]

The point of the second post of mine was to point out that it is possible to exceed to NP rating of the transformer without damaging it. Other than a violation of art 110 the more important issue would be exceeding the rating of the secondary conductors which size is normally based upon the NP rating of the transformer. Would you be intentionally overloading these conductors?
Even if the transformer rating can be exceeded without exceeding the 220degC insulation class of a drytype transformer there still is a concern for exceeding the capacity of the conductor and connections within the transformer.

 

[Smart $]

Not complying with the listing and labeling. It is labeled a 30KVA transformer.

Compliance is based on meeting a requirement. Where is the requirement that the transformer must be listed and labeled at not less than the calculated load?

 

[steve66]

Compliance is based on meeting a requirement. Where is the requirement that the transformer must be listed and labeled at not less than the calculated load?

110.3(A)(7)

 

[david luchini]

110.3(A)(7)

110.3(A)(7) isn't part of the instructions included in the listing or labeling.

 

[steve66]

110.3(A)(7) isn't part of the instructions included in the listing or labeling.

38KVA load on a 30KVA transformer.

The question was:

So is the transformer compliant or not?

The transformer is labeled 30KVA. 110.3a7 says that in judging equipment, current capacity shall be evaluated. With no other information provided, its obvious to me an inspector or AHJ would have the right to reject this installation.

But again, if the designer could provide calculations showing that the transformer is capable of handling the load, I would be all ears.

 

[Smart $]

110.3(A)(7)

That can go either way depending on who is judging... covered earlier by iwire.

 

[Smart $]

38KVA load on a 30KVA transformer.

The transformer is labeled 30KVA. 110.3a7 says that in judging equipment, current capacity shall be evaluated. With no other information provided, its obvious to me an inspector or AHJ would have the right to reject this installation.

But again, if the designer could provide calculations showing that the transformer is capable of handling the load, I would be all ears.

Seems clear cut when stated like that...

However, when we make the distinction the 38kVA calculated load is noncontinuous and the transformer 30kVA rating is continuous, the judgement is not so clear cut. If we take into consideration transformer protection rating is permitted up to 125%* of rated current seems to indicate a noncontinuous load is permitted up to 125% of the transformer rating.

*Where 125% of rated current does not correspond with a standard OCPD rating, we are permitted to round up to the next standard rating. We could feasibly have a protection rating in excess of 150% of rated current.

 

[david luchini]

38KVA load on a 30KVA transformer.

The question was:

The transformer is labeled 30KVA. 110.3a7 says that in judging equipment, current capacity shall be evaluated. With no other information provided, its obvious to me an inspector or AHJ would have the right to reject this installation.

What's the "current capacity" of a 30kVA transformer? If its tested at 200% of rated current per UL 1561, doesn't it have the "capacity" to carry twice its rated current?