Transformer Secondary Side - Bus sizing & Breaker sizing (NEC)

[cherryg]

I believe that NEC Article 450.3 Overcurrent Protection (and Table 450.3) directly determines the secondary side breaker sizing (and bus sizing).
For example, if I have a 2500kVA, 13.8-0.48kV Transformer, the rated secondary side current is 3007A. As per Table 450.3, for 480V secondary (<1000V), the breaker maximum rating or setting shall be 125% of Transformer rated current i.e 3007 x 1.25 = 3759A. The closest standard rating is 4000A. Thus breaker and bus rating on secondary side MCC shall be 4000A each.
So far so good.
Now let us say that this Transformer is rated at 30degC and undergoes a derating for 50degC ambient.
As per IEEE C57.91 Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators’, Section 6, the derating shall be 1.5% for each degree rise above 30degC (ONAN only).
Thus for 50degC ambient i.e 20degC rise, the derating shall be 20 x 1.5 = 30%.
Thus the derated kVA of the Transformer shall be 2500 x 0.7 = 1750kVA. The corresponding current is 2105A.
Going back to NEC Article 450.3 requirement, max. secondary side breaker rating or setting shall be 2105 x 1.25 = 2631A.
Closest higher standard rating or setting is 3000A. Thus both CB and bus bar can be sized at 3000A.

Is the above correct? By not following the nameplate Transfrmer rating but following the derated rating, am I flouting NEC requirement?

 

[xnuke]

You wrote it yourself - the NEC calculations provide maximum ratings. Why are you concerned about going with lower ratings?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dpc]

As noted, you can size the breakers for less than the maximum allowable rating, as needed for any derating.

I just wanted to note that you need to be careful about assuming you can always go to the "next larger size" breaker. In the NEC, that rule only applies up through 800 A, unless something has changed recently. You can get rating plugs for some breakers that would meet the requirement, even if a larger breaker is used. So a 4000 A breaker with a 3000 A rating plug is essentially a 3000 A breaker for NEC purposes.

 

[cherryg]

Thanks xnuke & dpc. The main reason for this query is: At a later point, say 10 years down the line, when we add loads, an adequacy check is always done. This check is only on the Transformer capacity and not on the downstream bus / breaker. If we choose lower than the 125% rated value of the derated capacity of the Transoformer, the hidden bottleneck is the bus / breaker, not the Transformer. Many engineers are not aware that in-panel rating of breakers is 80% of their nampelate rating (unless the breaker is rated and duly marked for 100% rating). Hence the concern.

 

[waross]

Anecdote alert:

At a later point, say 10 years down the line, when we add loads, an adequacy check is always done. This check is only on the Transformer capacity and not on the downstream bus / breaker.

I was involved with such a check once.
The customer hired an engineering firm to conduct the check.
As the contractor who did troubleshooting and repairs for the plant we were contracted to install recording equipment on the mains in order to accumulate data to show the inspection department that the service had adequate capacity to support the intended upgrade.
Data was accumulated for several months.
Some time later, when the customer knew us better, they asked us to correct their power factor.
Made a phone call to the customer's accounts payable department.
"Is it possible to get copies of the power bills for the last two years?"
The next day I picked up two years of past power bills, and in an hour or so knew how many KVARHr per month we needed to install.
And incidentally, also had the maximum monthly demand for the last two years, at revenue metering accuracy.
This may not apply to your installation, cherryg, but I mention it for the possible benefit of others following this thread.


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Ohm's law
Not just a good idea;
It's the LAW!