Matching Transformers and 208 V Panels

Suggestion/Comments: Please, provide additional info:
1. Do you intend to have one best matched panel per the transformer or more of them?
2. Sometimes, nature of the power distribution system and loads determine the size(s) of panels and number(s) of branch circuits including spaces for circuit breakers/fuses
3. Spare capacity or reserved design margins
4. One could possibly aim to have the panel main circuit breaker protecting the transformer secondary and feeder to avoid extra protection devices
5. In some cases, the short circuit current level must be considered to have the panel buses rated for higher short circuit current, including motor short circuit current contributions from the motor load.
6. Cost factors
7. Etc.

Suggestion/Comments: Please, provide additional info:
1. Do you intend to have one best matched panel per the transformer or more of them?
2. Sometimes, nature of the power distribution system and loads determine the size(s) of panels and number(s) of branch circuits including spaces for circuit breakers/fuses
3. Spare capacity or reserved design margins
4. One could possibly aim to have the panel main circuit breaker protecting the transformer secondary and feeder to avoid extra protection devices
5. In some cases, the short circuit current level must be considered to have the panel buses rated for higher short circuit current, including motor short circuit current contributions from the motor load.
6. Cost factors
7. Etc.

1) Yes - best matched panel per transformer.
2) Don't worry about size of panel. Assume that you can load it to maximum 80% of panel main breaker. Unless of course panel main breaker is 100% rated (which I spec on panels 800 amp an above).
3) In my opinion it is bad design not to assume panel will be fully loaded to max rating in the future - reguardless of current load. I have seen too many problems where they only look at panel size before adding load and not transformer or feeder. Therefore feeder and transformer to support full load capability of panel.
4) Yes, panel main breaker serves dual purpose - protecting panel, feeder, and transfomer secondary. I spec all panels with mains even if transformer not involved and next to switchboard.
5) Panel will be fully rated, and short circuit rating of breakers determined by study.
6) No cost issues.
7) Just wanted to see what others thought.

Thanks

Thanks for the navigation.
Panel 100A , Tranformer 45 kVA
<<125A rated panel with 125A rated protective devices is the closer match since the transformer maybe somewhat overloaded within the 125A rated protective device tripping range and within the Utility 480V±5% voltage tolerance.>>

Panel 225A, Transformer 112 kVA
<<The transformer secondary current is 302A, which would allow higher sized panel than 225A, possibly 400A panel with 300A protective devices. The cost and space are not an object.>>

Panel 400A, Transformer 150 kVA
<<The transformer secondary current is 416A, which leads to the 400A sized panel.>>

Panel 600A, Transformer 225 kVA
<<The transformer secondary current is 624A, which leads to the 600A sized panel.>>

Panel 800 A, Transformer 300 kVA
<<The transformer secondary current is 833A, which leads to the 800A sized panel.>>

Panel 1200A, Transformer 500 kVA
<<The transformer secondary current is 1388A, which could lead to a custom made panel higher than 1200A (industry standard) or 600A pannel and 800A panel which appears to be a better solution considering other factors such as the motor load short circuit contributions.>>

The NEC will allow the transformer secondary protective devies to the &quot; next higher standard device&quot;. Using that criteria the sizes for panels can be increased even more.
For example the full load current of a 112.5 kva transformer is 313 amps and 125% of that is 391 amps. The &quot;next higher device&quot; is a 400 amp breaker. That allows you to use a 400 amp panel and load it to 320amps with an 80 % breaker or almost 400 with a 100% breaker.
The same reasoning will allow you to use a 600 amp panel with a 150 kva and an 800 amp panel with a 225 kva transformer.
Secondary breaker space is generally more of a problem for the work I have done. Generally I would max the panel size and use a panel with a 100 % breaker and feed through lugs. A MLO panel next to it will give you 84 spaces.

Suggestion: Reference:
1. NFPA 70-1999 National Electrical Code (NEC)
Reference 1 Article 90-1(b) Adequacy. &quot;This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.&quot;
This means this Code does not cover aspects such as aging due to higher currents that were calculated since the circuit breaker would not trip at 80% of its rating thus causing the transformer to be supplying higher current than its rated 100% current. This may lead to premature aging of the insulation and equipment, which not covered by the NEC. However, the installation is &quot;essentially free from hazard.&quot; Some Clients do not prefer such on &quot;the dot design,&quot; which may age prematurely (pay now or pay later).

Most of the client I work for like &quot; the dot design&quot; when It comes to the bottom line ( $$$ that is). Equipment can be used as it is rated and that is the most economical use of it. It's part of good engineering ( remember a good engineer can do for a dollar what any one else can do for two.
Most all transformers can be operated at 125 % of rated nameplate with no damage ( check the ANSI stands and it will show some shortning of life.) If you use an 80 degree C transformer and put it in an envrioment of less that 104 degrees farenhite your within the code and common sense.
If you know that a particular transformer is going to be in a high ambient temperature setting and opperating at 100% load cycle then of course it should be sized accordingly.
If breakers have a useful life do you have a reference for it ? It's good practice to test them perodically but I am not aware they should be replaced based on time.

Comment to the previous BJC May 18, 2001.
1. Certainly, there will always be some Clients who will like &quot;the dot design&quot; just like there are some 10Ton truck drivers who drive over the 10Ton rated bridges, but not all, obviously for the bottom line ($$$ that is).
2. Often &quot;Clients&quot; who deal with someone else money do not care much, just to get their share and bottom line, which is the name of the game.
3. In some cases, &quot;the equipment can be used as it is rated and that is the most economical use of it&quot; holds true; however, there is posted the 80% circuit breaker derating above. It appears that there is whole wide area that has not been addressed yet, namely a lifecycle business. The dot design has been often challenged by the lifecycle bottom line. Supposing that the transformer is designed and maxed, and another transformer is not maxed and has the longer lifecycle, then one has to perform an analysis, which choice is better and leading to the long-term preferred bottom line (some Clients may be out of business).
4. One has to make distinctions between conductor temperature that is linked to the ampacity rating and RI**2, and ambient temperature that is linked to the derating, aging etc.
5. Breakers are protective devices that may or may not stay very long, since they are not meeting UL standards after clearing three fault (UL does not test them for more than three short circuit faults.) Circuit breakers tend to corrode and crack in time.