I am working on a lengthy project and have one little area that I am not sure how to answer. I need to know how to size a transformer for a building. Does anybody know the equation or method for doing this? The service coming in is 480Y/277V and the step-down is to a 240/120 single phase panel. Any help is great! Thanks
There is no actual formula that will size the x-former for a building but I can give you some help::: Add up all the loads lets say you have(main IDEA of the question) 20000 watts at 120 v 10000 watts at 240v for a total of 30000 watts calculating the secondary amps you use ohms law for single phase PI x E solving for I 20000/120167 amps solving for I 10000/240 41 amps You have to go to the NEC code book to look up the wire size but its about 4/0 (you MUST have a local expert do this for you never trust anyone on the net) Now you know what size wire feeds your panel from the x-former lets work this backwards The 480/277 Y is a 3 phase system Im sure you will select the higher votage of 480 to supply the transformer this lowers the wire size proportionally. The formula for OHM's 3 phase is P I x E x Sqrt of three or 1.732 the output kva is 30kva so you need 30 kva going in minimum solving 30000 I x 480 x1.732 I 30000/831.3636 amps now lets go bac to the code book to size the wire 10 is 30 amps #8 is 40+ so lets feed the primary side with #8. Sizing the overcurrent device is done the same way Since we did all that fluff.to answer your question Let me say this, Always oversize the x-former 10-20% . and always check your addition for the totals on the loads.There are a bunch of appliances that can be devaluated and in winter your /ac doesn't run and all that. so before you jump right into this try to get a real good handle on what's going on.Its not complicated at all you just need to be accurateWell Have a good day.From the E
Are you talking about sizing the 'dry-pack' step down from the 480V 3 phase to the 240 volt 1 phase? Or the overall service transformer? The overall transformer must carry all the loads of the building. The 'fudge factor' given by a previous answerer doesn't take into account enough details. If the transformer is located in a vault inside the building, it must be de-rated because it will not get as much cooling from airflow as it would outside. Do heating loads or cooling loads dominate? Cooling loads call for further de-rating as the load will be at maximum when the temperature is at maximum. Heating loads are better as they peak when temperatures are lowest, helping cool the transformer windings. Check with the local utility company. Load growth? You want to have capacity for the loads to grow without having to upgrade the transformer. If you're sizing the step down, you need to know the loads it will be serving plus the efficiency of the dry pack transformer itself (typically fairly low) as that will be part of the load placed on the main transformer. kVA is an acronym for kilovolt-amperes, roughly analogous to kW - kilowatts. If you have 10,000 watts of equipment connected, that's 10 kW or approximately 10 kVA, but you would still need to make all the necessary adjustments above to properly size the transformer. Check with an electrician or the buildings electrical engineer, failure is definately an option, but won't make anyone happy. It's as much art as science and varies greatly on information not given here.
buildings come in all sizes. Some use a lot of power, perhaps because of lots of machinery. Others use a much smaller amount. So you need to calculate or estimate the amount of power the building will be using. If you know the square foot total and use of the building (industrial, office, etc) there are probably tables that will give you a guess. One 240/120 panel for the entire building? That implies a very small building, as must buildings distribute 3 phase 208 volts, and have numerous subpanels. .
