Amperage from a transformer

[1st Class Husker]

Had a brain fart the other day and couldn't remember when finding total capacity on a transformer u divide kva by phase to phase or phase to ground example 50 kva,,, is 50000/ 240 is 208 amps!

[wtdoor67]

the Kurtz book for you.

[T-Man]

1000 kva divided by 240v secondary equals 4.2 amps which is the only number you need for single phase transformers.

Take any single phase KVA transformer and multiply the KVA times 4.2

25KVA X 4.2 - 104 amps
50KVA X 4.2 = 208 amps
37 1/2KVAX 4.2 = 157.5 amps
and so on.

These are all day long full load amps

[spark and bark]

Well, its kva times a thousand divided by induced coil voltage, so if its a 7.2/12.47 kv WYE system, its only 7.2 if you are hooking it up wye, and if on a 14.4 delta system and hook it up delta its 14.4. For secondary its kva times the full secondary allowable voltage, so 120/240, use 240.

[lewy]

50 kva is 50000 volt amps which at 240 volts gives you 200 amps, but we all know that you would use a 50 kva to feed more than 1 200 amp service.

[T-Man]

Let's keep this single phase primary. . .If you use 4.2 times the KVA of the transformer It will give you the full load current of the secondary.
25KVA X 4.2= 104 amps

Now if you want to know how much current is on the primary of that transformer, then you can figure out the ratio of the tub by dividing the single phase primary voltage (let's say 4800V) by the the secondary voltage 120/240) use the 240. 4800/240=20 so the ratio is 20:1

Now you already know that the tub if it's a 25KVA it makes 104 amps per leg you can divide the 20 into the 104 and viola the primary amps when the tub is fully loaded is 5.2 amps.

We would use a 15K link in the fuse door for the above transfromer because you can load the tub higher than the full load rating for shorter periods of time, like cold load pick up and temporary bridging the main when you loose a tub downline.

If you are talking 3 phase then you need to multiply the secondary voltage by 1.732 then divide it into the primary and that's a whole other ball game.

[wtdoor67]

would only work with 120/240 volt house transformers. There's other single phase around.

For the rated full load primary current divide the KVA by the number of primary turns for the primary current.

For the rated full load secondary divide the KVA by the number of secondary turns for the secondary current.

The Kurtz book makes it clear.

[lewy]

Maybe I am missing something here & it would not be the first time, but when finding out the amps for a particular kva I always used watts law P = VxA. 240v x 200a = 48000va,(50 kva) 240v x 100a 24000 va, (25 kva). I realize that this is keeping things real simple not worrying about power factors or the fact that we & I am guessing most run there transformers over rated capacity. It is normal for us to have 12 to 14 100 amp customers on 1 50 kva. As far as fusing for us it depends on the primary voltage feeding the transformer, for a 50 kva fed on our 8 kv we would use a 25k on our 27.6 kv we would use a 6k

[wtdoor67]

probably getting off topic here but the KVA divided by the number of turns either primary turns or secondary will give you an accurate figure of the 100% current of the primary or secondary.

If you look in a GE transformer book, or any other for that matter you will find their charts agree with your division.

Fusing of transformers is developed by each co. according to their gurus and all will differ a little. Setting of relays in the subs even play a part but that's out of my knowledge.

Anyway it's sometimes interesting. Actually I've seen people who really don't understand how to determine if a pot's actually overloaded. Those old cast iron pots (very old) would take extreme overload for a long time.

Anyway a review of the Kurtz book will help if you have any doubts.

[T-Man]

50 kva is 50000 volt amps which at 240 volts gives you 200 amps, but we all know that you would use a 50 kva to feed more than 1 200 amp service.

The way the engineers figure it is nobody uses their full service allocation all at one time, so you can have 10 or 15 customers on a 50KVA tr and only early in the morning and at dinner do the loads spike, the rest of the time the load is spuratic and never at it's peak for long periods of time.