Transformer Sizing Question

Hello Jcall,

Your load is only 3kva and hence 6 kva size is OK.

Additional ventilation using fans would help.

I will not suggest reverse wiring, hence go for a straight 208 pri and 480 sec 6 kva transformer.

If you see carefully, your designed secondary winding rating was 208 volts and you have used it at 480 volts.

This is a classic trap which electricians fell into in real world.

Cheers.

Would you please instruct me as to the correct proceedure to use for sizing transformers (please excuse my ignorance). Would the amp draw from each of three phases be added together to get the total load? Or do you only consider the one highest (one) phase amp draw as the load? I was thinking that (in this case) if the amp use was 6.1 per phase, that would total 18.3 amps at 480 volts.

Please advise.

Thank you !!!!!

jcall

The currents do not sum.. 6.1A per phase is correct

Tell me... what grounding provisions have been made on the 480 volt secondary?

A 208/480 transformer is normally a delta primary with a wye secondary. This would allow the star point of the 480 volt winding to be grounded. If your transformer is in fact wye/delta (which is what you should have if you are using a 480/208Y120v unit in reverse), you would have to corner ground one phase of the 480 volt three phase system to get this separately derived system grounded.

I would never do it this way nor approve one if I ever came across such a setup during an inspection.

I'm a little concerned because this needs to be done properly on anything associated with a swimming pool!! Call a licensed electrician, then get it inspected by the AHJ in your region. And for what it is worth... do it quickly.

This parragraph should read...

"I would never do it this way nor approve one if I ever came across such a setup during an inspection of any equipment associated with a swimming pool".

hello jcall

You may have a bigger problem that , on the surface , looks relatively easy to resolve-

One easy question----Was the 6.1 amp draw you stated derived from actual readings taken with your current setup (using a "step up" transformer in reverse) or are these numbers stated on the load (sanitizer)?

1--If the sanitizer states this load (current) at 480 v then do as has been previously suggested and purchase the proper "step up" transformer with at least a 25* cushon above the required secondary (load) rating which is your sanitizer.

2--If the readings (6.1A)/phase were derived using your present transformer in reverse, the readings are not correct and therefore you cannot "calculate" what size transformer you need--You need to find out what the sanitizer(s) actually draw under normal (proper) operation. Then purchase the appropiate transformer.

As was also previously stated, you very well could be using a transformer originally designed for D/Y operation, which provides your system (load) with a center tap ground for safety --reversing this system (D/Y) negates this feature, a very important feature.

As a matter of information, there's a 99.9* of burning out a "step up" or "step down" transformer if you attempt to use them in reverse --so be cautious when your told this!---I put money on it--if your read the windings of your transformer with an "VOM" prior to discarding it, you'll find one or more of the "line" side (208V) , which was originall the "secondary" side, burned open or shorted.

The reason your transformer was running "hot" is because they were running a "step down" transformer as a "step up"--It just wasn't designed for that.

Good luck--Donzi

The actual amp draw while in use of the UV sanitizer was 6.1, 5.7 and 5.8 amps on the different phases. I noticed no suspesious hum from the transformer and the unit had been working for almost two years (al be it did run too hot in my opinion 232 degrees F. This UV unit is an import unit (ETS-Engineered Treatment System, ATG Willand brand) and there is no wattage or amp ratings on the data plate (or anywhere else i looked). This thing is wall mounted and weighs about 100 pounds.

Any more info?

Thank for ALL the responses !!!!!!

jcall

North of 60 is right in his safety evaluation. Grounding issues are very important in this application. Make sure you use a licenced AND competent electrician. They are not always both.

As far as using a transformer in step-up / down direction, it doesn't matter as long as the windings are exposed to the voltages they are designed for. The transformer has no idea what is the primary or secondary. There are a few exceptions for exotic designs but normally the biggest constraints is the delta/wye direction that is optimized to reduce the third harmonics group and facilitate the secondary grounding.

Now for the cause of the original failure. UV systems generate a lot of harmonics just like most variable frequency drives. The harmonic currents can easily reach 30%-40% THD at full load. This produces an overheating in the transformer. The 20%oversizing should have been OK even if a little tight. This leads to mis-wiring of the equipment or a resonance with a capacitor bank nearby. This can be evaluated by the electrician.

Finally, a broken diode in the UV system would impose a DC current on the transformer and lead to failure within days or weeks depending on the conditions. You can usually identify a problem with DC current in a transformer by the very loud humming coming out of its core.

Good luck and be careful.

Hi Jcall,

Here are some things for you to think about.

3 PH. KVA is calculated KVA=(Volts X Amps x 1.732)/ 1000 = 5.07KVA; Chatterpillar is incorrect he used the 1PH formula not the 3ph formula to calculate the KVA. It is recommended 60-70% loading not 90% for transformer, I would recommend a 9Kva transformer. If you run it at 85%, it will run warmer than normal.

Reverse fed transformers - generally 6-15kva 3ph transformers are encapsulted and can be reverse fed but not always I would contact the transformer manufacture to be sure.

What do you consider hot; this class of transformer usually has a 115c rise with a 180c insulation class and thats above ambient room temperautre of 40c. If you place your hand on the top of the transformer and it is around 125F; I would not consider it hot.

Thanks Apps man, it is indeed 5.07 kva...( i must have missed my coffee..)

But feeding 480 volts to 208 volts rated winding is definitely not advisable.

Hence the new transformer should be 10 kva 3 phase 60 hz 208 v primary and 480 V secondary.

Cheers

Thanks Apps Man,

I was under the impression looking through some literature that you would add the amp draw of each phase to get the TOTAL load in amps to plug into the KVA 3 phase formula. So this is indeed incorrect, you do use the highest amp draw of the highest single measured phase.

So in this case, the highest measured amp draw on one of the phases was indeed 6.1 amps. If I up the amp rating to allow the transformer to run at 65% - 6.1 x .65% = 3.965 + 6.1 = 10.065 amps x 480 volts x 1.732 / 1000 = 8.367 KVA so indeed a 9 KVA transformer would be just right. Apps man, is all this correct?

Also, the 208 primary boost up to 480 secondary, do I insist on this configuration or fo a 9KVA unit does it indeed matter?

Thanks fo the many responses !!!! How great it is to have access to such expertise!!!

jcall

Make sure that the transformer you purchase has a 480 volt secondary winding that is configured as "wye". This will allow you to ground the star point.

North of 60,

The configuration, do I need a wye primary to wye secondary? It looks like a delta primary and wye secondary is offered but not wye to wye. Can I connect a transformer thats delta primary to the existing 208 wye primary power and be OK?

Thank you,

jcall

What you want is a delta primary with a wye secondary.

To ground this setup properly...

The star point, the "X0", of the secondary winding is "grounded" to the main building ground using a "grounding conductor", this "insulated" green conductor should be sized according to the applicable code in your region. If this was in Canada, that sizing data would come from Table 17 of the CEC.

Also from the X0 connection, take a "bond" jumper to the case of the transformer. The size of the "bonding jumper", again if in Canada, would be based on Table 16 of the CEC. This is then the starting point of the "bonding system" for this separately derived system.

As your load is 480v and not 277v you have no need of a neutral conductor, which would connect to the "X0" as well. If you did need the neutral, no other connection between the neutral and ground is permitted to be present then the ones mentioned above.

Please get a certified and capable electrician to do this under a valid installation permit issued by the AHJ in your area. Doing less then this exposes you and the property owner to all sorts of liability issues.

Rick...