3-Phase Load Calculation

[A-1Sparky]

I've got a 3-phase 225 KVA transformer (480-208/120) that's running pretty hot. I think it's overloaded, but want to make sure. I've taken load readings on each phase conductor at the 208/120 panel with an ammeter. They were 142 amps on phase A, 160 amps on phase B, and 135 amps on phase C. How do I determine the overall amp draw and thus the actual KVA? Thanks in advance.

 

[Mayimbe]

Van*Ia=120 V*142 A=17.04 kVA = Pa
Vbn*Ib=120 V*160 A=19.20 kVA = Pb
Vcn*Ic=120 V*135 A=16.20 kVA = Pc

P3f=Pa+Pb+Pc=52.440 kVA

thats assuming that those currents are in the secundarie side. and you have balanced voltage as well. you should meassure the voltage also and the power factor.

whats the overall amp draw? isnt that what you are taking readings of?

 

[winnie]

You don't need to determine the 'overall' KVA; each phase of the transformer is limited in the current that it can carry. With a 225 KVA transformer, then each phase can supply 75 KVA (to a good approximation...if you had load on only a single phase, then the reduced total heating of the transformer enclosure would mean that you could have somewhat more than 75 KVA on the loaded phase...but not much more).

225000 / 208 / 1.732 = 625 This transformer should be able to supply 625 amps per phase.

-Jon

 

[jim dungar]

I've got a 3-phase 225 KVA transformer (480-208/120) that's running pretty hot. I think it's overloaded, but want to make sure. I've taken load readings on each phase conductor at the 208/120 panel with an ammeter. They were 142 amps on phase A, 160 amps on phase B, and 135 amps on phase C. How do I determine the overall amp draw and thus the actual KVA? Thanks in advance.

First why do you think it is running hot? When operating at full load most transformers are extremely hot (many are not even 'touchable'). I knew one plant electric shop that used a transformer as a hot plate to keep their coffee pot warm.

For quick and dirty evaluations you can use the maximum phase current only in your calculations. (160A x 208V x 1.732)/1000 = 58kVA.
If this estimate approaches the nameplate rating of the transformer, you would want to use a more precise method.

 

[Paul the BIG Finn]

It does not sound as if you have a current draw problem, but you may have a harmonics problem. What does the transformer serve?

You should also identify what you mean by hot, the room temperature and the means of ventilation or cooling.

 

[A-1Sparky]

Thanks for all the replies, guys. Those amperage numbers were for only one set of phase conductors. I've got 3 sets of phase conductors fed off of the secondary side of this transformer. The transformer is in a 5'X8' room with zero ventilation and zero cooling. There's a faint burning smell that's noticeable when you enter the room. It's most noticeable near the vents on the transformer. I just don't want to end up with an unplanned shutdown because this thing was overloaded. Also, this transformer is over 40 years old. Is there a general rule of thumb for transformer life span (based on percentage of max. load)? Thanks again.

 

[Mayimbe]

I suggest you buy a new one...

 

[jatrottpe]

A 225kva Transformer at 208V will carry about 624A per phase.

Transformers have temperature ratings based on their insulating material. Typical ratings are 80 deg C rise above ambient, 115 deg C rise above ambient, 150 deg C rise above ambient. (Assuming 40 deg C ambient).

You will need to see what temp rise your transformer has.

Transformers can last upwards of 30 years if they are well maintained. Temperature is the enemy of transformer life.

Every 10 deg C rise in operating temperature over rating reduces the life expectancy by approx half.

 

[jim dungar]

Every 10 deg C rise in operating temperature over rating reduces the life expectancy by approx half.

And their life is doubled for every 10?C reduction in temperature. If you run a unit 10?C higher than design for 1 hour you have reduced its life by 2 hours. But if you then run it 10?C cooler than design for 1 hour you have increased its life by 2 hours (this ignores the thermal mass and heat transfer rates). The design half-life of transformer insulation systems is about 20,000hrs (2.3yrs) when run continuously at maximum operating temperature (hot spot = 220?C for Class H dry type units). Few transformers run continuously at maximum temperature which is why actual life easily can exceed 30-40yrs.

 

[drbond24]

We would need the total current the transformer is supplying (all conductors coming off of the secondary, not just one set) to comment on whether it is overloaded, and we would need the temperature of the transformer and the ambient temperature in the room to comment on whether it is overheated.

40 years means that transformer is at the end of its life regardless of how it was treated during that time. I'd equate that to a 90 year old human. They might last longer, but they might go at any time, regardless of how they lived their lives.

 

[Besoeker]

The transformer is in a 5'X8' room with zero ventilation and zero cooling.

Zero ventilation is bad, particularly in such a small room.
For that rating of transformer, you'd ideally need an airflow of around 900 cfm.

 

[jatrottpe]

Jim D
Thank you for completing my sentance.

 

[Paul the BIG Finn]

I think you can count yourself lucky that a generator lasted 40 years in a closed room. If it is running hot and you have a faint buring spell I think you need to schedule a replacement ASAP.

I would also provide a small ventilation fan to cool the room the tansformer is in. A 225 kVA transformer is likely giving off 5 kW of heat. Is the room itself hot when you enter it?

 

[LarryFine]

Those amperage numbers were for only one set of phase conductors. I've got 3 sets of phase conductors fed off of the secondary side of this transformer.

If you mean three paralleled sets, then you can triple the reading and get close, but you must read each conductor and add the readings for each phase for it to mean anything otherwise.

You've actually got 480a on B phase if these are paralleled feeders, which will get a transformer capable of 625a quite warm, but shouldn't damage it. You may be smelling hot dust.

Having the thing tested is the only way to certify that it's capable of remaining with the present load, and lasting much longer. It may live forever, and it may do a melt-down tomorrow.

 

[Karl H]

"Hot Dust", That's a perfect description of the smell of an old transformer.

 

[jim dungar]

Having the thing tested is the only way to certify that it's capable of remaining with the present load, and lasting much longer.

I know of no non-destructive test that can be used to predict the servicable life of a dry type transformer.

 

[Besoeker]

I would also provide a small ventilation fan to cool the room the tansformer is in. A 225 kVA transformer is likely giving off 5 kW of heat.

As it happens, that's the figure I used to calculate the 900 cfm. But 900 cfm isn't such a small fan. That said, any ventilation would be better than none.

 

[A-1Sparky]

Thanks for all the info., guys. I'm going to suggest that the room be supplied with ventilation and cooling, and ask that the transformer be replaced in the near future as preventive maintenance.