Transformer kVA Rating

[stevee]

Is it true that transformers are rated to run continuously at 100% of their rated kVA?

In other words if I had a transformer with a secondary current output of 100 amps could connect 100 amps of continuous load to that transformer? (Not that I would but is it rated to handle that volume of current continously?)

The reason I am asking is this. I was told that transformers were rated to run continuously at their kVA rating. So if I had a transformer with a rated output current of 100 amps and protected the secondary at 125% using a 125 amp circuit breaker and connected the maximum allowable continuous current (100A) to the panel it supplied, the system would be legal and functional. Is this true?

 

[petersonra]

Is it true that transformers are rated to run continuously at 100% of their rated kVA?

In other words if I had a transformer with a secondary current output of 100 amps could connect 100 amps of continuous load to that transformer? (Not that I would but is it rated to handle that volume of current continously?)

The reason I am asking is this. I was told that transformers were rated to run continuously at their kVA rating. So if I had a transformer with a rated output current of 100 amps and protected the secondary at 125% using a 125 amp circuit breaker and connected the maximum allowable continuous current (100A) to the panel it supplied, the system would be legal and functional. Is this true?

In reality. Transformers are as much rated for temperature as for current. If you keep the temperature down, they can handle more current. The KVA rating is based on a specific ambient temperature, and is the maxiumu current the thing is rated for at the temperature. At lower temperatures, the manufacturers have curves to show you how much current the transformer can take.

This is not trivial to apply in the real world though.

And normally they have some overload capacity for some period of time.

 

[stevee]

In reality. Transformers are as much rated for temperature as for current. If you keep the temperature down, they can handle more current. The KVA rating is based on a specific ambient temperature, and is the maxiumu current the thing is rated for at the temperature. At lower temperatures, the manufacturers have curves to show you how much current the transformer can take.

This is not trivial to apply in the real world though.

And normally they have some overload capacity for some period of time.

That makes sense as heat is a major factor in most electrical device failures. That is why in my original post I stated that loading a transformer at 100% of it's rating continously is not something I would do or recommed. I've seen transformers that we're probably overloaded for the ambient temperature in the area they were installed and were so hot you could almost not touch them. I was simply curious whether it was permissible to load them at 100% according to the manufacturers specs.

So, I guess the correct answer would be yes, so long as you didn't exceed the temp rating of the transformer otherwise you would essentially cutting down the life expectancy of the transformers similar to exceeding the temp rating of a motor.

 

[petersonra]

That makes sense as heat is a major factor in most electrical device failures. That is why in my original post I stated that loading a transformer at 100% of it's rating continously is not something I would do or recommed. I've seen transformers that we're probably overloaded for the ambient temperature in the area they were installed and were so hot you could almost not touch them. I was simply curious whether it was permissible to load them at 100% according to the manufacturers specs.

So, I guess the correct answer would be yes, so long as you didn't exceed the temp rating of the transformer otherwise you would essentially cutting down the life expectancy of the transformers similar to exceeding the temp rating of a motor.

If you can touch them, they are probably not "too" hot.

IIRC, the spec is that the surface cannot exceed 65 degree C above ambient. Normally that would be something like 200 degree F.

 

[Pierre C Belarge]

If you look at this picture (a utility transformer) you can see the cooling fins on the back. In effect, the fins help to reduce the operating temperature.

What about this contraption? Does it count towards keeping the transformer cooler/
It was installed so no one would leave anything on top of the transformer...:grin:

I thought it was a heated cheeze grater.

 

[micromind]

Any xfmr can be operated continuously up to its rated KVA provided a few conditions are met.

1) adequate ventilation is provided (rare in an enclosed room). This includes no accumulation of dust, etc. on the core and coils, and no blocked ventilation openings.

2) Ambient temperature is at or below nameplate rating (usually 40C, 104F).

3) Voltage is not higher than nameplate.

4) on 3PH units, phase balance must be reasonable, though I can't remember what the tolerance is. This is especially true involving delta connections.

On larger units, there's usually 2 (or more) KVA ratings. One is AA (atmospheric aspiration), another is FA (Forced aspiration). The difference can be significant. An example of forced aspiration is a utility switchyard. If you look closely at the xfmrs there, alot of them have fans mounted on the cooling fins. This boosts the KVA rating considerably. Some manufacturers have fan kits you can install in the bottom of smaller dry-type xfmrs. The ones I've installed come with a new label to stick next to the existing one that lists a higher KVA rating for FA cooling.