Transformer Loading

[aelec84]

Hello Forum,

Can you please assist with this scenario? Customer Owned transformer feeds 1200A, 4160V Gear. Metered load + 125% (220.87) and loads added/removed is less than 1200A on the gear. but what if if the kVA value of calculated load as described exceeds the transformer kVA capacity? Is that 125% applicable to transformer as well?

Thank you.

 

[jim dungar]

Hello Forum,

Can you please assist with this scenario? Customer Owned transformer feeds 1200A, 4160V Gear. Metered load + 125% (220.87) and loads added/removed is less than 1200A on the gear. but what if if the kVA value of calculated load as described exceeds the transformer kVA capacity? Is that 125% applicable to transformer as well?

Thank you.

Transformer loading is not covered by the NEC.
Transformer life is about heat and time (heat transfer).

If you can keep the heat, from load amps, down you can have the transformer in a hotter ambient. Conversely, if you can keep the heat down, from the ambient air, you can load your transformer more. If the transformer is allowed to cool down, it can handle higher loading for a short period of time.

Your transformer manufacturer should be able to help you, once you have developed a loading/time profile.

I remember installing a 3000kVA unit that had a continuous overload capacity of 150% when its cooling fans were working.

 

[Elect117]

In agreeance with Jim, the nameplate on the transformer can sometimes have an allowable overload percentage on it, which might help speed things up or ease concerns.

Just remember to oversize the secondary or primary accordingly. The transformer has oil/fans/ etc. to dissipate the heat, but the conductors do not.

 

[aelec84]

Transformer loading is not covered by the NEC.
Transformer life is about heat and time (heat transfer).

If you can keep the heat, from load amps, down you can have the transformer in a hotter ambient. Conversely, if you can keep the heat down, from the ambient air, you can load your transformer more. If the transformer is allowed to cool down, it can handle higher loading for a short period of time.

Your transformer manufacturer should be able to help you, once you have developed a loading/time profile.

I remember installing a 3000kVA unit that had a continuous overload capacity of 150% when its cooling fans were working.

Jim you mean article 220.87 is not applicable to a transformer when calculating loads on the transformer?
Would I just use peak demand from meter and add new loads without the 125%?
Is the 125% only applicable to switchboard it feeds?

 

[jim dungar]

Jim you mean article 220.87 is not applicable to a transformer when calculating loads on the transformer?
Would I just use peak demand from meter and add new loads without the 125%?
Is the 125% only applicable to switchboard it feeds?

The 125% is part of the conductor sizing. Because Over Current Protective Devices, both breaker and fuses, are installed in enclosures and are intended to protect conductors, they are also impacted by the 125%.

Some people believe the 220.87 sizing needs to be carried over to the transformer primary side, using the turns ratio of the transformer, while others say sizing starts over again on the primary side.

 

[GoldDigger]

Some people believe the 220.87 sizing needs to be carried over to the transformer primary side, using the turns ratio of the transformer, while others say sizing starts over again on the primary side.

And the input current on the primary side of the transformer will be the vector sum of the load current, transformed by the turns ratio, and the magnetizing current for the transformer core. The magnetizing current for a transformer of a particular power capacity can vary with the transformer design.
As long as the magnetizing current is small compared to the load current, the 90 degree phase difference will make the effect of magnetizing current on the total magnitude of the input current relatively small.

 

[jim dungar]

...transformer will be the vector sum of the load current,...

And this is part of the issue.
Do you use the design load, the probable load, the NEC load, or something else.

 

[PaulEd]

aelec84 what is the calculated load of what you need to serve? You need to start there