Transformer OCPD Incorrect?

[chrisreisinger79]

I work as a maintenance electrician in metal foundry, started in April, and I have a question concerning an existing transformer that I believe the OCPD is not properly sized.

It is a 100kva single phase transformer with primary voltage of 480vac and secondary voltage of 240vac. The secondary is feeding a panel, less than 10ft away. The panel has 200amp main with 17 of the 20amp breakers being used out of the 40 available slots.

The transformer primary is being protected by a 100amp breaker in a 400amp protected power distribution cabinet. My calculations for primary only protection show that this transformer should be protected by a 300amp OCPD.

Am I missing something to where the 100amp breaker is adequate protection?

 

[topgone]

I work as a maintenance electrician in metal foundry, started in April, and I have a question concerning an existing transformer that I believe the OCPD is not properly sized.

It is a 100kva single phase transformer with primary voltage of 480vac and secondary voltage of 240vac. The secondary is feeding a panel, less than 10ft away. The panel has 200amp main with 17 of the 20amp breakers being used out of the 40 available slots.

The transformer primary is being protected by a 100amp breaker in a 400amp protected power distribution cabinet. My calculations for primary only protection show that this transformer should be protected by a 300amp OCPD.

Am I missing something to where the 100amp breaker is adequate protection?

That's a case of overly protecting the transformer, IMO. Nothing prevents the designer from limiting the transformer loading to 100/120 (83% of transformer capacity). Does the transformer trip often?

 

[JFletcher]

I work as a maintenance electrician in metal foundry, started in April, and I have a question concerning an existing transformer that I believe the OCPD is not properly sized.

It is a 100kva single phase transformer with primary voltage of 480vac and secondary voltage of 240vac. The secondary is feeding a panel, less than 10ft away. The panel has 200amp main with 17 of the 20amp breakers being used out of the 40 available slots.

The transformer primary is being protected by a 100amp breaker in a 400amp protected power distribution cabinet. My calculations for primary only protection show that this transformer should be protected by a 300amp OCPD.

Am I missing something to where the 100amp breaker is adequate protection?

V_p x A_p = V_s x A_s

where p is primary and s is secondary

480V x 100A = 240V x 200A = 48kVA

Using a lower amperage breaker than needed is not a safety concern

 

[chrisreisinger79]

As we start to utilize more circuits in the panel... the 100amp breaker will be insufficient then correct? I do not believe the intent was to limit the transformer loading, but rather to install it the cheapest way possible for what was needed at the time. Apparently that was the mantra for the previous owners of the plant. I would like to utilize this particular installation to its full potential.

 

[topgone]

As we start to utilize more circuits in the panel... the 100amp breaker will be insufficient then correct? I do not believe the intent was to limit the transformer loading, but rather to install it the cheapest way possible for what was needed at the time. Apparently that was the mantra for the previous owners of the plant. I would like to utilize this particular installation to its full potential.

Correct. If the conductors are sized properly for rated primary current, you can just replace the main circuit breaker with a bigger fuse/breaker. If you want, you can use fuses instead.

 

[Sahib]

If the conductors are sized properly for rated primary current, you can just replace the main circuit breaker with a bigger fuse/breaker.

Are the transformer supply conductors protected in that case?

 

[ramsy]

Your Transformer Nameplate is 140A

According to 2011 NFPA-70, ANNEX D Example D10

a) Calculate the nameplate rating of the transformer:
I = 100,000VA = 140A
√3(460v x 0.9eff)

 

[david luchini]

Your Transformer Nameplate is 140A

According to 2011 NFPA-70, ANNEX D Example D10

a) Calculate the nameplate rating of the transformer:
I = 100,000VA = 140A
√3(460v x 0.9eff)

100kVA/480V=208A. The transformer primary current rating is 208A.

 

[iwire]

100kVA/480V=208A. The transformer primary current rating is 208A.

Wouldn't that be 120 amps?

100,000/480/1.73=120

 

[iwire]

As we start to utilize more circuits in the panel... the 100amp breaker will be insufficient then correct?

Maybe, that depends on the load that is there now vs what you add right?

I do not believe the intent was to limit the transformer loading, but rather to install it the cheapest way possible for what was needed at the time. Apparently that was the mantra for the previous owners of the plant.

I will point out that saving money is not a bad thing and something that those signing the checks may need to do.

I would like to utilize this particular installation to its full potential.

100 Kva transformer. you could put up to 300 amp breaker on the primary but you would need to up size the conductors to match.

On the secondary side you have 240 amps usable and could put up to a 300 on the secondary as well.

If you do up size the primary you might want to consider leaving the 200 amp panel as it is and add a second 100 amp panel to this transformers secondary connections.

 

[chris kennedy]

Wouldn't that be 120 amps?

100,000/480/1.73=120

OP states single phase.

 

[kwired]

Wouldn't that be 120 amps?

100,000/480/1.73=120

OP said this is a single phase transformer


just like chris pointed out while I was replying

 

[iwire]

OP said this is a single phase transformer

:lol:

That would do it. Thanks.

I almost never touch a single phase transformer so I don't even consider it. That also explains why 100 kVA instead of 75 or 112.5.

 

[iwire]

OP states single phase.

Don't bother me with facts!

 

[kwired]

Originally Posted by topgone If the conductors are sized properly for rated primary current, you can just replace the main circuit breaker with a bigger fuse/breaker.


Are the transformer supply conductors protected in that case?

If the conductors are sized properly for the rated primary current and you don't exceed maximum allowed overcurrent setting why not?

 

[petersonra]

If the conductors are sized properly for the rated primary current and you don't exceed maximum allowed overcurrent setting why not?

You can use the primary overcurrent protection device to protect both primary conductors and the primary side of the Transformer but the wires have to be big enough to be protected by the overcurrent protection device. It's not like you get any special allowance to make the conductor ampacity less than the rating of the overcurrent protection device just because it's going to a Transformer.

 

[kwired]

You can use the primary overcurrent protection device to protect both primary conductors and the primary side of the Transformer but the wires have to be big enough to be protected by the overcurrent protection device. It's not like you get any special allowance to make the conductor ampacity less than the rating of the overcurrent protection device just because it's going to a Transformer.

You are correct.

OP has not said what size primary conductors are. If they are sized to transformer rated current there is an ability to increase overcurrent protection without changing anything else. But if 100 amp primary does hold when energizing there probably isn't much need for more since the secondary is already protected at 200 amps. He has a lot of transformer capacity but has limited it by the devices used, possibly used something already readily available? If not, spent more money on a transformer then needed to.

 

[chrisreisinger79]

Maybe, that depends on the load that is there now vs what you add right?



I will point out that saving money is not a bad thing and something that those signing the checks may need to do.




100 Kva transformer. you could put up to 300 amp breaker on the primary but you would need to up size the conductors to match.

On the secondary side you have 240 amps usable and could put up to a 300 on the secondary as well.

If you do up size the primary you might want to consider leaving the 200 amp panel as it is and add a second 100 amp panel to this transformers secondary connections.

Not following... where are you getting 240 amps available for the secondary?

Thanks for the help so far everybody. And no, the primary conductors are only sized for 100amp.

 

[ramsy]

Not following... where are you getting 240 amps available for the secondary?

Thanks for the help so far everybody. And no, the primary conductors are only sized for 100amp.

That was my mistake, assuming a 3-phase transformer.

I believe Fletcher had your Transformer dialed in, back in post #3.

 

[kwired]

Not following... where are you getting 240 amps available for the secondary?

Thanks for the help so far everybody. And no, the primary conductors are only sized for 100amp.

iwire's 240 amps was a mistake because he thought you had a three phase transformer. Your 100kVA single phase unit has a secondary capacity of 417 amps, but is currently limited by the 100 amp primary as well as the 200 amp secondary devices. No safety issues because of that, but you can not get the full potential of the unit either.