Size transformer

[marcinrudnik30]

Hi
Im looking to install a 75kva 3 phase 480v transformer. I have a 125a breaker at the primary, and i need 200a at the secondary. Can I install 3/0 or is that too little. Calculations show me i got to use 300mcm.
Just wanted to make sure.
If I wanted 200A on the secondary, would I have to use a 50kva 3 phase trans instead

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[david luchini]

Hi
Im looking to install a 75kva 3 phase 480v transformer. I have a 125a breaker at the primary, and i need 200a at the secondary. Can I install 3/0 or is that too little. Calculations show me i got to use 300mcm.
Just wanted to make sure.
If I wanted 200A on the secondary, would I have to use a 50kva 3 phase trans instead

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3/0 is fine for the secondary.

 

[marcinrudnik30]

So I can use 75kva?

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[iwire]

So I can use 75kva?

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A 75 kva has a 208 amp max output at 208 volts, so yes for sure.

75 kva x 1,000 / 208 volts / 1.73 = 208.425 amps of secondary current

A typical 75 kva install for us would be a 100 to 150 amp primary breaker and a 225 amp secondary breaker using 4/0 copper.

Buy the way, in your first post about the 300 kcmil vs 3/0 were you talking about copper or aluminum?

 

[marcinrudnik30]

3/0 is fine for the secondary.

3/0 is fine for 75kva?

Hi
Im looking to install a 75kva 3 phase 480v transformer. I have a 125a breaker at the primary, and i need 200a at the secondary. Can I install 3/0 or is that too little. Calculations show me i got to use 300mcm.
Just wanted to make sure.
If I wanted 200A on the secondary, would I have to use a 50kva 3 phase trans instead

Sent from my SM-G925T using Tapatalk

Sent from my SM-G925T using Tapatalk

 

[marcinrudnik30]

A 75 kva has a 208 amp max output at 208 volts, so yes for sure.

75 kva x 1,000 / 208 volts / 1.73 = 208.425 amps of secondary current

A typical 75 kva install for us would be a 100 to 150 amp primary breaker and a 225 amp secondary breaker using 4/0 copper.

Buy the way, in your first post about the 300 kcmil vs 3/0 were you talking about copper or aluminum?

Cooper

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[Carultch]

3/0 is fine for 75kva?



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200A (160A continuous) at 208V is not fully utilizing the 75 kVA. There is room for another 17 kVA on the transformer, which could be installed on another secondary circuit.

The kVA of the transformer doesn't directly govern what size the conductors have to be, unless you are counting on the primary OCPD to protect the secondary as you can do with a select few transformer topologies. 240.21(C) governs transformer secondary conductors. In general, you need as much wire Amps as you have secondary OCPD. There are context rules depending on length/inside vs outside.

 

[marcinrudnik30]

Thank you

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[marcinrudnik30]

Also, I know that I am putting a disconnect at the primary side before it hits the trans. Do I need it to be fusable or non?

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[Smart $]

200A (160A continuous) at 208V is not fully utilizing the 75 kVA. There is room for another 17 kVA on the transformer, which could be installed on another secondary circuit.

Transformer ratings are based on continuous load. You can actually go up to 100% of the transformer rating with an all continuous load or 125% of the transformer rating with an all noncontinuous load. For the latter, that's 260A per line or 93.7kVA with a 75kVA-rated transformer.

 

[Smart $]

Also, I know that I am putting a disconnect at the primary side before it hits the trans. Do I need it to be fusable or non?

Depends on whether this disconnect is providing the overcurrent protection for primary conductors and or the transformer primary. Obvious it has to be fused if it is. If not, it can be either.

 

[Carultch]

Transformer ratings are based on continuous load. You can actually go up to 100% of the transformer rating with an all continuous load or 125% of the transformer rating with an all noncontinuous load. For the latter, that's 260A per line or 93.7kVA with a 75kVA-rated transformer.

Thanks for clarifying. I seldom use noncontinuous loads, so that's why I specified.

So what is (125% continuous + 100% noncontinuous)/125% the proper sizing algorithm for a transformer KVA? Or is it 100% continuous + 100% noncontinuous?

 

[kwired]

Hi
Im looking to install a 75kva 3 phase 480v transformer. I have a 125a breaker at the primary, and i need 200a at the secondary. Can I install 3/0 or is that too little. Calculations show me i got to use 300mcm.
Just wanted to make sure.
If I wanted 200A on the secondary, would I have to use a 50kva 3 phase trans instead

Sent from my SM-G925T using Tapatalk

If you intend to supply a 200 amp device no need for more then a 200 amp conductor. Your transformer still has slightly more capacity then 200 amps on the secondary - you just won't ever quite use all of it if you have a 200 amp device (sort of).

If you went with a 50 kVA transformer you only have a full load rating of ~139 amps on the secondary, you can easily have up to 175 amps overcurrent protection on it, but still shouldn't intentionally connect more then 139 amps of load to it.

 

[Smart $]

Thanks for clarifying. I seldom use noncontinuous loads, so that's why I specified.

So what is (125% continuous + 100% noncontinuous)/125% the proper sizing algorithm for a transformer KVA? Or is it 100% continuous + 100% noncontinuous?

Proper is debatable. All I can tell you is Code permits up to 125% for transformer protection, and that OCPD can be loaded per Code to the first algorithm.