Sizing conductors and overcurrent protection on both the primary and secondary of a 45 KVA transformer

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Terry J Loiacano

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Most projects are engineered for me but I have a specific piece of equipment that I am trying to connect a 208 3 phase to a 45 KVA transformer which is a step up to a 60 amp piece of equipment that is 480-volt. I am struggling trying to determine what size overcurrent protection I need on the primary side for the breaker feeding the 45 KVA transformer
 
45 KVA 3 phase @ 208 is 125 amps.
But I would want to know more details about the exact transformer specs. If you are reverse feeding a standard delta/Y this is not a good idea for a number of reasons.
 
That is very helpful, I was running into the 600% rule because I'm installing a circuit breaker in a four hundred amp Square D qob panel.....
Which still concerns me on if I will be able to find the breaker necessary for the primary to come out of that panel, which according to my math came up 312.5 amps, and I don't know if they make a 3-phase Square D qo bolt in 350 amp breaker
 
the company that supplied the $250,000 machine also recommended the 45 KVA Step up Transformer. It is a MIT-DRY-133-M....
208-480Y/277
 
Terry J Loiacano said:
the company that supplied the $250,000 machine also recommended the 45 KVA Step up Transformer. It is a MIT-DRY-133-M....
208-480Y/277
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Because your transformer is a step-up design you may be able to use as small as a 154A breaker on the primary, using the next size up would take you to175 or 200A. I believe QO panels are limited to 150A branches, unless they were built to allow separately mounted sub feed breakers.
 
That is good to know I will see what size breaker I can get. If they do not make a 175 or 200 for the qo Bolt in panel would I be safe to go with the wire size to 150 amp and the breaker 150 amp
 
Terry J Loiacano said:
That is good to know I will see what size breaker I can get. If they do not make a 175 or 200 for the qo Bolt in panel would I be safe to go with the wire size to 150 amp and the breaker 150 amp
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You may have nuisance tripping when the transformer is energized.
 
Terry J Loiacano said:
That is very helpful, I was running into the 600% rule because I'm installing a circuit breaker in a four hundred amp Square D qob panel.....
Which still concerns me on if I will be able to find the breaker necessary for the primary to come out of that panel, which according to my math came up 312.5 amps, and I don't know if they make a 3-phase Square D qo bolt in 350 amp breaker
Click to expand...
Lemme try:
Primary current = 45,000/(1.732X208) = 124.9A. 250% of that is = 2.5 X 124.9 = 312.3, therefore, use a 350A breaker!
Secondary current = 45,000/(1.732X 480)= 54.1A. 125% of that is = 67.6A, therefore, use a 70A breaker!
 
Topgone that's exactly what I came up with. The primaries coming out of a 208 120 panel that is 400 amps and has very few Breakers in it but how would I possibly feed the Transformer what's a 350 amp breaker out of that panel.... what are my options
 
topgone said:
Primary current = 45,000/(1.732X208) = 124.9A. 250% of that is = 2.5 X 124.9 = 312.3, therefore, use a 350A breaker!
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The NEC has a maximum of 250% for the primary, rounding up to the next standard size would not be allowed.
 
topgone said:
Lemme try:
Primary current = 45,000/(1.732X208) = 124.9A. 250% of that is = 2.5 X 124.9 = 312.3, therefore, use a 350A breaker!
Secondary current = 45,000/(1.732X 480)= 54.1A. 125% of that is = 67.6A, therefore, use a 70A breaker!
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Why would you go to the maximum permitted on the primary? I would be looking at a 175 or 200 amp primary OCPD. If you upsize the primary OCPD, you also have to upsize the primary conductors to match.
 
When transformers get energized you have to deal with inrush current. That is why the NEC permits the vast oversize on the primary OCPD.

But usually you don't need to oversize this much. Unfortunately there really isn't a good way to know how little you can get away with without trying.

Once the transformer is energized successfully then the minimum acceptable size for the circuit is based on load (eg 125% continuous + 100% max non-continuous.....). IMHO in your case 150A would be sufficient if it successfully energizes the transformer.

You can make some guesses about how well this will work if you evaluate the upstream impedance of the circuit. Do you have long supply runs or upstream transformers that will reduce available current?

Also evaluate how frequently this transformer will be switched, will it be on once and forever, or will it be turned off and on frequently?

Jon
 
don_resqcapt19 said:
Why would you go to the maximum permitted on the primary? I would be looking at a 175 or 200 amp primary OCPD. If you upsize the primary OCPD, you also have to upsize the primary conductors to match.
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I agree, I can't recall ever seeing a design where the primary was sized at 250%. A 200 amp primary is probably large enough for the inrush current.
 
Thank you that is extremely helpful. It will be for a specific piece of equipment that will be turned on and off quite often but it is a very short supply run and there are no other Upstream Transformers. Now if I can only find a 200-amp breaker to bolt in the 400 amp panel I feel confident then will be sufficient
 
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