Transformer OCPD - Primary-Only Protection, Table 450.3(B)

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Sizing Transformer Primary OCPDs using the Primary-Only Protection method for 1,000 Volts and Less. I reference NEC T-450.3(B) & 240.6(A). [Code references are from the 2020 NEC, but most will correspond with prior editions.]
  1. Dave Gordon
  2. Transformer OCPD
  3. Transformer Primary-Only OCPD
  4. Transformer Primary-Only Protection
  5. NEC 450.3(B)
  6. 450.3(B)
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Комментарии
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Thank you!
I was in doubt of which breaker to use for a 150 kVA dry type transformer 3 phase @ 440V primary.
According to the NEC table the max size should be 250 amps breaker, but cost was considerably higher than 225 amps installation.
We used a 225 amp I-Line breaker with 4/0 awg wire.
Transformer is working without a problem.
Your video saved me money :)

gerardotrevino
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Thank you sir, your videos are helping me stay afloat during my electrical apprenticeship.

adriano
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Awesome video. Excellent way of explaining. Thank you!!

bigjesskennelz
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Your explanation is so clear and concise!!

sungkim
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A very informative video and you are well spoken. Many thanks!

paulmendez
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Dave I really like your explanation and clarification of a difficult subject in a difficult book to understand just by reading the text. By chance have you done a similar video for a 75kva delta delta 480/240 three phase 3 wire transformer?

clevereams
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I have watched both videos in regards to Table 450.3 (B), Primary Method and both Primary and Secondary Method. The teachings were excellent and i understand completely how to use this table for overcurrent protection in regards to transformers. However there is one thing that remains unclear to me and that is, how to choose which method to use? Primary only protection or Primary and Secondary Method?

keithbrinton
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Thanks a lot sir! I'm watching your video for my review. Im from the Philippines and will take Registered Master Electrician Exam next year. This will help a lot. I'm studying our Philippine Electrical Code is like your NEC.

legendaryl
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Hello, great video. I have a 400 amps service open delta made from two single phase transformers 120/240 ( 250 and 150 amps main breakers. I m buying a new step up transformer 112.5 kva 3 phase to power a 100 amps 480v machine. what OCP do I need on my primary?

josealfaro
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Great video Dave, one thing that confuses me is what is the standard method for sizing OCPD for pri. And sec…. I have seen ocpd’s out in the field where the 125% rule was applied for pri but not sec.

TheJesusr
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Isn't phase voltage and line voltage the same in a delta transformer? why are you multiplying 480 by 1.73?

Happy-czft
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Hi mr dave..
Ques.when and how i will used the right percentage of multipliyer..

xyronefilvarelajr.
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Question: why did you go down from 3.75A to the next size ocpd 3A? Note 1 on 450.3(B) says go up to the next size? I’m an apprentice so I’m still learning. Great video by the way.

koryabel