Transformer OCPD - Pri. & Sec. Protection, Table 450.3(B)

Sizing Transformer OCPDs on both Primary and Secondary sides using the Primary and Secondary Protection method. I reference NEC sub-sections 450.3(B), 240.6(A), and their tables. [Code references are from the 2020 NEC, but most will correspond with prior editions.]

[PeaceHi]

Thank you so much. I am learning a lot from you. I have an Electrical Engineering degree, but never been lucky to get a good professor like the way you explained in your videos. Thank you again !!! You are the best !!!

[bigjesskennelz]

Just came across your channel and right away I subscribe. Love your videos. I'm currently going through this in class but you definitely explain it better. Keep them videos coming!! Thanx for you help.

[stepanvinnikov]

Had him as teacher now watching the video installing a step down transformer

[EonTide]

I guess my other question would be why is the primary ocpd 250% not 125 % . Your videos are so thorough and helps a lot

[andrewvallejo]

Your bad ass bro like how you break stuff down nice and simple make it easy for us to understand thanks for the video 👍

[tonymag]

Thank you so much for this EXTREMELY useful information. You are a savior

[luismota]

Thank you Dave your explanation very clear. I didn’t understand why we have two time the standard 15 amps in table 240.6(A)

[altuber_athlete]

Thanks, excellent video. How would the primary and secondary conductors be sized in this case (primary and secondary protection)? I suppose we’d use the max OCDP (60 A for the primary and 125 A for the secondary), not the FLA (25 A for the primary and 100 A for the secondary), right?

What if I use a secondary OCPD of 100 A (the same as the secondary FLA), would I still be required to size the secondary conductors to 125 A, or can I now size them to 100 A?

[carloscifuentes]

i understand the Max OCPD is 250%. Is it incorrect if I use 125% for both Primary and Secdary side of the transformer?

[edwrd]

I’m confused. Aren’t you suppose to use 240V for the secondary?

[pkprrsg]

Why can't you go over the primary Max. OCPD, but you can go to the next size up on the secondary Max. OCPD?

[EonTide]

So the maximum ocpd is 250%. the minimum is 125% or 25% over of the fla ?

[michaelhinchey]

THANKS for the refresher. You are a very good instructor.

[koryabel]

Helped me navigate the tricky task of sizing transformer equipment

[oscarorozco]

In the denominator of both those formulas u need to multiply by 1.73

[jonmonte]

Now, do you have to size the conductors for 250% or based on the FLA?

[eibiepb]

Trying to understand why we calculate the Max OPCD. Am I correct in my following assumptions? Cost wise you would want to select the OCPD as close to the FLA as possible . However, in some applications intermittent overcurrent can cause a headache with too much tripping of the breaker so some applications would want to go as high as possible which is what the Max OPCD provides the answer to.

[normanvictor]

What about working in a building that only has 208Y/120 and you are back feeding the transformer to create 480 volt for a piece of equipment.
The 208 volt input will have a bit of inrush current that would nuisance trip the breaker when turning on. Example 75 kva transformer, what breaker size and what formula to determine allowable ocpd?

[jolyonwelsh]

25KVA is not a standard size for a 3 phase transformer. For a single phase yes. I would go with a 30KVA which is next closest example.

[matthewrwalker]

Can you add sizing of the conductors based on primary only calc and when calculating pri & secfor both single phase and 3- phase. Also, speak on the nuance of sizing based on ocpd vs downsizing the conductor using 240.4B in reverse.