3ph Transformer FLA + OCP

[Mustang125]

I'm trying to make a simple quick reference chart. Does someone mind confirming my math? See attached picture

 

[Tulsa Electrician]

If you look at your 75 kva line. I wonder why 250 amp secondary OCPD. No saying wrong just why.
Seams to be higher than most installations. Most are 200 or 225.
After 225amp you jump in rating to 400 amp buss equipment.
Again nothing wrong with that. Cost of job will increase as well.
With that said you could have a heavily loaded lighting panel. So the 250 would makes sense. 250*.80= 200
The transformer would run at darn near 100% not good for transformer efficiency though. Just thinking out loud.
As you go up in size you GEC,SSBJ and secondary conductors also increase along with raceway and equipment.
The way you headers are set up seam misleading. Sec voltage is 208 however you show secondary amps. Of course I know what you mean.
Maybe; sec amps@ 208 volts.
I find you chart interesting.

 

[HEYDOG]

I'm trying to make a simple quick reference chart. Does someone mind confirming my math? See attached picture

Your fla are good on primary and secondary. Are you basing your breaker sizes from the NEC ?

 

[Mustang125]

My math is basically as follows... if you have a breaker on both side then pri. is 250% of FLA but have to round breaker down, sec. breaker is 125% rounded up to next size. If breaker only on primary then its 125% rounded up to next size. I'm pretty sure that's how its done, hence the need for me to get it checked by other eyes haha. i do understand that some don't seem "typical" but this is more of a chart for what's allowed by the NEC, you can always resort back to more typical but this is a "not to exceed" type of thing.

 

[Tulsa Electrician]

I'm going do some reading

My math is basically as follows... if you have a breaker on both side then pri. is 250% of FLA but have to round breaker down, sec. breaker is 125% rounded up to next size. If breaker only on primary then its 125% rounded up to next size. I'm pretty sure that's how its done, hence the need for me to get it checked by other eyes haha. i do understand that some don't seem "typical" but this is more of a chart for what's allowed by the NEC, you can always resort back to more typical but this is a "not to exceed" type of thing.

 

[Tulsa Electrician]

Ok did some reading. Running some numbers. Sticking with what you posted. Transformer over current protection as shown in 450.3(B).

 

[Engser18]

I'm trying to make a simple quick reference chart. Does someone mind confirming my math? See attached picture

1. Every thing 800A and below you can round up to next standard size, you will need to round down everything above 800A.
2. Primary CB only can be used for delta to delta transformer (see 240.21 (C))
3. If you size primary CB 250%, then your primary conductors become tap conductor if their ampacity less than primary CB. 25 foot rules apply both primary and secondary. if you sized the primary CB 125%, then 25 foot rule only for 2nd side.