45 kVA transformer primary OCPD & conductor

[cppoly]

In Mike's recent article, he has an example with 45 kVA transformer 480V primary and calculates a 70A CB for the primary with #6s.

Question, 45 kVA 480V primary = 54.12A x 1.25 = 68A, round up and you have 70A OCPD.

But how are #6's (rated 65A) OK?

Assuming max load 54A and continuous load, how do #6's rated 65A work out when you need conductors to be rated at least 68A? Am I missing something?

Here's the article:
https://www.ecmweb.com/national-ele...da|4922A3440078H4Y&oly_enc_id=4922A3440078H4Y

 

[infinity]

Assuming max load 54A and continuous load,

In Mike's example does it mention a continuous load?

 

[HEYDOG]

In Mike's recent article, he has an example with 45 kVA transformer 480V primary and calculates a 70A CB for the primary with #6s.

Question, 45 kVA 480V primary = 54.12A x 1.25 = 68A, round up and you have 70A OCPD.

But how are #6's (rated 65A) OK?

Assuming max load 54A and continuous load, how do #6's rated 65A work out when you need conductors to be rated at least 68A? Am I missing something?

Here's the article:
https://www.ecmweb.com/national-ele...da|4922A3440078H4Y&oly_enc_id=4922A3440078H4Y

I am looking at 240.4 (B) Overcurrent devices rated 800 amps or less.

 

[cppoly]

The article does not mention continuous load. Good point. But to make it clear it should differentiate

 

[don_resqcapt19]

The primary full load current is only 54 amps. The overcurrent protection selected is 125% of the primary full load current. That does not change the load current. The OCDP is 70 amps, and 240.4(B) permits a 65 amp conductor to be protected at 70 amps as long as the load does not exceed 65 amps.

 

[cppoly]

The primary full load current is only 54 amps. The overcurrent protection selected is 125% of the primary full load current. That does not change the load current. The OCDP is 70 amps, and 240.4(B) permits a 65 amp conductor to be protected at 70 amps as long as the load does not exceed 65 amps.

Thanks Don, but if the primary current is continuous wouldn't the minimum wire size be 54A x 1.25 = 68A, which #6's aren't rated to carry?

The example doesn't say continuous load, but if this was a continuous load I would like to know the wire size. Thanks.

 

[HEYDOG]

Thanks Don, but if the primary current is continuous wouldn't the minimum wire size be 54A x 1.25 = 68A, which #6's aren't rated to carry?

The example doesn't say continuous load, but if this was a continuous load I would like to know the wire size. Thanks.

The same size. Have you read 240.4(b)

 

[cppoly]

Yes, I see 240.4(b) allows the next higher size OCPD.

Using the continuous load as an example, wouldn't 215.2(A)(1) (feeders) require 125% of 54A = 68A minimum = #4 conductor using 75 degree column. What am I missing?

 

[infinity]

Yes, I see 240.4(b) allows the next higher size OCPD.

Using the continuous load as an example, wouldn't 215.2(A)(1) (feeders) require 125% of 54A = 68A minimum = #4 conductor using 75 degree column. What am I missing?

The circuit to the transformer primary by definition would not be a feeder.

 

[david luchini]

Yes, I see 240.4(b) allows the next higher size OCPD.

Using the continuous load as an example, wouldn't 215.2(A)(1) (feeders) require 125% of 54A = 68A minimum = #4 conductor using 75 degree column. What am I missing?

Yes, you would need #4 conductors if the load on the primary side was 54A continuous load.

 

[david luchini]

The circuit to the transformer primary by definition would not be a feeder.

Huh? Of course it would be a feeder.

 

[cppoly]

Huh? Of course it would be a feeder.

Yes, you would need #4 conductors if the load on the primary side was 54A continuous load.

Thanks! I thought I was going crazy.

 

[Tulsa Electrician]

As you read thru the artical is says.
" This Note does not require the next size up protection; it " permits" the next size up."
Important difference.

 

[cppoly]

As you read thru the artical is says.
" This Note does not require the next size up protection; it " permits" the next size up."
Important difference.

Article 450 only covers the transformer protection not the conductors.

 

[Tulsa Electrician]

Article 450 only covers the transformer protection not the conductors.

In the artical it ask a question and gave an ansawer.
I seen nothing in the question or artical to suggest you need larger than #6 a@ 75c.

The load in the questions 54 amps.
#6 @ 75 c = 65 amps.

What's the issue.
240.6 A list 60 than 70. 450 table note allows you to round up if you wish.

It does say you must, it says you can. In this case a 70 amp breaker on the #6 if allowed and fine. It will carry 54 amps for ever at normal operating.comditions.

For all we know the panel is all recptical branch circuits for general use.
No need to assume continuse use untill it give a reason to.
When it does than the equation changes.

I simply stated it permits not required. Since the load is 54 amps I would choose to use the 70 so my breaker is operating with in it's 80%.

After all don't know what OCD is being used other than the pic of a breaker in the picture.
Your question was how can you use #6.
Easy, 54 amp load.

Here is how I look at from a field point of view.

Most likely it has a secondly OCD at 125 amps in the panle.
Using the 80% of that is 100 amps.
That relates to a primary current of around 44 amps.
So 44*1.25=55 amps
Then a 60 amp breaker on primary will work.
Then the 45 kva transformer is operating at around 80% fully loaded.
That in my opinion is how #6 is fine even with a lower sized primary breaker.

Load amps, type of load and secondary main OCD for me would help determine the difference between permitted not required.

Now run the numbers if the load was 54 amps of continues use load and I would say bad choice of transformer size.

Just my take on it.

 

[Tulsa Electrician]

Forgot to add, if in deed continuse load than the wire size would be required to be larger as you suggest.

 

[augie47]

I'm not in disagreement but with transformers Ioads and OCP I find it a bit unnerving.
There is likely something wrong with my thought process but.....
A 45kva 480/208 transformer install would be legal with a 175 amp secondary OCP_ device ("next size up").
That would allow a primary load of 75 amps above the ampacity of a #6.

 

[david luchini]

I'm not in disagreement but with transformers Ioads and OCP I find it a bit unnerving.
There is likely something wrong with my thought process but.....
A 45kva 480/208 transformer install would be legal with a 175 amp secondary OCP_ device ("next size up").
That would allow a primary load of 75 amps above the ampacity of a #6.

The primary ocpd would limit the primary to 70A.

But you're not permitted to load the #6 above 65A.

 

[wwhitney]

Yes, you would need #4 conductors if the load on the primary side was 54A continuous load.

I agree this is what the NEC requires per 215.2(A)(1)(a).

But there's no reason for it to require that. 65A ampacity conductors are sufficiently protected by 70A OCPD per 240.4(B); a 54A continuous load with 70A OCPD satisfies 215.3; and a 65A ampacity conductor can safely carry 54A continuous all day long, per the definition of ampacity.

215.2(A)(1)(a) serves no purpose.

Cheers, Wayne

 

[infinity]

Of course it would be a feeder.

Yes you're correct. I was thinking... I have no idea what I was thinking. Might have been the 5 o'clock somewhere Martinis I was drinking at 2 in the afternoon.