cable sizing on a 300-amp breaker

[electrofelon]

I would like to further comment on the idea that has been mentioned several times that the secondary OCPD will protect the primary conductors. Clearly this may provide some overload protection but no short circuit or ground fault protection. There is no allowance that overload protection is all that is required. Further, note the rules for primary OCPD protecting secondary conductors: you can only do it for select transformer types.

 

[infinity]

I think the problem you may encounter is most inspectors are going to look at things in a simplistic form. 110.14(C) tells us "Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment
termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7)."
Your circuit breaker will likely be 75° termination so, as an inspector, I'm going to looking at the 75° rating of 4/0 of 230 amps and the fact that 240.6 shows a 250 amp breaker to be the next standard size and, as David, has pointed out 240.4 tells us to protect the conductor at its ampacity (next size up rule taken into account)

In my world, I would listen intently to your explanation, look at your 4/0 and the 300 amp breaker and issue a rejection.

Good luck !

That's make two of us. This isn't a gray area of the NEC open to AHJ interpretation it's black and white.

 

[kingpb]

Whether an inspector has the fore-site and knowledge to understand it is another issue entirely; but it is a verifiable engineered solution including proof using a TCC with cable damage curves plotted. Granted it is maybe outside the typical cookbook solution, but it meets all wording of the NEC.

Honestly, the industry I'm in, the Owner's don't need permits, and everything is engineered and checked/verified. Our designs are way over the minimum requirements needed to meet the NEC safety codes and thankfully I haven't seen or had to deal with an inspector since the early 80's. Back then though a nice quality jelly filled doughnut and some coffee pretty much got you past every issue.:hug:

 

[iceworm]

...; but it is a verifiable engineered solution including proof using a TCC with cable damage curves plotted. ...

Yes, very likely true

... but it meets all wording of the NEC. ...

No it does not. You have been given the code sections that show your method to not be per NEC.

... , the industry I'm in, the Owner's don't need permits, and everything is engineered and checked/verified. Our designs are way over the minimum requirements needed to meet the NEC safety codes ...

That's great. And I am sure your designs/installations are safe and reliable. Not having an AHJ inspection does not change NEC relevance.

Here is my translation of your transformer primary design method (1000V or less):
Size the primary Over Current Device to where it does not trip on inrush - up to 250% xfm FLA (300% in some cases)
Size the primary conductors at 125% xfm FLA
Secondary conductors/ocp sized per individual application. Generally 125%, next size up CB is okay.

You seem certain you are correct. That's fine. Please cite the applicable code sections. If I have been doing this wrong for the last 15+ years. I need to know and correct the error of my ways.

Don't just tell us it meets code - Show us.

 

[topgone]

Yes, very likely true


No it does not. You have been given the code sections that show your method to not be per NEC.


That's great. And I am sure your designs/installations are safe and reliable. Not having an AHJ inspection does not change NEC relevance.

Here is my translation of your transformer primary design method (1000V or less):
Size the primary Over Current Device to where it does not trip on inrush - up to 250% xfm FLA (300% in some cases)
Size the primary conductors at 125% xfm FLA
Secondary conductors/ocp sized per individual application. Generally 125%, next size up CB is okay.

You seem certain you are correct. That's fine. Please cite the applicable code sections. If I have been doing this wrong for the last 15+ years. I need to know and correct the error of my ways.

Don't just tell us it meets code - Show us.

Agreed. Transformers and other inductive equipment will draw high current when energized! That's why we are allowed to size the OCPD to consider the inrush!

 

[newtonwb]

You seem certain you are correct. That's fine. Please cite the applicable code sections. If I have been doing this wrong for the last 15+ years. I need to know and correct the error of my ways.

Don't just tell us it meets code - Show us.

I believe Kingpb's method meets code as long as the conditions in 240.21(B)(3) are met.

 

[infinity]

Here is my translation of your transformer primary design method (1000V or less):
Size the primary Over Current Device to where it does not trip on inrush - up to 250% xfm FLA (300% in some cases)
Size the primary conductors at 125% xfm FLA
Secondary conductors/ocp sized per individual application. Generally 125%, next size up CB is okay.

You seem certain you are correct. That's fine. Please cite the applicable code sections. If I have been doing this wrong for the last 15+ years. I need to know and correct the error of my ways.

Don't just tell us it meets code - Show us.

I'm guessing that you see the problem as the part in bold.

 

[iceworm]

I'm guessing that you see the problem as the part in bold.

Ahhh :?... Who is the "you"?

Would that be me, or newton, or somebody else?

 

[infinity]

Ahhh :?... Who is the "you"?

Would that be me, or newton, or somebody else?

"You" is the person I quoted in the post #47.

 

[iceworm]

"You" is the person I quoted in the post #47.

Hummmm .......... I'm lost.

My position is summarized in post 40, 02/08/19 - two months ago. I even covered newton's issue.

I have not added anything new.

I'm not understanding the context of your post 47.

 

[infinity]

The part in red is not code complaint, using an OCPD larger than the conductor size. I believe that was your point.

Size the primary Over Current Device to where it does not trip on inrush - up to 250% xfm FLA (300% in some cases)
Size the primary conductors at 125% xfm FLA

 

[iceworm]

The part in red is not code complaint, using an OCPD larger than the conductor size. I believe that was your point.

Okay. That's close enough to what I said in post 40.

This conversation was finished 2 months ago. Newton add a comment about something already covered. That's okay with me, but it's not anything I'm planning to respond to.
I am still clueless about your context. Where you are going?? :?

 

[infinity]

Okay. That's close enough to what I said in post 40.

This conversation was finished 2 months ago. Newton add a comment about something already covered. That's okay with me, but it's not anything I'm planning to respond to.
I am still clueless about your context. Where you are going?? :?

You're right the thread is old so no need for me to go any further.