Over current protection

Status
Not open for further replies.
Mike01 said:
. . . yes 500?s (paralleled) are compliant on an 800A OCPD, it helps that the load does not exceed the allowable 760A
Click to expand...
No, that is not a "it helps" situation. It's a deal breaker. If the load is 761 amps or higher, then a pair of 500's is a code violation. Furthermore, if you don't actually have a load calculation, if you do not know whether the load is above or below 760, if all you know is that the OCPD is 800 amps, then IMHO a pair of 500's is still a code violation. The violation is in article 316, not 240.
 
charlie b said:
Once again, you haven't been pickin' up what I have been puttin' down::wink:
Click to expand...

Don't say I didn't warn you :D

charlie b said:
Nothing in 240 is going to say anything about what conductor I can, or cannot use, for a given load. 240 is all about the breaker or fuse. I am talking about selecting a conductor to serve a load.

Posit: Calculated load is 91 amps. Conductor size is #4 cu. OCPD is set at 70 amps. (OK, I concede this is not a good design. But the NEC is not a design manual.)
Question: What NEC articles, if any, have I violated?


celtic said:
So we would be limited to to 90A OCPD [240.6(D) ]
Click to expand...
Click to expand...


charlie b said:
I see where we are having trouble communicating.
Click to expand...
It's gonna happen with me....yes, it's me, not you. :D


charlie b said:
The question is not whether an 800 amp breaker can be used to protect a pair of 500 MCM copper conductors. It can, for the reasons you gave. I agree with you on everything you have said. Happy now?
But again, that is not the question. The question is whether a pair of 500 MCM copper conductors can be used for this application. You don’t answer that question by looking at the breaker. You answer that question by calculating the load.
Click to expand...
hmmm.....what about :
(above the ampacity of the conductors being protected)
Click to expand...
EDIT TO ADD:
above quote from 240.4(B)
 
OK. Promise me that for the rest of this discussion, you won't look in article 240. Please?

You can protect a conductor beyond its ampacity. You can't load a conductor beyond its ampacity. That's the difference. And you won't find the difference in 240 alone.
 
charlie b said:
OK. Promise me that for the rest of this discussion, you won't look in article 240. Please?
Click to expand...
The best I can do is try not to.



charlie b said:
You can protect a conductor beyond its ampacity. You can't load a conductor beyond its ampacity. That's the difference. And you won't find the difference in 240 alone.
Click to expand...

OIC said the blind man....kind of like with A/C equipment, right?
 
agree

agree

Charlie B I agree with you 100% regarding the load issue, the load is what drives the conductor size and in turn drives the OCPD size, I just wanted to point out some of the other issues to be aware of and that it needs to be looked at on a project by project basis to make a blanket statement that 500?s are good for 800A would be incorrect.
 
Mike01 said:
... yes 500?s (paralleled) are compliant on an 800A OCPD, it helps that the load does not exceed the allowable 760A...
...But to me the short of it is yes 500?s are acceptable...
...so all I am saying is yes it is code compliant...
Click to expand...

__________
 
The way that I look at it is that one should establish what the load is first, then that cable is sized to carry the load after which the breaker is selected to protect the cable.
As such it was determined that (2) 500kcmil cables was sufficient to carry the load where you are allowed to protect that cable with a breaker rated equal to the rating of the cable. If it is not a standard rating then you are allowed to select the next higher standard rated breaker, which would be 800at. With cable ampacities greater than 800a then the OCPD chosen would be equal to that ampacity, if it's not a standard rating then you have to select the an OCPD with a closest standard rating below.
 
charlie b said:
Posit: Calculated load is 91 amps. Conductor size is #4 cu. OCPD is set at 70 amps. (OK, I concede this is not a good design. But the NEC is not a design manual.)
Question: What NEC articles, if any, have I violated?
Click to expand...

How about Article 310? Sections 310.2(B) > 310.10 > 310.15(A)(1) > 310.15(B) > Table 310.16? :D
Do I win?
 
Volta said:
How about Article 310? Sections 310.2(B) > 310.10 > 310.15(A)(1) > 310.15(B) > Table 310.16?
Click to expand...
Volta said:
Do I win?
Click to expand...

Sorry, not a winner. Please play again. :smile:

310.2(B) says I can use AL wire. No help there. 310.10 comes close to what I was looking for. But in my posit statement, I say I am protecting the #4 with a 70 amp breaker, so there is no risk of overheating the insulation. 310.15(A)(1), (B)(1), and the table all speak of ampacity. But where, exactly, does the NEC say that I have to pick a conductor whose ampacity meets or exceeds the calculated load? :-?
 
That's it. Thanks.

And that brings us back to the original question. If the load is under 760 amps, then you can use a pair of 500's. Otherwise, you need to go with a larger conductor, or more parallel conductors of a smaller size.
 
charlie b said:
That's it. Thanks.

And that brings us back to the original question. If the load is under 760 amps, then you can use a pair of 500's. Otherwise, you need to go with a larger conductor, or more parallel conductors of a smaller size.
Click to expand...


Yup,

Now what happens when you have twin 500's with a 760 load calc and 800A breaker, and want to add a 10A load?...or more like what will happen?...change out the conductors, right?:smile:
 
If it were my job to add the load, I would first revisit the load calculation, to see if I could justify a smaller number. If that failed, I would take a 30 day load measurement, and use the results to justify adding load. That has a good chance of success.

Or did you ask what is most likely to happen in the "real world"? :wink:
 
charlie b said:

Sorry, not a winner. Please play again. :smile:

310.2(B) says I can use AL wire. No help there. 310.10 comes close to what I was looking for. But in my posit statement, I say I am protecting the #4 with a 70 amp breaker, so there is no risk of overheating the insulation. 310.15(A)(1), (B)(1), and the table all speak of ampacity. But where, exactly, does the NEC say that I have to pick a conductor whose ampacity meets or exceeds the calculated load? :-?
Click to expand...

Oops. I meant to type 300.2(B), not 310.2(B), limiting temperatures.
That would lead down the path I showed, which I think would work. However, Wasasparky hit it on the nose directly with the 215.2 and 210.19. Much better.
 
charlie b said:
Celtic, this is trickier than I had expected, especially since I am struggling to find code articles to prove my point.

I think we would all agree that if you had a load of 91 amps, you could not use a #4 copper conductor to serve that load. Am I right? The 310.16 ampacity is 85 amps, and the load is 91 amps. Bad choice of conductors, perhaps, but where exactly in the code does it say I can?t do that?

Let me start there, and I?ll tell you later where this question leads me.
Click to expand...

Charlie B

I was doing a search and happend to come across this thread.

I dont understand why you are saying that in the OP if the load is greater than 761 A (cable rating) then it is a violation, but in your example above you are saying it is o.k, or not a code violation to use a 85A rated cable for a 91A calculated load. Are you saying in your example that it is o.k. if the load is greater then the cable rating.
 
philly said:
. . . but in your example above you are saying it is o.k, or not a code violation to use a 85A rated cable for a 91A calculated load. Are you saying in your example that it is o.k. if the load is greater then the cable rating.
Click to expand...
I was not saying that it is OK. It's not OK. I know that it's not OK. I was asking if anyone knew exactly which code article it would violate.

(Edited to add: Wasasparky answered that question for me in post #31, as I acknowledged in post #32.)
 
Last edited:
charlie b said:
I was not saying that it is OK. It's not OK. I know that it's not OK. I was asking if anyone knew exactly which code article it would violate.
Click to expand...


240.4 Protection of Conductors.
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
 
Status
Not open for further replies.
Top