Over current protection

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We have a 800 amp sub panel being feed with a 800 amp breaker. The engineer is saying us 2x500 mcm cu per phase, is this correct or should 2x600 mcm cu be used?
 

charlie b

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Lockport, IL
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Why is nobody asking the only question that really matters: "What is the calculated load"? :-?

That is always "step 1." Once you know the load, you select a conductor that has sufficient ampacity (a term that takes into account the conditions of use, such as ambient temperature and number of current-carrying conductors in a raceway) for that load. Picking the overcurrent device setting is the final step.

So, what is the load? If it is 760 amps or lower, then you can use a pair of 500's. If it is 761 amps or more, you cannot.
 

celtic

Senior Member
Location
NJ
Why is nobody asking the only question that really matters: "What is the calculated load"? :-?


So, what is the load? If it is 760 amps or lower, then you can use a pair of 500's. If it is 761 amps or more, you cannot.
:-? Charlie...hold my hand for bit?

2005 NEC said:
240.4(B) Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a multi outlet branch circuit supplying receptacles for cord and- plug-connected portable loads.

(2)
The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

I know what you are saying...I just don't see it?

Let's assume [to keep things simple]....it's an 18" nipple w/o any derating of any sort needed...

240.4(B)(2) makes mention of "ampacity"...not "calculated load".
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
Celtic, you are on step 3. I am talking about step 2. If the load is, for example, 750 amps (as calculated in step 1), then I can use a pair of 500's. That is because the ampacity of a pair of 500's is 760 amps, and that is more than the load.

Now that I have picked the conductors, let's pick the OCPD. 760 amps is not a standard setting, and it is lower than 800, so I can use an OCPD setting of 800, which is the next higher standard value.
 

celtic

Senior Member
Location
NJ
Celtic, you are on step 3. I am talking about step 2. If the load is, for example, 750 amps (as calculated in step 1), then I can use a pair of 500's. That is because the ampacity of a pair of 500's is 760 amps, and that is more than the load.

Now that I have picked the conductors, let's pick the OCPD. 760 amps is not a standard setting, and it is lower than 800, so I can use an OCPD setting of 800, which is the next higher standard value.

Ever talk to a block of wood?
Now's your chance
icon14.gif


Why are we calculating step 1?


(B) Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord and- plug-connected portable loads.
If we have an 800A OCPD, it stands to reason this is not going to be a BC ~ but rather a feeder ~ that will be supplying "multi outlet branch circuit supplying receptacles for cord and- plug-connected portable loads.".
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
Why are we calculating step 1?
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.
 

celtic

Senior Member
Location
NJ
Celtic, this is trickier than I had expected, especially since I am struggling to find code articles to prove my point.

I appreciate your time and efforts here Charlie.


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?

Assuming all basic stuff [short run, no derating, 75?, etc]...
Yes...
A #4 is 85A [T310.16]...and would not be allowed for 91A...240.4(B)(3) disallows it:
(3) The next higher standard rating selected does not exceed 800 amperes.

So we would be limited to to 90A OCPD [240.6(D) ]



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

No problem...we can pick it up later on.
Thanks again.
 

Mike01

Senior Member
Location
MidWest
500mcm vs 600mcm....

500mcm vs 600mcm....

I guess I would start with if the documents called for 500’s I would install 500’s, looking deeper into the question yes 500’s (paralleled) are compliant on an 800A OCPD, it helps that the load does not exceed the allowable 760A, the only problem you might have with 600’s depending on the manufacturer and the type of terminations you are required to provide (mechanical lugs or compression lugs if compression single indent or double, long barrel or standard) the problem you will run into if you are landing on a breaker is will the breaker accept parallel 600mcm cable some will not depending on the termination style, another problem could be is the required wire bending space for a 500 vs a 600mcm cable and the available space within the panel, this is more a problem with panelboard construction vs switchboard, just another item to consider when making decisions nothing worse than getting the phone call that they pulled the 600’s and are ready to terminate however it might void the UL label if not installed properly. But to me the short of it is yes 500’s are acceptable, not knowing the entire situation amount or current carrying conductors, conductor length, specified terminations, ambient temperature, type of equipment you are terminating in, temperature rating of terminations, all these play a role, 600’s might be nice but not necessarily required., and if it’s not required why should the owner pay for it? all of this understanding that the code is a design minimum so all I am saying is yes it is code compliant. jsut my 2 cents..for what it's worth..
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Once again, you haven't been pickin' up what I have been puttin' down::wink:
A #4 is 85A [T310.16]...and would not be allowed for 91A...240.4(B)(3) disallows it:
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?
 

charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
No problem...we can pick it up later on.
OK, now to finish the story. I see where we are having trouble communicating. 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.
 
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