800 amps or less next standard size ocp

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Jraef

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800 amps or less next we use next standard size ocp.... 800 amps or more we use next standard size down ocp? correct?
No.

And I have no idea where that concept came from, makes no sense. Might have been some local company standard for some other reason, though I can't imagine wjhat that would be. If you NEED 860A for a load, putting in an 800A breaker is a waste of time.
 

augie47

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800 amps or less next we use next standard size ocp.... 800 amps or more we use next standard size down ocp? correct?

almost... devices over 800 the conductor ampacity must be equal or greater
(the way u stated it "800" was is both categories)

simple rule.. calculate the load, select the conductor, select the OCP device.
IN any event the conductor must be adequate to carry the load)
 

Smart $

Esteemed Member
Location
Ohio
As noted... but looking at it the other way around

800A or less OCPD rating, the conductor ampacity value has to be greater than the next lower standard OCPD rating value.

Over 800A OCPD, the conductor ampacity must equal or exceed the OCPD rating value.

In all cases, the conductor ampacity must equal or exceed the calculated load.
 
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mayanees

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Electrical Engineer and Master Electrician
Supervised Industrial Installations

Supervised Industrial Installations

But there's also 240.91(B) that allows a conductor ampacity of 95% of the ocpd for Devices Rated over 800 Amps for Supervised Industrials. This is the one that allows (4) 500 kCMs (1520A ampacity) to be protected by a 1600-A trip breaker.
 

iwire

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Staff member
Location
Massachusetts
But there's also 240.91(B) that allows a conductor ampacity of 95% of the ocpd for Devices Rated over 800 Amps for Supervised Industrials. This is the one that allows (4) 500 kCMs (1520A ampacity) to be protected by a 1600-A trip breaker.

That section is new for 2014 and seeing as you are aware of it I have to assume you know the limitations of it. :)

But for the rest of us that do not work in industrial here is some more info.

VIII. Supervised Industrial Installations

240.90 General.
Overcurrent protection in areas of supervised
industrial installations shall comply with all of the
other applicable provisions of this article, except as provided
in Part VIII. The provisions of Part VIII shall be
permitted only to apply to those portions of the electrical
system in the supervised industrial installation used exclusively
for manufacturing or process control activities.

I will also add that IMO 'supervised' means qualified engineering spervision not just a 'Shift supervisor Vinnie'. ;)
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
That section is new for 2014 and seeing as you are aware of it I have to assume you know the limitations of it. :)
I think it's new for 2011.
But your clarification is an important point to make - that the application is not only limited to a supervised installation, but also that it's used exclusively for manufacturing.
 
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