Feed through panel question

[jsherwin]

I am not an electrician by trade, but rather an researcher/course developer on PV electrical systems. Often times, I see things that are code compliant and have to explain or are asked questions as to why they are compliant. Such as:
Per 240.21, overcurrent protection devices must be placed at the point where the feeder conductors receive their power.
A common practice is to have feeders from "feed through panels" supply sub panels that have a MOCPD. Given 240.21, why is an ocpd not required at those feed through lugs from the main panel? The general statement that I hear is that the feeder is an extension of the bus, but I am looking for the article or explanation that allows this practice to be code compliant.

 

[augie47]

240.21(B) Tap Rules address situations where the feeder does not have OCP at the source.
In most cases the feeder is protected against overcurrent at the "load" end and is sized in such a manner (length-size) that the supply device provides adequate short circuit protection.

 

[infinity]

A common practice is to have feeders from "feed through panels" supply sub panels that have a MOCPD. Given 240.21, why is an ocpd not required at those feed through lugs from the main panel? The general statement that I hear is that the feeder is an extension of the bus, but I am looking for the article or explanation that allows this practice to be code compliant.

If all of the components are the same minimum size then the OCPD at the beginning of the feeder will protect everything downstream.
For example: (assuming that all conductors are a minimum of 200 amps)

200 amp OCPD>200 amp panel>feed-through>200 amp panel.

If you look at the example above you will see that the 200 amp OCPD at the origin of the feeder will protect all of the components. If any of the components are less than 200 amps then one of the tap rules may apply.
For example:

200 amp OCPD>200 amp panel>feed-through>100 amp conductors>100 amp panel.