Name that Conductor!

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Dale Hayes

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An Industrial Control Panel wired as follows:

1. The load side of a 250Amp Circuit breaker is connected to the primary side of a Power Distribution Block (PDB) using 250MCM wire (rated at 255Amp).

2. From the Power Distribution Block there is a 12GA tap (rated at 25A) wired to to a 25Amp fuse.

All of these components are located inside the Industrial Control Panel. The 12GA tap conductor connected between the secondary of the PDB and the line side of the 25Amp fuse is less than 10FT.

A remote heater is connected to the 25Amp fuse. The conductor used to make the connection between the 25Amp fuse that is located inside the control panel and the remote heater ... is it considered a tap, a branch circuit conductor or a feeder?
 
The conductor that leaves the enclosure would be a branch circuit if the 25A fuse is the final protective device. It would be a feeder if there is another protective device downstream. These NEC terms are independent of the wiring internal to the control cabinet.

I think you are concerned with the wire sizing. The conductors that leave the cabinet need to sized per the NEC. The wiring internal to the cabinet may be sized using different standards, such as UL508A. It is very common to see a 'field connection terminal block' with internal wiring of one size and external wiring significantly larger. So in your case the #12 internal wiring would need to be changed to #10 as it leaves the cabinet.
 
Jim,

How does the "location" of the OCPD at the the termination of the tap conductor affect the 10FT and 1/10th rule on tap conductors?

Is there a difference if the tap terminates on the OCPD (in my case the 12Amp fuse) that is located inside the Industrial Control Panel or if the tap conductor were to leave the enclosure and is terminated on a 12Amp fuse 10Ft away? Is the conductor considered a tap conductor until it reaches the OCPD and from there is defined as a branch conductor?
 
Dale Hayes said:
Jim,

How does the "location" of the OCPD at the the termination of the tap conductor affect the 10FT and 1/10th rule on tap conductors?

Is there a difference if the tap terminates on the OCPD (in my case the 12Amp fuse) that is located inside the Industrial Control Panel or if the tap conductor were to leave the enclosure and is terminated on a 12Amp fuse 10Ft away? Is the conductor considered a tap conductor until it reaches the OCPD and from there is defined as a branch conductor?
Click to expand...

For the most part, NEC tap rules do not apply 'inside' to equipment built and listed to specific standards such as UL508A.
The #12 conductor form the PDB to the fuse is a tap. Tap conductors are usually not allowed to leave the Listed equipment, conductors external to the equipment are subject to NEC article 240.
 
Jim,

My 12GA tap terminates inside the Industrial Control Panel on the 25Amp fuse. Therefore the connection from the load side of the 25Amp fuse to the heater is considered a branch conductor and is covered in NEC Article 240?
 
Dale Hayes said:
Jim,

My 12GA tap terminates inside the Industrial Control Panel on the 25Amp fuse. Therefore the connection from the load side of the 25Amp fuse to the heater is considered a branch conductor and is covered in NEC Article 240?
Click to expand...
Yes for the portion from the 'field' terminations outward.

Is your panel "Listed"? Most listing require some type of explanation concerning sizing or provisions for 'exterior' wiring.
 
Dale Hayes said:
Jim,

How does the "location" of the OCPD at the the termination of the tap conductor affect the 10FT and 1/10th rule on tap conductors?

Is there a difference if the tap terminates on the OCPD (in my case the 12Amp fuse) that is located inside the Industrial Control Panel or if the tap conductor were to leave the enclosure and is terminated on a 12Amp fuse 10Ft away? Is the conductor considered a tap conductor until it reaches the OCPD and from there is defined as a branch conductor?
Click to expand...

there is no such rule if the tap conductor does not leave the enclosure.

BTW, UL508a was "harmonized" along with NFPA79 several years ago so wire sizes are all the same now.
 
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