Question about Table 250.122.

reyamkram

Senior Member
Location
Hanover park, il
If the OCPD is ahead of the circuit I use table 250.122 for the EGC. ????
I have a plug in bus bar non-fused disconnect that feeds a 75 amp disconnect breaker for a CNC machine, So.
will the OCPD be behind.??? Do I used table 250.102 for the EGC???

There is no fuse in the bus plug disconnect, and the next OCPD is the 75 amp disconnect breaker for the CNC machine. Or do I use 250.102, for the
sizing of the EGC.

Thank you all, for any and all information.
 

kwired

Electron manager
Location
NE Nebraska
OCPD on the feeder ahead of the 75 amp device is what you need to base EGC on before that 75 amp device.

If you have a tap conductor situation the EGC running with tap conductors shouldn't need to ever be larger than the ungrounded tap conductors though.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Since your using tap conductors 250.122 is the proper table. 250.102(C) is for bonding jumpers which would not apply.
 

reyamkram

Senior Member
Location
Hanover park, il
If the OCPD is ahead of the circuit I use table 250.122 for the EGC. ????
I have a plug in bus bar non-fused disconnect that feeds a 75 amp disconnect breaker for a CNC machine, So.
will the OCPD be behind.??? Do I used table 250.102 for the EGC???

There is no fuse in the bus plug disconnect, and the next OCPD is the 75 amp disconnect breaker for the CNC machine. Or do I use 250.102, for the
sizing of the EGC.

Thank you all, for any and all information.
If there is no fuse then are you using the tap rule? What size breaker is protecting the bus?

Thank you for your reply,
The bus duck is feed from switch-gear, 208Y/120 volt 3 phase 4 W, supply is 2000 amps main. and that is the only bus duck on that switch.
I can not see what OCPD is on that switch gear, I would need to turn it off, and look inside, and that would shut down a lot of machines.

Thank you.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Thank you for your reply,
The bus duck is feed from switch-gear, 208Y/120 volt 3 phase 4 W, supply is 2000 amps main. and that is the only bus duck on that switch.
I can not see what OCPD is on that switch gear, I would need to turn it off, and look inside, and that would shut down a lot of machines.

Thank you.
So the bus duct is protected at 2000 amps and you're installing a tap. The EGC is sized based on the 2000 amp OCPD protecting the bus duct but the EGC is not required to be larger than the tap conductors. Using the 10' tap rule the minimum size tap conductors would be #3/0.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Curious as to the manufacturer.. ?? (I've not seen a NF Buss duct disconnect)
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Curious as to the manufacturer.. ?? (I've not seen a NF Buss duct disconnect)
I think that he's referring to something like this, a tap box that installs on the bus duct. This bus duct is from the early 1970's.

Bus Duct Tap Box.jpg
 

reyamkram

Senior Member
Location
Hanover park, il
So the bus duct is protected at 2000 amps and you're installing a tap. The EGC is sized based on the 2000 amp OCPD protecting the bus duct but the EGC is not required to be larger than the tap conductors. Using the 10' tap rule the minimum size tap conductors would be #3/0.
Thank you, for your insight, and infromation,
 

reyamkram

Senior Member
Location
Hanover park, il
Curious as to the manufacturer.. ?? (I've not seen a NF Buss duct disconnect)
Thank you for your reply,
you do have a good point, come to think about it, I do not recall working on any bus duck disconnect that was not not fused, I asked my boss, and he replayed there no fuses in the disconnect, after this weekend, and at start up, I can open the disconnect and see, and I will let you know,

again thank you.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Thank you for your reply,
you do have a good point, come to think about it, I do not recall working on any bus duck disconnect that was not not fused, I asked my boss, and he replayed there no fuses in the disconnect, after this weekend, and at start up, I can open the disconnect and see, and I will let you know,

again thank you.
He said " there are no fuses in the disconnect BUT does that mean they don't come with fuses or is it a fuseless disco. haha
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I did find out there are 3- 100 amp fuses, thank you again.
What size the conductors feeding the 75 amp breaker? You need 100 amp or larger conductors on the load side of the bus duct disconnect and an EGC selected from T250.122 as Dennis stated.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
What size the conductors feeding the 75 amp breaker? You need 100 amp or larger conductors on the load side of the bus duct disconnect and an EGC selected from T250.122 as Dennis stated.
There is no 75 amp breaker but perhaps one can be ordered
 
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