Circuit Breaker Rating / Conductor Size

Status
Not open for further replies.

Mike01

Senior Member
Location
MidWest
When sizing overcurrent protective devices (low voltage power cirucit breakers with adjustable LSIG) 240 of the NEC requres 800A or more the conductors are required to match the trip setting. I looked at a drawing today (one-line) that indicated 4-sets of 500kcmil with a 1600AF/1600AT setting, however the 75 degree rating of the conductors would make it 380A @ 4-sets is only 1520A, so the long time setting would have to be set to 1520A or below. Are there any terminations avaliable for 90 degrees that would allow me to get the full 1600A rating. I have been searching but have not been able to locate a 90 degree termination lug? Any suggestions or is 4-500's acceptable for a 1600A setting?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The important thing is the rating of the termination, not that of the rating of the lug.

Any protective device, listed to UL489, will be limited to conductor sizing per the 75?C column.
 

Mike01

Senior Member
Location
MidWest
main lugs

main lugs

Jim, I agree however the connection to the low voltage switchgear would be to the bus bar stabs, and the connections to the switchboard downstream is a main lug only so there is no UL489 OCPD in the cirucit I assume since the "terminations" are to the bus assembly that using 90 degree terminations / lugs would be acceptable in achieving the 90 degree rating?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Jim, I agree however the connection to the low voltage switchgear would be to the bus bar stabs, and the connections to the switchboard downstream is a main lug only so there is no UL489 OCPD in the cirucit I assume since the "terminations" are to the bus assembly that using 90 degree terminations / lugs would be acceptable in achieving the 90 degree rating?
If this is switchgear (i.e. breakers listed to UL1066) then it would be supplied with 90?C terminations.

Switchboard construction (i.e. UL489 breakers and UL89 structure) will have the 75?C limitation regardless where the conductors are landed, unless 'special' provisions have been made.

Your OP made no mention of the standards to which the equipment is built.
 

kwired

Electron manager
Location
NE Nebraska
Jim, I agree however the connection to the low voltage switchgear would be to the bus bar stabs, and the connections to the switchboard downstream is a main lug only so there is no UL489 OCPD in the cirucit I assume since the "terminations" are to the bus assembly that using 90 degree terminations / lugs would be acceptable in achieving the 90 degree rating?
I think the breaker is going to be designed for 75oC no matter what is attached to it.
 

Mike01

Senior Member
Location
MidWest
If this is switchgear (i.e. breakers listed to UL1066) then it would be supplied with 90?C terminations.

Switchboard construction (i.e. UL489 breakers and UL89 structure) will have the 75?C limitation regardless where the conductors are landed, unless 'special' provisions have been made.

Your OP made no mention of the standards to which the equipment is built.

Jim, I apoligize the equipment is swtichgear ANSI rated switchgear with UL1066 listed LVPCB's, if I recall the terminations are made directly to the bus stabs in the back of the gear not to the breaker itself, also the downstream switchboard is a UL891 rated switchboard with a main lug only assembly, I assume this would be a compression type lug secured to the bus bars in the switchboard not to the terminals of a UL489 device (ICCB, or MCCB) so one could provide 90 degree lugs for connection to the main bus in the switchboard. The easier thing may be to just set the long time of the upstream device to protect the cables (1520A).
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
...the downstream switchboard is a UL891 rated switchboard with a main lug only assembly, I assume this would be a compression type lug secured to the bus bars in the switchboard not to the terminals of a UL489 device (ICCB, or MCCB) so one could provide 90 degree lugs for connection to the main bus in the switchboard. The easier thing may be to just set the long time of the upstream device to protect the cables (1520A).

I am not sure of the rating of the termination pad at the UL891 switchboard, but I believe it is rated for '75?C sized conductors' even though it is 100% for ampacity. It is all about the heat rise allowed in the termination compartment.

I would go with adjusting the LT.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Jim, I think you need to swap the rating plug or CT's, not just dial down the LTPU?
The NEC allows 'dialing down' as long as the access to the dial is restricted, even with something as simple as a cable tie.
 

zog

Senior Member
Location
Charlotte, NC
Huh. Learn something new everyday. Not sure I agree with it though, accessable only to "qualified personel". How many times have you seen someone "qualified" respond to a breaker trip and say "Well here is your problem, this isn't dailed up all the way"?
 

kwired

Electron manager
Location
NE Nebraska
Huh. Learn something new everyday. Not sure I agree with it though, accessable only to "qualified personel". How many times have you seen someone "qualified" respond to a breaker trip and say "Well here is your problem, this isn't dailed up all the way"?

I can probably name quite a few that thought they were qualified to do something like that:happyyes:
 
Status
Not open for further replies.
Top