# Copper Bus Bar Ampacity

#### infinity

##### Moderator
Staff member
I have a panelboard with 2" wide by 1/4" thick copper bus (the straight portion in center of the photo). How would one determine the ampacity of that bus?

#### drktmplr12

##### Senior Member
I have a panelboard with 2" wide by 1/4" thick copper bus (the straight portion in center of the photo). How would one determine the ampacity of that bus?
https://www.copper.org/applications/electrical/busbar/bus_table1.html

1/4" x 2", 60 Hz, 30 degree C Rise: 710 A
1/4" x 2", 60 Hz, 50 degree C Rise: 940 A
1/4" x 2", 60 Hz, 65 degree C Rise: 1100 A
Skin effect: 1.04

The table assumes emissivity of 0.4 for the copper, indoors, 40 degree C ambient

#### infinity

##### Moderator
Staff member
I did take a look at that table online, what would be the temperature rise for a panelboard?

#### electrofelon

##### Senior Member
This gets into the busbar debate. I don't think we can size bus bars in general. IIRC there is only one place it seems to be allowed, auxiliary gutters maybe? Maybe you can convert the size to AWG and size it like it was an article 310 conductor? Off the top of my head I can't think of any prohibition from doing that.

#### sameguy

##### Senior Member
I used to have a chart that showed cooper pipe amps to replace a fuse , it was lost in many hard drive crashes.

#### Jraef

##### Moderator
Staff member
I did take a look at that table online, what would be the temperature rise for a panelboard?
UL requires 50C max. rise, NEMA is 60C. As a general rule on a UL listed panelboard, 1/4 x 2” is typically 800A max.

#### don_resqcapt19

##### Moderator
Staff member
While is doesn't apply to this application, the NEC specifies the ampacity of copper bus at 1000 amp per square inch in 366.23(A).

#### infinity

##### Moderator
Staff member
While is doesn't apply to this application, the NEC specifies the ampacity of copper bus at 1000 amp per square inch in 366.23(A).
I looked at that number too which would make this 2"x.25" bus good for 500 amps which to me seems about right when I compare it the bus in some other panels on site. The bus rating of this panelboard is supposed to be 1200 amps.

#### jim dungar

##### Moderator
Staff member
UL requires 50C max. rise
Most UL Listed panelboards are rated based on how hot they get on the inside, rather than by current density.

#### electrofelon

##### Senior Member
I looked at that number too which would make this 2"x.25" bus good for 500 amps which to me seems about right when I compare it the bus in some other panels on site. The bus rating of this panelboard is supposed to be 1200 amps.
Rob, so this isnt anything you are modifying or making, just "double checking" a factory panelboard?

#### infinity

##### Moderator
Staff member
Rob, so this isnt anything you are modifying or making, just "double checking" a factory panelboard?
Yes, according to the submittal 2 panelboards were supposed to be rated for 1200 amps each including the one with the 2" bus in the photo in the OP. When comparing that bus to several other panelboards of various sizes it seems a little small for that ampacity.

#### ggunn

##### PE (Electrical), NABCEP certified
Something we have found out is that not every differently rated panelboard has differently rated busbars. We have documentation from Eaton that all their Cutler-Hammer panelboards with ratings from 150A to 225A have 225A busbars.

#### ATSman

##### ATSman
For copper, all of the panelboards and switchgear we have inspected thru the years usually
follows the 1000A per square inch cross-section rule. So for that panel it would be 500 A per phase.

#### infinity

##### Moderator
Staff member
For copper, all of the panelboards and switchgear we have inspected thru the years usually
follows the 1000A per square inch cross-section rule. So for that panel it would be 500 A per phase.
That was the number I used in my initial analysis. Even if it's highly inaccurate it still gets you nowhere near the 1200 amp rating that the panelboard is supposed to have.