# How to find ampacity of 1/2 by 4 copper buss bar

1. Jan 29, 2014

### Crazyhorse2882

How to find ampacity of 1/2" by 4" copper buss bar

I have a question about finding ampacity of buss bar. I have a 1/2" by 4" copper buss bar and the only formula I can find in the NEC is 1000/in^2 but when I use this formula I come up with 500 amps and I'm being told that 1/2" by 4" copper bus bar is good for 2000 amps. Can somebody help me please and explain what I'm missing. Thank you

2. Jan 29, 2014

### sandy.bridge

As far as I know, $1000/in^2$ is merely a "rule of thumb".

3. Jan 29, 2014

### nsaspook

4. Jan 30, 2014

### wirenut

Am I missing something here? You have 2in^2 of copper bus bar(4*.5=2) and 1000 A/in^2 should be 2000A.
Correct? And ,yes, as per NEC (US) {250A/(1/4 in^2)} is acceptable. So 1000A/in^2 is correct.

5. Jan 31, 2014

### Crazyhorse2882

I tell you why I'm confused, I read 1000/in^2 as 1000 divided by inches squared. I think the NEC would be right to rewrite that so it looks like multiplication

6. Feb 2, 2014

### subtech

7. Feb 7, 2014

From ABB Switchgear Manual Table 13-4 Continuous current carrying capacity [AC up to 60 c/sec]
For indoor installation at 35oC.copper bar max.temp. 65oC.Continuous load.
10*100 mm [0.4”*4”] 1810 A
10*120 mm [0.4*5”] 2110

8. Feb 7, 2014

### the_emi_guy

Your interpretation would imply that the larger the buss bar the less current it could carry, that should clue you in that somethings wrong.

Read it as 1000 amps per square inch for 2 square inches. (Driving 1000 miles per hour for 2 hours = 2000 miles).

Last edited: Feb 7, 2014
9. Feb 7, 2014