Main Breaker Problem

[xbanone]

I just installed a Square D QO, 100 amp sub panel in the garage. I connected to the main panel with mobile home feeder 2-2-4-6. There are only 5 circuits currently wired in the garage:

(2) 20 amp outlet runs,
(1) circuit for outside lights,
(1) for inside lights and
(1) 30 amp air compressor

First we turned on the Main panel breaker, then the 100amp sub panel breaker and began turning on the other breakers. At some point the sub panel 100 amp breaker sparked and than began buzzing, one of the over head lights flickerd (the last one in the chain) and the compressor tried to start but wouldn't turn over. We turned off all of the breakers. The 100amp main breaker in the sub panel smells burnt, bench checking it with an ohm meter, it shows no continuity between the lugs and the terminals,

What could cause this? Have you heard of new breakers being faulty out of the box? or was it something I did? All of my work was overseen by an electrician and I have a good bit of experience with house wiring and i believe everything to be installed correctly.

 

[Gary S]

If you have any significant resistance across the breaker when closed, it is trashed. You need a new one. First check that you have things wired correctly. I'm not an electrician, but everything I read says that #2 aluminum isn't adequate for 100amps., and on top of that, your neutral appears undersized for the job.
My first guess is that you have something wired incorrectly between the main and sub.

 

[xbanone]

I forgot, the breaker at the Main Panel is an 80 amp feeding the 2-2-4-6 MHF. I edited my post to show that. The 100amp breaker in the sub panel is the one that fried. I emailed square D and they are sending a new breaker.

 

[Gary S]

A new breaker is the right place to start. This time, before you start putting any loads on the new panel, check the voltage in and out of the new breaker. Missing voltage indicates a mis-wireing. Low voltage indicates a poor connection. Then start with a breaker with small 120v load like some incandescent bulbs. Measure voltage again at the breaker. Do this for both sides of the 240v main breaker in the sub panel, and make sure you put loads on both sides for the measurements.

 

[rlitman]

When you bought the breaker, was it tripped (did you see the orange trip indicator in the window)? ALL Square D breakers ship tripped from the factory (from testing). If it wasn't tripped when you got it, someone futzed with it before it reached your hands.

Sounds like a defective breaker to me too. The main breaker should never spark.

 

[pattenp]

If you have any significant resistance across the breaker when closed, it is trashed. You need a new one. First check that you have things wired correctly. I'm not an electrician, but everything I read says that #2 aluminum isn't adequate for 100amps., and on top of that, your neutral appears undersized for the job.
My first guess is that you have something wired incorrectly between the main and sub.

The #4 neutral is okay. #2 MHF comes sized as 2-2-2-4 or 2-2-4-6.