Originally Posted by R Funk
There are 3 issues that come to play in a situation like this:
1) What does the manufacturer say. In other words does the UL listing of the panel include a breaker in that position? Read the info on the inside of the panel/panel cover or go out on the web. The information should be there on the manufacurer's site.
2) You need to go through the National Electric Code (NEC) calculations for the connected load and application. This is carefully laid out in the NEC. These calculated values are usually well under 50% of the connected load (sum of all loads conected)
3) The actual measured load on the panel. It will most likely be well below the capacity of the main breaker and probably below the NEC calcualtions.
Editorial Note: It is amazing how low the total typical electrical load on panel is due to diversity of the load. Most of us will not be running a welder, drill press, plasma burner etc at the same time. Go to to Harbor Freight and buy their amp meter (I paid $10.00 or $12.00 for mine if I remember correctly but is now superceded by a $100 meter I purchased later) and measure the typical amp draw. Of course I should recomend a Fluke or other major US company but you will spend almost 10 times more. However I found the HF meter's performance suprisingly acceptable.
Buy the way to pass UL listing the breaker must trip at no more than 120% of rated load after 30 minutes. I doubt if you will continously load the main at full load for more than 5 minutes at a time or so....
Unofficial Comment: Is the electrical inspector going to be involved? If not install it (if allowed by the panel manufacturer) and let her rip. Most panels are so lightly loaded in typical non industrial applications the main will never trip. If it does what does it hurt as long as you don't have your mother in law on life suport equipment in the house or your wife is in a bad mood, if this panel feeds your house also? However make certain that you still follow the NEC for the materials and workmanship.
Trust this helps
There will not be much info from the manufacturer since Challenger has been gone for around 20 years & they built shitty panels too, but there is not enough of a bus stab to install another branch breaker next to the backfed main, the only breakers to my knowledge that are UL classified to fit Challenger panels are Eaton (formerly Cutler-Hammer) since OEM breakers are no longer made. Since this is a old thread, if someone needed to add a breaker, then check if the panel has notched bus stabs, and if so get a couple of twin breakers & use them replace 4 single pole breakers which will leave 2 spaces open for a 240 volt feed.
OEM circuit breakers are UL listed for the same make of panel, some makes are UL classified to be used in some competitive makes, such as Eaton and Siemens make UL classified replacements for SQ D QO breakers, but no SQ D breaker is classified to be used in competitive panels, if a SQ D HOM, is found in a ITE/ Siemens, GE, Bryant/Westinghouse/Cutler-Hammer/ Eaton BR,or Murray, panels then there is a sure sign that hack electrical work has been done.