Sheet capacity (the primary rating used for comparison) is determined by feeding letter size paper into the throat, 8½" edge first. The weight of the paper will determine how may sheets can be shred in one pass. Some competitors have been known to use onion-skin stock for testing to make their models seem more powerful. Dahle's sheet capacity ratings are based on standard #20 and #16 bond, hence an approximate rating of 32 / 28 sheets.
Even though the same number of sheets is being fed through the machine, a jam occurs when feeding the 11" edge instead of the 8-1/2" edge because the cutting surface has been increased by 30%. This explains why a shredder having a 12 sheet rating will constantly jam on 12 sheets of 14-7/8" wide computer printout.
Remember that the sheet capacity rating is intended as a reference for comparison, similar to the MPG rating in new car showrooms. The user's actual sheet capacity (like the MPG of his car) will vary with operating conditions. Actual sheet capacity can vary 1 or 2 sheets, up or down, depending on the voltage available to the motor at the time, atmospheric conditions or quality of the paper being shred.
On any shredder, a sudden drop in sheet capacity usually signals a need for cutter lubrication. Oiling the cutting head will instantly bring the sheet capacity back into its normal range.
A final point is that in actual use, the operator does not count out sheets before dropping them into the feed opening. A 'feel' for thickness is developed rather quickly by the operator, knowing that overfeeding and a subsequent jam, will add more time to the overall task. A good rule of thumb is to try and operate a shredder at 80% or less of its capacity rating.