Why can't I make the distribution system be exactly the size I want?
Usually it is because Pennsylvania regulates the aspect ratio of the distribution system. Longer-skinnier systems work better hydrologically than squarish systems, especially as the slope increases and the regulations reflect this.
When you enter size constraints for a distribution system, the software gives you the system dimensions that both meet Pennsylvania regulations AND are the closest match to what you asked for. Closest match means the system that has the least amount of area outside of the bounds you specified. The software uses the specific size you give as the target, but will pick the system that has the least amount of overlap outside of the box you specify, but it also has to be a valid configuration.
Why isn't this what I thought I was going to get?
The software increments width in one foot increments (except Eljen's where it is 4 ft or 6 ft). For example, you cannot generate a 600 sq ft sand mound that is 12.5' by 48'.
Eljen GSF's are modular. Widths will bump out in 4 ft or 6 ft increments (depending on perc rate), lengths in 3 ft increments. And there are aspect ratio constraints on systems with slopes over 8% to exasperate the problem.
Are you asking for an aspect ratio that is allowed? All system types have minimum aspect ratios for steeper slopes and some, such as at-grades, have minimum aspect ratios for all slopes.
How do I get around this?
Make sure you specify both width and length constraints. The internal sizing algorithm needs both most of the time.
IF the system is for a repair you can toggle on “Best Technical Guidance” (BTG) on the site data page of the design wizard. BTG relaxes some of the rules to allow off-regulation configurations for repair systems. Invoking Best Technical Guidance is allowed ONLY for repairs, it is NOT allowed for new construction.