Some couplings result in a 1/8″ or more pipe separation, after they are added to pipe. On long runs of pipe, the multiple separations accumulate, and result in a pipe that is too long overall. For example, a run of pipe with 16 couplings can result in the pipe creeping 1/8″ per coupling for a total of 0-2″. A pipe segment somewhere in the run of main should be adjusted 2″ shorter so the overall main length will be correct.
The example below shows mains that are a total of 168′-0″ long from end to end. The main on the bottom has couplings cut in at exactly 10′-6″ – resulting in 16 pieces exactly 10′-6″ long. The main on the top has couplings cut in at 10′-6 1/8″ – resulting in 15 pieces rounded to 10′-6″ long and 1 piece 10′-4″ long. Pipe dimensions are rounded to the nearest 1/2″ or 1/4″ so the extra 1/8″ in each pipe segment does not show up in the reported length. The INSTALLED length of both mains will end up being 2″ longer than the total of the added “pipe length dimensions”.
After installation the bottom main will be 168′-2″ long.
After installation the top main will be 168′-0″ long as required.
The H-Factor 
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