This mainly applies to residential, but is somewhat applicable to commercial as well. When doing main panel change-outs, I've learned to pull the cover on the existing panel and count the number of conductors entering the panel. I've bid a couple of jobs by looking at the number of breakers, only to find when I started the work that most of the breakers had two wires connected to them - and they weren't Square D load centers. Rather than terminating 15 circuits, I may actually be terminating 20 or 30. The cost of additional circuit breakers, plus the additional time of terminating the wires, adds up pretty quick.
Also, I've learned to carefully check to see how easy it will be to move existing circuits or install new ones - even if I have to climb in the attic or crawl under the house. Having an obstacle in your way could add several feet to each circuit run. Multiply that by 30 circuits and you could easily have several hundred feet of additional wire.
Kevin
