Calculating conduit-fill tells you how large your conduit should be. Here are some principles that govern the calculations . . . .
There are two primary principles that govern conduit sizing. The first is making sure the cables won’t be damaged during installation. If their insulation is damaged their conductors can be exposed to their surroundings and cause faults. The second is ensuring that the cables have enough “breathing room” so that they don’t overheat during normal operation. There is also a third: don’t use a bigger conduit than you need because the bigger they are, the harder they are to install (especially in congested areas).
To avoid installation damage, you’ll typically need to evaluate pull tension and sidewall pressure on your cables through your planned route. Pull tension measures how much force is applied to pull the cables through, and sidewall pressure is a crushing force applied to cables around conduit bends. The longer the route and the more bends there are, the higher the pressure and tension. You don’t want to exceed 360-degrees of conduit turns without adding a pull box. This is an NEC requirement (Article 358.26) that often informs the requirements documented in plant procedures.
There’s also another factor to consider that’s directly related to conduit size, and that’s jam ratio (see NEC Table 1, Chapter 9, Informational Note 2). Jam ratio is calculated by dividing the inner diameter of the conduit by the outer diameter of the cable. Jam ratio applies when pulling three cables of the same diameter through a conduit. There is a common range — 2.8 to 3.2 — that you should avoid. If you size your conduit based on the NEC allowable raceway fill, but then you calculate your jam ratio and discover that it’s within that range, then you probably need to up-size your conduit.
A pull box should be installed after every third or fourth conduit bend at a minimum to mitigate the risk of cable damage. You’ll want to analyze these three factors — pull tension, sidewall pressure, and jam ratio — when doing your design to ensure that the cables will be as easy to install as possible.
A good resource that introduces you to the calculation basics is here: http://www.pdhonline.org/courses/e122/e122.htm