EXTRAN uses a point junction formulation for calculating the depth of water in a junction. The use of a point junction formulation has been questioned by Joliffe  who suggests a momentum junction formulation is superior for predicting the combination of pipe flows, and a weir formulation is superior to the point junction formulation for dividing flows.
The point junction formulation is only adequate when the head loss at pits are small [Price, 1981]. An energy balance at manhole (j) for all incoming and outgoing conduits (i) can be formulated as:
The lost head is given by:
The loss coefficient of the junction is related to the following parameters and variables:
- The depth of flow in the connecting conduits.
- The depth of flow in the manhole.
- The angle between the incoming and outgoing conduits. This is assumed to be 180° in the EXTRAN Model.
- The type of flow (subcritical or supercritical flow).
- The geometry of the manhole and conduit.