AT&T MERLIN LEGEND Installation & Maintenance Manual page 108

Preparation
Unit Loads for the Hybrid/PBX
Mode
The 391A1 power supply module generally supports six modules
of any type in a Hybrid/PBX system. However, the unit loads on a
carrier can exceed 54 if both of the following exist:
all six carrier slots have only MLX telephone or analog
multiline telephone modules installed
the carrier has a total of more than 45 MLX-20L or 34-button
analog multiline telephone stations installed
Unit Loads for the Key or Behind
Switch Mode
In a Key or Behind Switch system with four or fewer modules, no
calculation is needed. The 391A1 power supply module
generally supports four modules of any type in Key or Behind
Switch mode.
Calculating Unit Loads
Calculate the unit loads of each carrier by completing the steps
on the worksheet in Appendix D.
2-16 Unit Loads