Appendix A: System calculations
246 EST3X Technical Reference Manual
Calculating the maximum CAB cable length
1. Calculate the total current (Itot) as the sum of the maximum operating
currents for all the appliances.
Itot = ΣIa
Where:
Ia = appliance maximum current
Refer to the appliance installation sheets for Ia. Remember to use the
maximum operating current specified for DC power.
2. Calculate the minimum voltage (Vm).
Vm = Vr − (Itot × K)
Where:
Vr = source voltage
Itot = total current (from above)
K = load factor
For the PS10-4B power supply, Vr is 22.4 V and K is 0.0 V/A.
For the 3-IDC8/4 power supply, Vr is 21.3 V and K is 0.0 V/A.
For the 3-ZA20/40 power supply, Vr is 21.7 V and K is 0.0 V/A.
3. Calculate the allowable voltage drop (Vd) between the NAC circuit source and
the appliances.
Vd = Vm − Va
Where:
Vm = minimum voltage (from above)
Va = appliance minimum voltage
For regulated notification appliances, Va is 16 V. For special application
appliances, Va is the lowest operating voltage specified on the appliance
installation sheet.
4. Calculate the maximum resistance (Rmax) the wire can have.
Rmax = Vd / Itot
Where:
Vd = voltage drop
Itot = total current
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