Liebert UPS System

Chapter 3 Electrical Assembly

This chapter introduces the connection methods of the power and signal cables.

3.1 Selection of cables

3.1.1 Current carrying capacity of cables

The current carrying capacity can be selected according to the voltage drop and

permissible temperature increase.

Table 3-1 Current carrying capacity of cables

Cross sectional area(mm2) Current

carrying

capacity (A) 1 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240

Rubber 21 27 35 45 58 85 110 145 180 230 285 345 400 470 540 660

25°C

Plastic 19 24 32 42 55 75 105 138 170 215 265 325 / / / /

Rubber 20 25 33 42 54 80 103 136 168 215 267 323 374 439 505 617

35°C

Plastic 18 22 30 39 51 70 96 129 159 201 248 304 / / / /

3.1.2 Selection of power cables

The power cables include input power cables of main circuit and that of the bypass,

the UPS output cables, battery input cables, protective earth cables and the

lightning protection cables.

1. Calculation of current of power cables

The first calculation method is done according to the following formula:

If the power factor is 0.80, the efficiency is 0.9, the minimum input voltage is 176

volts, output line to neutral voltage is 220V, minimum battery voltage is 324 V,

bypass overload capability is 135% and that of the inverter is 125%,

2. Selection of cable cross sectional areas

Power cables are selected according to two factors - 3 – 5 A/mm2 current density

and the maximum voltage drop be less than 3 W.

The cross-sectional areas of bypass input neutral cable and UPS output neutral

cable should be 1.5 times of the phase cable. The cross sectional area of the