# Installation, connection and sizing of cables with UPS

The low power UPSs, for micro computer systems for example, are compact ready-to-use equipement. The internal wiring is built in the factory and adapted to the characteristics of the devices.

For the other UPSs, the wire connections to the power supply system, to the battery and to the load are not included.

Wiring connections depend on the current level as indicated in Figure N28 below.

Fig. N28 – Current to be taken into account for the selection of the wire connections

### Calculation of currents I1, Iu

• The input current Iu from the power network is the load current
• The input current I1 of the charger/rectifier depends on:
• The capacity of the battery (C10) and the charging mode (Ib)
• The characteristics of the charger
• The efficiency of the inverter
• The current Ib is the current in the connection of the battery

These currents are given by the manufacturers.

## Cable temperature rise and voltage drops

The cross section of cables depends on:

• Permissible temperature rise
• Permissible voltage drop

For a given load, each of these parameters results in a minimum permissible cross section. The larger of the two must be used.

When routing cables, care must be taken to maintain the required distances between control circuits and power circuits, to avoid any disturbances caused by HF currents.

## Temperature rise

Permissible temperature rise in cables is limited by the withstand capacity of cable insulation.

Temperature rise in cables depends on:

• The type of core (Cu or Al)
• The installation method
• The number of touching cables

Standards stipulate, for each type of cable, the maximum permissible current.

## Voltage drops

The maximum permissible voltage drops are:

• 3% for AC circuits (50 or 60 Hz)
• 1% for DC circuits

### Selection tables

Figure N29 and N30 indicates the voltage drop in percent for a circuit made up of 100 meters of cable. To calculate the voltage drop in a circuit with a length L, multiply the value in the table by L/100.

• Sph: Cross section of conductors
• In: Rated current of protection devices on circuit

### Three-phase circuit

If the voltage drop exceeds 3% (50-60 Hz), increase the cross section of conductors.

### DC circuit

If the voltage drop exceeds 1%, increase the cross section of conductors.

Fig. N29 – Voltage drop for three-phase circuits, 50-60 Hz, 380 V / 400 V / 415 V, cos φ = 0.8, balanced system 3L+N. Values in percent for a circuit of 100m (copper conductors)
In (A) Sph (mm2)
10 16 25 35 50 70 95 120 150 185 240 300
10 0.9
15 1.2
20 1.6 1.1
25 2.0 1.3 0.9
32 2.6 1.7 1.1
40 3.3 2.1 1.4 1.0
50 4.1 2.6 1.7 1.3 1.0
63 5.1 3.3 2.2 1.6 1.2 0.9
70 5.7 3.7 2.4 1.7 1.3 1.0 0.8
80 6.5 4.2 2.7 2.1 1.5 1.2 0.9 0.7
100 8.2 5.3 3.4 2.6 2.0 2.0 1.1 0.9 0.8
125 6.6 4.3 3.2 2.4 2.4 1.4 1.1 1.0 0.8
160 5.5 4.3 3.2 3.2 1.8 1.5 1.2 1.1 0.9
200 5.3 3.9 3.9 2.2 1.8 1.6 1.3 1.2 0.9
250 4.9 4.9 2.8 2.3 1.9 1.7 1.4 1.2
320 3.5 2.9 2.5 2.1 1.9 1.5
400 4.4 3.6 3.1 2.7 2.3 1.9
500 4.5 3.9 3.4 2.9 2.4
600 4.9 4.2 3.6 3.0
800 5.3 4.4 3.8
1,000 6.5 4.7
• For a three-phase 230 V circuit, multiply the result by √3
• For a single-phase 208/230 V circuit, multiply the result by 2
Fig. N30 – Voltage drop for DC circuits. Values in percent for a circuit of 100m (copper conductors)
In (A) Sph (mm2)
- - 25 35 50 70 95 120 150 185 240 300
100 5.1 3.6 2.6 1.9 1.3 1.0 0.8 0.7 0.5 0.4
125 4.5 3.2 2.3 1.6 1.3 1.0 0.8 0.6 0.5
160 4.0 2.9 2.2 1.6 1.2 1.1 0.6 0.7
200 3.6 2.7 2.2 1.6 1.3 1.0 0.8
250 3.3 2.7 2.2 1.7 1.3 1.0
320 3.4 2.7 2.1 1.6 1.3
400 3.4 2.8 2.1 1.6
500 3.4 2.6 2.1
600 4.3 3.3 2.7
800 4.2 3.4
1,000 5.3 4.2
1,250 5.3

## Special case for neutral conductors

In three-phase systems, the third-order harmonics (and their multiples) of single-phase loads add up in the neutral conductor (sum of the currents on the three phases).

For this reason, the following rule may be applied:

neutral cross section = 1.5 x phase cross section

### Example

Consider a 70-meter 400 V three-phase circuit, with copper conductors and a rated current of 600 A.

Standard IEC 60364 indicates, depending on the installation method and the load, a minimum cross section.

We shall assume that the minimum cross section is 95 mm2.

It is first necessary to check that the voltage drop does not exceed 3%.

The table for three-phase circuits on the previous page indicates, for a 600A current flowing in a 300 mm2 cable, a voltage drop of 3% for 100 meters of cable, i.e. for 70 meters:

${\displaystyle 3\times {\frac {70}{100}}=2.1\%}$

Therefore less than 3%

A identical calculation can be run for a DC current of 1,000A.

In a ten-meter cable, the voltage drop for 100 meters of 240 mN2 cable is 5.3%, i.e. for ten meters

${\displaystyle 5.3\times {\frac {10}{100}}=0.53\%}$

Therefore less than 3%