Motorola GTR 8000, Manual

Page: 103 / 346

3.4.3.2 DC Power Connection Wire Gauge Calculations for Integrated Voice and Data
3.4.3.2 DC Power Connection Wire Gauge Calculations for
Integrated Voice and Data
Since the power supply disconnects itself from the DC input when it senses that DC voltage has dropped to 42 VDC,
it is important to minimize the voltage drop in the DC power supply loop (the total length of the 48 VDC hot wire
and the DC return wire) to no more than 1 V total. This ensures that the maximum energy is removed from the
battery prior to disconnecting the power supply from the DC input line.
A base radio transmitting at 100 W will draw up to 10 A* current when operating from a 54 V source (nominal 48
VDC system). As voltage decreases (due to the standby battery discharging) the current will increase proportionally
(since the base radio appears to be a constant power load). At the low voltage disconnect point (42 V for a nominal
48 VDC system), the current will be up to 13 A*. If a single pair of 2 AWG wire is used to connect the battery to the
back panel, the maximum length of a single conductor would be 75m (245 ft). Use of smaller gauge wire would
reduce this length depending on the resistance of the wire.
* = The actual current value can be calculated from the power consumption value in the specifications tables. See
1.9 GTR 8000 Base Radio/GPW 8000 Receiver Specifications, page 1-12 .
To determine the maximum length of wire for wire other than 2 AWG, the following relationship can be used:
• Length (meter/feet) = V/I/R
where:
• V = voltage drop in one leg of the loop (max = 0.5V)
• I = current drawn by the base radio during DC operation
• R = resistance of the wire being considered (in Ohms per foot)
For common wire sizes, the maximum distances shown in Table 3-3 apply.
Table 3-3 DC Power Connection Wire Gauge Maximum Distances for Integrated Voice & Data
AWG
Resistance (ohm/304.8 meter/
1000 ft)
Maximum Distance (for 13A)
2 0.1563 75m (245 ft)
3 0.1970 60m (195 ft)
4 0.2485
47m (155 ft)
5
0.3133 37m (120 ft)
6 0.3951 30m (95 ft)
3.4.3.3 DC Power Connection Wire Gauge Calculations for HPD
Since the power supply disconnects itself from the DC input when it senses that DC voltage has dropped to 42 VDC,
it is important to minimize the voltage drop in the DC power supply loop (the total length of the 48 VDC hot wire
and the DC return wire) to no more than 1 V total. This ensures that the maximum energy is removed from the
battery prior to disconnecting the power supply from the DC input line.
A base radio transmitting at 50 W will draw up to 7.4 A current when operating from a 54 V source (nominal 48
VDC system). As voltage decreases (due to the standby battery discharging) the current will increase proportionally
(since the base radio appears to be a constant power load). At the low voltage disconnect point (42 V for a nominal
6871022P86-A - November 2012 3-19