Cisco Systems MGX 8230 manual DC Current Distance in Feet

Chapter 3 Site Preparation

Power and Grounding

As Figure 3-3shows, the non isolated system has a 48 VDC return that internally connects to the backplane. (This design calls for a hard-wired return and so does not allow for an optional or alternate ground connection.) The internal connection provides a low-impedance connection between 48 VDC return and frame ground. This grounding scheme protects the signals on the backplane from corruption by transients that can result from lightning or electrostatic discharge.

To improve protection against transients, the loop area (and resultant loop impedance) should be made as small as possible by locating the –48 VDC supply, 48 VDC return, and protective earth conductors as close to each other as possible.

As recommended in ITU-T K.27, the multi point grounding in a mesh bonding network provides the best protection for equipment by providing the lowest impedance in the ground system. For more detailed information, refer to the recommendation itself.

Conductor Characteristics for Carrying Current and Ensuring Low Voltage Drops

To prevent signal degradation, a conductor must be large enough to prevent its impedance from creating a voltage drop equal to 2 percent of the reference voltage. Also, the protective earth conductor must be large enough to carry all the current if the 48 VDC return fails. This latter requirement is for safety. Full fault redundancy is achieved by having equal size conductors for the protective earth ground and the 48 VDC return of the switch.

For wire gauges that prevent unacceptable voltage drops over different lengths of copper wire, see Table 3-2. For the resistance of 1000 feet of copper wire for each gauge of wire, see Table 3-3. These references are for planning purposes and may be further subject to local laws and practices.

Table 3-2 Wire Gauge for Current Loads Over Copper Wire Lengths

DC Current	Distance in Feet
	25 feet	50 feet	75 feet	100 feet	150 feet	200 feet	400 feet
5A	18	14	14	12	10	8	6
10A	14	12	10	8	8	6	2
15A	14	10	8	8	6	4	2
20A	12	8	8	6	4	2	0
25A	12	8	6	4	4	2	0
30A	10	8	6	4	2	2	00
35A	10	6	4	2	2	1	000
40A	8	6	2	2	2	0	000
45A	8	6	4	2	1	0	0000
50A	8	4	4	2	1	00	______
55A	8	4	2	2	0	00	______
60A	8	4	2	2	0	00	______
65A	6	4	2	1	0	000	______
70A	6	4	2	1	00	000	______
75A	6	4	2	1	00	000	______
100A	4	2	1	00	000	______	______

	Cisco MGX 8230 Edge Concentrator Installation and Configuration
3-20	Release 1.1.31, Part Number 78-11215-03 Rev. B0, May 2001