-48V DC cabling examples?

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Question by Anonymous visitor posted 28 Apr 2000
icon -48V DC cabling examples?
Does anyone know of any information sources on wiring up -48 V equipment?
I'm curious to see what cable and wiring practices people use.
The only site I've actually seen so far used some sort of marine grade of stranded cable. All the cable was black and the only color coding was numbering at the end of each cable.
Does this fall under any regulations?

Currently I've got all the equipment in one rack. I have a ServerTech Sentry-48 distributing power from the A and B DC busses. I've used black, red and green stranded cable to supply power from the Sentry to the devices. It works but seems almost too colorful compared to the few telco sites I've seen.

Anyone?

Answer by Dmitri Abaimov posted 28 Apr 2000
icon In some circumstances it may be more convenient to use telecom cables (4-pairs UTP for ex.) to bring power to your equipment. Especially in those cases when a device needs to send some signal to central equipment and up to 3 pairs is enough pair count for that (as RS 232).
Generally, cable manufacturers specify 1 Amp limitation per 1 conductor in 24 AWG UTP cable or 3.3 Amps total through all 4 pairs. Telecom cables are considered low-voltage so usually there is also voltage limit of 40V.
Telecom cables are coming out of the closet, forming star topology, so it's very convenient to have low-voltage power supply unit in a closet, serving floor(s), corresponding to the closet. To do that, however, you'll have to make some calculations to find out, what's the maximum distance you may bring devices away from power supply unit.
Consider device's operational current, allowable voltage drop and UTP's DC resistance, which is typically 57.2 ohms per 1000ft or 18.8 ohms per 100 m.
Dividing allowable voltage drop by device's current you'll get your circuit resistance. Then dividing circuit resistance by cable DC resistance, you'll get maximum distance.
For color code: use tip (white wire) for positive, ring (colored blue, orange, green or brown) for negative conductor.
If you really have to use marine grade power cable due to environmental issues, please disregard everything above because those telecom UTP's are usually indoor use only unless specially created for OSP (but still not marine) applications.

With best regards,
Dmitri Abaimov, RCDD

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