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I need to feed 12 volts into an Ethernet cable. The vendors datasheet states:

Power Method: Passive Power over Ethernet (pairs 4, 5+; 7, 8 return)

Does that mean I connect both the 4 and the 5 to the same 12 volts positive cable and 7 + 8 to the negative? Is this standard procedure? It seems weird to me to split up the volts and send them through 2 cables.

Please note that I'am a beginner in this field and thus I have only a very basic understanding of how electronics work.

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  • \$\begingroup\$ I'm assuming that the datasheet doesn't have instructions on how to install the ethernet cable? If not, then that's weird and very unusual for a vendor to give you something that would help you install this. \$\endgroup\$ – KingDuken Mar 3 '18 at 20:24
  • \$\begingroup\$ You're supposed to use a PoE switch or injector. This is a standard way of providing power; you aren't expected to hook it up yourself. \$\endgroup\$ – duskwuff Mar 3 '18 at 20:27
  • \$\begingroup\$ The datasheets suggests I buy another product to get power into the cable. I don't want to do that ;) I have already bought the device on the other end of the cable. And since I use that device on a boat where everything is 12 volts I thought I might as well hook it up myself. \$\endgroup\$ – Mats Mar 3 '18 at 20:34
  • \$\begingroup\$ Which vendor? Vendor of what? We're not mind readers so give us a clue. \$\endgroup\$ – Finbarr Mar 4 '18 at 23:52
  • \$\begingroup\$ PoE uses 48 volts so make sure there's no misunderstanding... \$\endgroup\$ – immibis Mar 6 '18 at 3:25
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Yes, you would connect 4/5 (blue/blue-white) both to VDC+, and 7/8 (brown/brown-white) both to VDC-. That's standard to the pinout for PoE.

Ethernet cables use very thin gauge wire (23-24AWG). Doubling them up increases the amount of current you can safely put over the cable.

Before you do that though, definitely look into how tolerant your input device will be to being directly connected to your boat's power system. Either your boat's power or the equipment would have to be carefully regulated. Otherwise, you could easily damage it by exposing the equipment to several volts above or below the target 12V between battery charge and discharge cycles.

It's also worth noting that 12V is not common in PoE systems. Most operate around 46VDC. Can't speculate further without having the datasheet for your equipment. Either way, the power adapter they recommend is probably for very good reason.

At the very least, you will want to put an inline fuse on the VDC+ side.

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  • \$\begingroup\$ Thanks for your reply! The device I'm powering in a Ubiquity Bullet M and can run with either 12 or 24v. As for regulating the power for the Ethernet cable, I will look into that. \$\endgroup\$ – Mats Mar 6 '18 at 11:57
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    \$\begingroup\$ You can get a DC-DC converter to step up the voltage and provide some regulation if necessary. A quick search found a few things for golf carts and cars that might work for under $20 and are waterproof. Can't say for sure without knowing a lot more about both. one example here Also make sure that you select the smallest fuse possible that allows it to run under normal conditions. Depending on the fault, you're risking both the antenna and your computer being shorted to full battery current. \$\endgroup\$ – Phil C Mar 6 '18 at 17:00

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