# Voltage Drop on POE



## Navyguy (Mar 15, 2010)

Anybody have a good source for voltage drop calculations for POE? Not seeing much online or anything like the Southwire or others that we can reference to for standard electrical requirements.

I suppose I could do the calcs by hand, but they do not have to bee too accurate, so was hoping for a quick and dirty online calculator.

Cheers
John


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## joe-nwt (Mar 28, 2019)

PoE calculator for Power over Ethernet


PoE calculator for Power over Ethernet, calculates required and delivered power and voltage drop for voltages from 5 to 56 volts



poe-world.com


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## Navyguy (Mar 15, 2010)

I was looking at that one, but it always comes up with the same answer... maybe I am using it wrong...

Cheers
John


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## splatz (May 23, 2015)

The calculator at Voltage Drop Calculator will do a DC calculation and lets you use the number of sets to punch in the number of pairs. The type of PoE in use (passive or negotiated between the switch or injector and the device) will determine the number of pairs active. I think all quality / compliant Cat5e is 24 gauge copper but some Cat6 is 23 awg. 

But I think this is really the device maker's headache, PoE devices are designed to function properly right up to the 100m ethernet distance on standard compliant cable.


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## Navyguy (Mar 15, 2010)

splatz said:


> The calculator at Voltage Drop Calculator will do a DC calculation and lets you use the number of sets to punch in the number of pairs. The type of PoE in use (passive or negotiated between the switch or injector and the device) will determine the number of pairs active. I think all quality / compliant Cat5e is 24 gauge copper but some Cat6 is 23 awg.
> 
> But I think this is really the device maker's headache, PoE devices are designed to function properly right up to the 100m ethernet distance on standard compliant cable.


It is really the switch I am worried about. While we (as electricians) tend to always look at the device and voltage at the end of the line (from an electrical perspective), some managed switches have the ability to control output; I am concerned about maxing out switches that exist in situ... If the device is pulling 100% at 100m, I should be looking at more then the +/-9W at each device and total watt output of the switch / port.

Well that is what I am thinking anyway.

Cheers
John


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## gpop (May 14, 2018)

Most of our stuff has the power injector mounted in the field so VD was never a issue.


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## R777V (May 16, 2016)

Navyguy said:


> Anybody have a good source for voltage drop calculations for POE? Not seeing much online or anything like the Southwire or others that we can reference to for standard electrical requirements.
> 
> I suppose I could do the calcs by hand, but they do not have to bee too accurate, so was hoping for a quick and dirty online calculator.
> 
> ...


How long are your runs really? The voltage is usually jacked up pretty high on most POE isn’t it? I mean that’s why they can power up devices that far downstream right? That voltage drop was figured in already. Unless you are exceeding your distance limits, then you’re asking for trouble. Then you’re going to have to make up a custom non UL listed Chinese rig, or find something listed for that excessive length.


Sent from my iPhone using Tapatalk


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## splatz (May 23, 2015)

Navyguy said:


> It is really the switch I am worried about. While we (as electricians) tend to always look at the device and voltage at the end of the line (from an electrical perspective), some managed switches have the ability to control output; I am concerned about maxing out switches that exist in situ... If the device is pulling 100% at 100m, I should be looking at more then the +/-9W at each device and total watt output of the switch / port.


That is true, the line losses will be more if all your runs are 300' rather than all your runs 20'. I'd bet the worst case would be like 10% of the device power consumption. 

I personally wouldn't worry that much about it because I definitely don't push the power supply capacity on switches, I want them using 50-65% of the rated capacity at idle. But then I think PoE switches are highly overrated, except for phones. I will take injectors for important equipment any day.


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## Navyguy (Mar 15, 2010)

For me, the highlight of POE is the fact of not having to run the 120V source to the device in the first place. If I am running out of capacity at a switch, I already have a power source there, so that is not to much of an issue.

Cheers
John


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## kb1jb1 (Nov 11, 2017)

Keep in mind that you might have to use CMLP or CMPLP cable. Or other cable substitutions because the cables are now carrying power and not just DATA.


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## splatz (May 23, 2015)

Navyguy said:


> For me, the highlight of POE is the fact of not having to run the 120V source to the device in the first place. If I am running out of capacity at a switch, I already have a power source there, so that is not to much of an issue.


Same here. I'd rather deal with regular switches which usually make it ten years without breaking a sweat, and PoE injectors installed at the distribution frame, near the switches. 

I have used multiport PoE injectors that use a separate DC power supply of your choice. I was able to use two power supplies for redundancy and run the switches (industrial type) off the same redundant power source.


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## Tonedeaf (Nov 26, 2012)

If you are using CAT 6 wire and if you stay under 300LF, you should not have a problem with POE. I have installed many WAP's and Cameras on the fringe of 300' and haven't had any problems...once you go over the issues arise and you need to use a boaster or have another IDF fed from a fiber.


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