Channel Length and PoE Calculator

How it works

It works out how long the permanent link is allowed to be, then checks yours against it.

1. It starts from a 90.0 m permanent link.
2. Above 20 °C ambient, it takes a temperature derate off that for every degree. Unscreened cable loses length faster than screened, and faster again above 40 °C. The derate stops growing at 60 °C. At 20 °C or below there's no derate.
3. The channel is 100 m end to end, so the allowed link is the shorter of the derated length and 100 m minus your cord total. With 10 m of cords or less, the derated length is always the shorter.
4. Your installed link is checked against the allowed length.

The PoE type doesn't change the length. It adds that type's power figures to the result, and a note on heat for the four-pair types.

The calculator opens on Unscreened (UTP) cable at 35 °C, 10.0 m of cords, 80.0 m installed and Type 4 (802.3bt) PoE. The heat takes 5.4 m off the 90.0 m baseline, so 84.6 m is allowed, and the link reads PASS, DERATED.

The derating follows ANSI/TIA-568.2-D and the PoE figures IEEE 802.3af, at and bt, cited by designation under every result.

Reading the result

The big number is your installed link, read against the allowed length. The allowed length sits under the bar, and the tick on the bar marks it. The lines below show the baseline, the derate and the cords it came from, then the PoE figures.

• PASS: the link fits, the cords are 10 m or less, and the ambient is 20 °C or below.
• PASS, DERATED: the same pass with the ambient above 20 °C, so less than 90.0 m is allowed.
• TOO LONG: amber when the link is over the allowed length by 5% or less. Red when it's further over, or when the cords are over 10 m, whatever the link length. Both are failures, each with a WON'T PASS line.

Over the allowed length, the fix line says what to shorten the link to. Make the opening job 88.0 m and it reads TOO LONG in amber, with the fix line: Shorten to 84.6 m, shorten the cords, or get the ambient down.

Anything that needs attention goes in one of three buckets under the stamp:

• WON'T PASS: the link is longer than the allowed length, shown as installed against allowed at your ambient. Or the cords are over 10 m, because cords over 10 m eat the permanent link one for one.
• WON'T LAST: any pass with a derate, because it passes at today's ambient and a sunlit tray or an un-cooled attic lowers the limit. And Type 3 or Type 4 PoE on a link that fits: they energise all four pairs, the cables in the middle of a big bundle run hottest, and that heat derates the length. Bundle size, ambient and PoE type have to be read together.
• JUST UGLY: this calculator doesn't raise one.

What it doesn't check

• Bundle size. There's no bundle term in the length. The four-pair PoE note is the reminder, and the bundle guidance it points to is TSB-184-A.
• Heat past 60 °C. The derate stops growing there, so a hotter ambient gets the 60 °C allowance, while the result still shows the temperature you entered.
• The cable or the test. It doesn't know the category and it doesn't certify anything. A tester still has to pass the link.

Questions

Does the PoE type change how long the run can be?

No. Every PoE type gets the same allowed length. The type adds its power figures to the result: watts at the PSE, watts guaranteed at the PD, pairs energised and the most current per pair. On a link that fits, Type 3 and Type 4 also add a WON'T LAST note, because they energise all four pairs and the cables in the middle of a big bundle run hottest.

What ambient temperature should I enter?

The hottest it gets where the cable runs, not the temperature on the day you're there. A pass above 20 °C carries a WON'T LAST note for that reason: a sunlit tray or an un-cooled attic lowers the limit.

What if my cords add up to more than 10 m?

The result is TOO LONG even when the link fits. The allowed link becomes the shorter of the derated length and 100 m minus the cords. At 20 °C with 15.0 m of cords, 85.0 m is allowed, and 80.0 m installed reads TOO LONG for the cords alone. Exactly 10 m of cords is fine.

Does a cold space give me more length?

No. At 20 °C or below there's no derate and no bonus: with 10 m of cords or less, the allowed link stays at 90.0 m. Below-zero ambients are accepted.

Why does a link right at the allowed length read TOO LONG?

The allowed length is compared before it's rounded to 0.1 m for display, and a few limits come out a hair under the figure shown. Screened cable at 40 °C shows 86.4 m allowed, and 86.4 m installed reads TOO LONG. A link sitting on the limit has no room left either way.