PoE Maximum Cable Length Calculator
Estimate whether a PoE device can run on a proposed copper channel, then see the maximum length allowed by voltage, current, PSE power, and the 100-meter Ethernet channel limit.
Maximum allowable channel length96.86
- Design status
- Passes the selected PoE type and channel length checks
- Estimated voltage at device
- 43.11
- Power drawn from PSE
- 25.52
- Cable and connection loss
- 3.52
- Channel loss percentage
- 13.78%
- Current per powered pairset
- 0.51
- Remaining PD power allowance
- 3.5
- Minimum passing conductor size
- 26 AWG
- Limiting constraint
- Minimum PD voltage
Uses Type 2 - 802.3at with 1 powered pairset and 50 V at the PSE.
Adjusted device demand is 22.00 W after headroom; selected-conductor resistance is 0.14442 ohm/m at 40 C.
How to use this calculator
- Choose the PoE type supported by both the switch or injector and the powered device.
- Enter the device maximum input power, then add headroom for startup loads and accessories.
- Enter the complete channel length, including patch cords at both ends.
- Select the actual copper conductor gauge and the expected cable temperature.
- Read the maximum length, device voltage, loss, current, and minimum passing gauge.
How the PoE length is calculated
This calculator treats the powered device as a constant-power load. The stated device load is first increased by the design headroom:
P = device power x (1 + headroom / 100)
The selected PoE type sets a conservative source voltage, a minimum voltage at the powered device, maximum delivered PD power, maximum PSE power, maximum current per powered pairset, and the number of powered pairsets. Type 1 and Type 2 use one pairset in this model. Type 3 and Type 4 use two pairsets, which represents four-pair operation.
Copper resistance is adjusted for temperature:
rT = r20 x [1 + 0.00393 x (temperature C - 20)]
For a channel length L, added connection resistance Rc, and m powered pairsets, the equivalent PoE power-loop resistance is:
R = rT x L / m + Rc
The device voltage is the high-voltage solution of the constant-power equation:
Vpd = (Vs + sqrt(Vs^2 - 4 x P x R)) / 2
If the term under the square root is negative, the simplified model has no steady operating point. Otherwise current is I = P / Vpd, current per pairset is I / m, PSE power is Vs x I, and cable loss is PSE power minus delivered device power.
How the maximum run is found
Instead of asking only whether one length works, the calculator reverse-solves the longest electrical run. It finds the required device voltage from the strictest of three limits:
Vreq = max(Vmin, P / (m x Imax), Vs x P / PSEmax)
The allowable loop resistance is then:
Rallow = Vreq x (Vs - Vreq) / P
After subtracting added connection resistance, the remaining resistance budget is converted back into length with the temperature-adjusted resistance for the selected gauge. The result is capped at 100 meters because that is the structured-cabling channel limit for Ethernet, even when the electrical voltage calculation could go farther.
What moves the result most
Device wattage and conductor resistance usually dominate. A hotter bundle raises copper resistance, so a marginal camera or access point can pass at room temperature and fail in a warm ceiling space. Four-pair PoE reduces resistance per watt by sharing current across two pairsets, but it only applies when both ends actually negotiate that mode.
What this leaves out
The calculation assumes solid annealed copper with balanced current sharing. It does not model copper-clad aluminum, damaged terminations, DC resistance unbalance, LLDP policy, switch power budgeting, stranded patch-cord heating, or downstream conversion efficiency inside the powered device. Use it for planning before checking the actual switch, injector, device, and cabling specifications.
Worked example
With the default inputs, a Type 2 802.3at device draws 20 W and the 10% headroom raises the design load to 22 W. A 26 AWG copper channel at 40 C has about 0.14443 ohm per meter per conductor. At a proposed 90 m plus 0.5 ohm of added connection resistance, the loop resistance is about 13.50 ohm.
The constant-power equation gives about 43.11 V at the device. The total current is about 0.51 A, PSE power is about 25.52 W, and cable plus connection loss is about 3.52 W. The reverse solve gives a maximum channel length of about 96.86 m, so the proposed 90 m run passes and 26 AWG is the thinnest listed conductor that passes.
Common questions
How far can PoE run over Cat5e, Cat6, or Cat6A?
Ethernet structured cabling is normally limited to a 100-meter complete channel, including patch cords. The PoE electrical limit can be shorter when the device load is high, the conductors are thin, the cable is hot, or connection resistance is high.
Why can a PoE device fail below the switch port wattage?
The switch port rating is measured at the PSE, while the device needs enough power and voltage at the PD input. Cable resistance turns some source power into heat, and the current limit can also be reached before the published wattage appears available at the device.
Why does conductor gauge matter more than cable category?
Category rating describes data performance, not a guaranteed DC resistance for every cable construction. A 26 AWG patch-style cable loses more voltage than a 23 AWG solid copper cable over the same length, even when both carry the same Ethernet category marking.
Does four-pair PoE reduce voltage drop?
Yes, when both the PSE and PD use four-pair 802.3bt operation. Current is shared across two powered pairsets, so the equivalent loop resistance in this model is lower than a two-pair run of the same conductor gauge.
Can PoE operate beyond 100 meters if the voltage calculation passes?
The voltage math may show extra electrical margin, especially with thick copper and a light load. That does not remove the Ethernet channel limit, so the calculator caps the allowable channel length at 100 meters.