Power over Ethernet Cable Length: How Far Can You Really Go?

The spec says 100 meters. That's the official answer for Ethernet cable length, and it's plastered across every textbook and training course.

But when you're deploying PoE cameras across a warehouse, or powering access points in a campus building, 100 meters starts to feel more like a suggestion than a guarantee. Because the real limiting factor isn't just signal integrity — it's power delivery. The cable has resistance. Resistance causes voltage drop. And if the voltage at the powered device drops below its minimum operating level, nothing works.

So the real question is: given your specific setup, how far can you actually go?

Why Cable Length Matters for PoE

PoE sends DC power over the same Cat5e/Cat6 cables that carry Ethernet data. The power travels over the copper conductors, which have a small but nonzero resistance. The longer the cable, the more resistance, the more voltage drops along the way.

There are three limits that constrain your maximum cable length:

Ethernet signaling limit — 100m per IEEE 802.3 (round-trip delay and signal integrity)
Voltage drop — the PD must receive at least its minimum required voltage
Power dissipation — too much loss in the cable causes heating and wastes energy

For data-only Ethernet, limit #1 is usually the binding constraint. For PoE, especially at higher power levels, limits #2 and #3 often kick in first.

Quick Reference: PoE Standards

Standard	IEEE	Max PSE Output	Min PD Voltage	Max PD Power
PoE	802.3af	15.4W @ 44V	36V	12.95W
PoE+	802.3at	30W @ 50V	42.5V	25.5W
PoE++ (Type 3)	802.3bt (Type 3)	60W @ 50V	42.5V	51W
PoE++ (Type 4)	802.3bt (Type 4)	90W @ 52V	41.1V	71.3W

The gap between "max PSE output" and "max PD power" is the power lost in the cable. That gap grows as cable length increases.

The Voltage Drop Equation

For PoE, power is delivered over two pairs (or all four pairs for 802.3bt Type 3/4). Each pair carries current through both the outgoing and return conductors, so the total loop resistance matters.

For 2-pair PoE (802.3af/at):

Vdrop = 2 × Iload × Rcable × L

Where:

Vdrop = voltage drop in volts
Iload = load current in amps
Rcable = resistance per meter of a single conductor (Ω/m)
L = cable length in meters
The factor of 2 accounts for the round-trip (out + return)

Wait — actually for 2-pair PoE, current flows through two parallel conductors on each side, so the effective resistance per meter is half the single-conductor resistance. The formula becomes:

Vdrop = Iload × Rcable × L

The current splits across two parallel wires in each direction, giving the same round-trip factor effectively.

For 4-pair PoE (802.3bt Type 3/4), current flows over all four pairs, further reducing effective resistance.

Cable Resistance Values

Cable Type	AWG	Resistance per meter (Ω/m)	Max length per TIA
Cat5e	24 AWG	0.0842	100m
Cat5e (thick)	23 AWG	0.0668	100m
Cat6	23 AWG	0.0668	100m
Cat6A	23 AWG	0.0668	100m
Cat6 (direct bury)	22 AWG	0.0530	100m

These are per-conductor values at 20°C. Resistance increases with temperature — roughly 0.39% per °C for copper.

Let's Run the Numbers

Example 1: PoE+ (802.3at) IP Camera at 90 Meters

Setup:

PSE output: 50V, 30W max
PD requires: 25.5W
Load current: 25.5W ÷ 50V = 0.51A (but the PSE supplies current based on the higher voltage)

Actually, let's be more precise. The PD draws the power it needs, and the current is determined by the PD's power requirement and the actual voltage at the PD.

For a simpler worst-case calculation:

PSE voltage: 50V
PD power: 25.5W
Cable: Cat5e (24 AWG), 90 meters
Loop resistance (2-pair): Rcable × L = 0.0842 × 90 = 7.58Ω

The load current at the PD:

I = P_PD / V_PD

But V_PD depends on the drop, which depends on current... it's iterative. For a reasonable approximation:

I ≈ P_PD / V_PSE = 25.5 / 50 = 0.51A
Vdrop = 0.51 × 7.58 = 3.87V
V_PD = 50 - 3.87 = 46.13V

PD minimum for 802.3at is 42.5V. We're at 46.13V with 90m of Cat5e. This works fine.

Example 2: PoE++ (802.3bt Type 4) at 100 Meters

Setup:

PSE output: 52V, 90W
PD power: 71.3W
Cable: Cat5e (24 AWG), 100 meters
4-pair delivery (lower effective resistance)

For 4-pair, the effective resistance is roughly half the 2-pair value:

R_eff = 0.0842 × 100 / 2 = 4.21Ω
I = 71.3 / 52 = 1.37A
Vdrop = 1.37 × 4.21 = 5.77V
V_PD = 52 - 5.77 = 46.23V

PD minimum for Type 4 is 41.1V. Still good at 100m.

Example 3: Pushing It — 150 Meters with PoE+

Now let's see what happens when you try to exceed the spec:

Setup:

PSE: 50V, 30W
PD: 25.5W
Cable: Cat5e, 150 meters
Loop resistance: 0.0842 × 150 = 12.63Ω

I ≈ 0.51A
Vdrop = 0.51 × 12.63 = 6.44V
V_PD = 50 - 6.44 = 43.56V

Still above 42.5V minimum — barely. You're at 150m with only 1V of margin. Any additional resistance from poor terminations, a patch panel, or temperature increase will push you under. And this completely ignores the Ethernet signaling limit, which almost certainly fails at this distance.

Verdict: Possible with very good cable and perfect conditions, but not recommended.

What About Temperature?

Cable resistance increases with temperature. If your cable runs through a hot attic or an industrial environment, this matters.

Copper's temperature coefficient is approximately 0.00393 per °C. The resistance values listed above are at 20°C.

At 60°C (a hot attic in summer):

R(60°C) = R(20°C) × [1 + 0.00393 × (60 - 20)]
        = R(20°C) × 1.157

That's a 15.7% increase in resistance. A cable run that works fine in your lab at 20°C might fail when deployed in a 60°C ceiling space.

Temperature-Adjusted Example

Revisit Example 1 at 60°C:

R_loop = 7.58 × 1.157 = 8.77Ω
Vdrop = 0.51 × 8.77 = 4.47V
V_PD = 50 - 4.47 = 45.53V

Still above 42.5V, but your margin shrank from 3.63V to 3.03V.

Practical Tips for Maximizing Cable Length

Use thicker cable. Cat6 (23 AWG) has about 20% less resistance than Cat5e (24 AWG). If you're near the limit, this alone might save you.
Minimize patch cables and connections. Every RJ45 jack and patch panel adds contact resistance. In long runs, terminate directly if possible.
Consider higher PSE voltage. Some managed switches let you adjust PSE output. A few extra volts at the source translates to more margin at the PD.
Use a PoE extender / repeater. These devices regenerate the signal and power at a midpoint, effectively doubling your range. They work, but they add cost and another point of failure.
Measure, don't guess. If you have a long run, measure the actual resistance with a cable tester before assuming it'll work.

Maximum Length Summary Table

Approximate maximum cable lengths based on voltage drop alone (not accounting for Ethernet signaling limits):

Standard	PD Power	Cat5e (24AWG)	Cat6 (23AWG)
802.3af	12.95W	~100m+	~100m+
802.3at	25.5W	~100m	~100m+
802.3bt Type 3	51W	~90m	~100m
802.3bt Type 4	71.3W	~85m	~95m

These assume 20°C. Subtract 10-15% for hot environments.

The takeaway: for standard PoE and PoE+ at typical power levels, 100 meters of Cat5e is generally fine. For PoE++ at high power, you might need Cat6 or shorter runs to stay within voltage margins.

When to Stop Calculating and Start Measuring

Math is great for planning. But real installations have splices, patch panels, tight bends, and temperature variations that calculators can't fully predict. If your calculation shows you're within 2-3V of the PD minimum, measure the actual voltage at the powered device with a multimeter after installation.

If the voltage is too low at the PD, you'll see symptoms like:

The device reboots randomly under load
The device won't power on at all
The PoE switch reports an "overload" or "fault" condition
The device works at low power but fails when it draws more (e.g., PTZ camera moves)

Skip the Spreadsheet

You can work through all these calculations manually with a spreadsheet, or you can use the PoE Power Budget Planner to model your specific setup — cable gauge, length, power level, and temperature — and see exactly what voltage and power you'll have at the far end.

Want to model your PoE deployment without breaking out the calculator? Try the PoE Power Budget Planner — enter your cable specs, power level, and distance, and get instant voltage drop and power delivery calculations. It runs in your browser and exports results as PDF.

Know your numbers before you pull cable. It's a lot cheaper to calculate than to re-run 100 meters of Cat6 through conduit.