PoE Cable Voltage Drop Calculator
A security camera may receive both data and power through a single Ethernet cable, but some voltage is lost as current travels through the cable’s copper conductors. The PoE Cable Voltage Drop Calculator estimates that loss so you can see how cable length, current, conductor size, and the number of powered pairs affect the voltage reaching a camera or another PoE device.
This estimate is useful when planning a camera installation or comparing cable options. It uses nominal copper loop resistance at 20°C and assumes that current is shared equally among the selected pairs. Actual results can differ because of cable construction, temperature, connectors, and other installation conditions.
What the PoE Cable Voltage Drop Calculator does
The calculator estimates the voltage drop, in volts, along a Power over Ethernet cable. Voltage drop is the difference between the voltage supplied at the source and the voltage remaining after the cable’s resistance has caused a loss.
For security cameras, the source is commonly a PoE switch or injector. The powered device may be an IP camera, a wireless access point, or another compatible device. A longer cable or higher current generally produces a larger voltage drop. A lower-resistance cable or more pairs sharing the current generally produces a smaller estimated drop.
The result covers cable voltage drop only. It does not establish whether a cable installation meets a PoE standard, whether a switch can supply enough power, or whether a particular camera is compatible with the equipment. Check the specifications of the camera, power source, and cable before making a final installation decision.
How to use the PoE Cable Voltage Drop Calculator
Enter the cable run and current, then select the conductor size and number of pairs sharing power. The result is displayed as Voltage Drop in volts, rounded to two decimal places.
- Enter the one-way cable length in meters. Measure the cable route from the PoE source to the powered device, including any extra length needed to follow walls, ceilings, or conduits. The calculator accepts lengths from 0 to 100 meters.
- Enter the total PoE current in amperes. Use the current supplied to the powered device, expressed in amps. The calculator accepts values from 0 to 2 A. A value of 0.5 A means half an ampere.
- Select the copper conductor size and loop resistance. Choose 24 AWG, with an assumed loop resistance of 16.84 Ω per 100 m, or 23 AWG, with an assumed loop resistance of 13.32 Ω per 100 m. These are nominal solid-copper values at 20°C.
- Select the pairs sharing the power current. Choose either two pairs or four pairs, according to the power arrangement you are evaluating. The estimate assumes the total current is shared equally among those pairs.
- Read the voltage drop. Use the displayed value to compare possible cable runs or conductor sizes under the same assumptions.
Enter the one-way distance, not twice the distance. The selected loop resistance already accounts for one outgoing conductor and one return conductor over that cable length. If you are unsure about the current or powered pairs, consult the relevant equipment documentation rather than treating an assumed value as a measured one.
How the PoE Cable Voltage Drop Calculator formula works
The calculator applies Ohm’s law, voltage drop = current × resistance, to the cable. It scales the selected loop resistance to the entered one-way length and divides by the number of pairs sharing the current:
Voltage drop (V) = one-way cable length (m) × loop resistance (Ω per 100 m) × total current (A) ÷ (100 × powered pairs)
The loop resistance describes the outgoing and return path for one pair over 100 meters of one-way cable. Dividing by the number of powered pairs models equal sharing among parallel paths. This is a simplified estimate; real installations may not share current perfectly equally.
Worked example: a 100-meter security camera cable
Assume a 100-meter one-way cable run, a total current of 0.5 A, 24 AWG solid copper conductors, and two pairs sharing the power current. The calculator’s assumed loop resistance for 24 AWG is 16.84 Ω per 100 m.
- One-way cable length: 100 m
- Total PoE current: 0.5 A
- Assumed loop resistance: 16.84 Ω per 100 m
- Pairs sharing power: 2
Using the formula: 100 × 16.84 × 0.5 ÷ (100 × 2) = 4.21 V. The calculator displays a Voltage Drop of 4.21 V.
If the voltage at the cable input were known and stayed constant, subtracting the estimated drop would give a simplified estimate of the voltage at the other end. The calculator itself does not ask for source voltage or display receiving voltage, and the actual voltage can differ from that simplified estimate.
Use cases for the PoE Cable Voltage Drop Calculator
Planning a security camera run: Estimate the effect of a proposed cable route before installation. A camera mounted on a distant exterior wall may require more cable than the straight-line distance suggests, so measuring the actual route can make the input more useful.
Comparing conductor sizes: Keep the length, current, and number of powered pairs the same, then compare the 24 AWG and 23 AWG options. Under the calculator’s nominal assumptions, the lower loop resistance of the 23 AWG option produces a smaller estimated voltage drop.
Comparing cable lengths: Test a shorter and a longer route while keeping the other inputs unchanged. Because length is directly proportional to voltage drop in the formula, doubling the entered length doubles the estimated drop.
Evaluating current sharing: Compare the two-pair and four-pair selections when both are relevant to the equipment being considered. At the same total current and resistance, the four-pair setting produces half the estimated drop of the two-pair setting. That comparison assumes equal current sharing and does not mean every PoE device or power source can use either arrangement.
Investigating installation choices: A voltage-drop estimate can help identify a long run worth examining more closely. It cannot, by itself, identify the cause of a camera restarting, disconnecting, or failing to power on. The power source, cable, connectors, and device all need appropriate checks.
Other factors to consider when calculating Voltage Drop
Conductor temperature: The listed resistance values are assumptions for copper at 20°C. Copper resistance changes with temperature, so a cable running through a hot attic or bundled with other cables may have a different voltage drop from the estimate.
Cable construction: The calculator assumes solid copper conductors and uses nominal values for the selected sizes. Actual conductor dimensions and construction affect resistance. Check the cable manufacturer’s specifications when a more precise estimate is needed.
Connections and terminations: Plugs, jacks, patch panels, couplers, and poor terminations can add resistance. The displayed result estimates the cable drop using the selected nominal loop resistance; it does not separately account for these connection losses.
Unequal current sharing: The formula assumes that each selected powered pair carries an equal share of total current. Differences between conductors or connections can make actual sharing uneven and change the resulting losses.
Equipment requirements: A voltage-drop figure alone does not tell you whether the camera will operate reliably. Review the camera’s input requirements, the PoE source’s capabilities, and the relevant cable and installation specifications. A cable length within the calculator’s 100-meter input limit is not, by itself, proof of compliance or compatibility.
Frequently asked questions
What is a good voltage drop for a PoE security camera?
There is no single acceptable value for every camera and power source. The voltage available to the device depends on the source, cable, connections, and operating conditions. Compare the installation with the specifications for the particular equipment, and use the calculator as a planning estimate.
Should I enter the cable length one way or round trip?
Enter the one-way cable length from the power source to the device. The selected loop resistance already includes an outgoing and a return conductor, so entering the round-trip distance would overstate the estimate.
Does 23 AWG cable have less voltage drop than 24 AWG cable?
With the calculator’s assumed nominal resistance values and all other inputs held constant, yes. The 23 AWG option uses 13.32 Ω per 100 m of loop resistance, compared with 16.84 Ω for the 24 AWG option. Actual cable properties and installation conditions can affect the result.
Does selecting four pairs guarantee that a camera will receive more power?
No. The four-pair calculation shows a lower estimated cable voltage drop for the same total current under equal-sharing assumptions. Whether equipment uses four pairs, and how much power it can supply or receive, depends on the devices and their specifications.
Can this calculator confirm PoE standard compliance?
No. It estimates cable voltage drop from the inputs provided. Confirming compliance or compatibility requires checking the applicable standard and the specifications of the PoE source, powered device, cable, and complete installation.