PoE Voltage Drop Calculator
Estimate voltage drop and power loss for PoE-powered devices over Ethernet cable. Useful for installers planning access points, IP cameras, VoIP phones, and IoT devices.
This calculator uses simplified resistance models for standard PoE (2-pair) mode. Actual values depend on specific cable quality, connectors, and temperature. Always measure real-world values for critical installations.
PoE Voltage Drop Calculator
PoE Power Results
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Cable Current
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Voltage Drop
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Voltage at Device
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Power Loss in Cable
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Status
Formulas
Current (A) = Device Power (W) ÷ Input Voltage (V)
Voltage Drop (V) = Current × Cable Resistance (Ω/m) × Cable Length (m)
Voltage at Device (V) = Input Voltage − Voltage Drop
Power Loss (W) = Voltage Drop × Current
Cable resistance values (round-trip, 2-pair PoE mode):
- Cat5e: 24 AWG, ~93.8 mΩ/m round-trip resistance
- Cat6: 24 AWG (tighter spec), ~87.9 mΩ/m round-trip resistance
- Cat6A: 23 AWG, ~66.9 mΩ/m round-trip resistance
PoE Standards Reference
- IEEE 802.3af (PoE): Up to 15.4W at PSE, 12.95W at PD. Input voltage 44–57V.
- IEEE 802.3at (PoE+): Up to 30W at PSE, 25.5W at PD. Input voltage 50–57V.
- IEEE 802.3bt Type 3 (PoE++): Up to 60W at PSE, 51W at PD. Uses all 4 pairs.
- IEEE 802.3bt Type 4 (PoE++): Up to 90W at PSE, 71.3W at PD. Uses all 4 pairs.
- Maximum cable length for all PoE standards is 100 metres, but voltage drop must be verified for high-power devices on long runs.
Practical Notes for Installers
- Patch cables add resistance — allow ~5% extra for connectors and patch cord sections.
- For APs drawing 20–25W, a 70-metre Cat5e run typically loses 3–5V. Most modern APs tolerate this.
- PTZ cameras drawing 50–60W on long runs should use Cat6A and keep runs under 60 metres.
- PoE budget at the switch matters — ensure your switch's total PoE budget is not exceeded.
- Avoid using thin-gauge patch cables (28 AWG) for long PoE runs — they have significantly higher resistance.
Frequently Asked Questions
How far can PoE power a device?
The IEEE standard maximum is 100 metres. For low-power devices (< 15W), voltage drop is rarely a concern at that distance. For 60–90W devices, consider keeping runs under 60 metres on Cat5e or use Cat6A to extend reach. Always calculate and verify.
What is the voltage drop formula for PoE?
Current = Device_Watts ÷ Input_Voltage. Then V_drop = Current × Resistance_per_meter × Length. For Cat5e at 1A current over 70m: V_drop = 1A × 0.0938Ω/m × 70m = 6.57V. If the PSE outputs 52V, the device sees 45.4V — acceptable for most devices specified down to 44V.
Does Cat6A have less voltage drop than Cat5e?
Yes. Cat6A uses 23 AWG conductors which are slightly thicker than Cat5e's 24 AWG. Lower gauge = lower resistance. Cat6A has about 30% less resistance per metre, making it the recommended choice for long runs with high-power PoE devices like Wi-Fi 6E APs and PTZ cameras.