Solutions > Industrial Ethernet > How Distance Affects PoDL Power Delivery — Why 1 km Is Not a Free Lunch
How Distance Affects PoDL Power Delivery — Why 1 km Is Not a Free Lunch
10BASE-T1L’s most attention-grabbing feature is its 1,000-meter transmission distance, but in a PoDL power delivery context, the longer the distance, the less available power reaches the powered device (PD) — a physical limitation that engineers must squarely address in system planning.
The root cause lies in the cable’s loop resistance. When current flows through a long-distance twisted pair, it produces a voltage drop (IR drop) across the conductor resistance, causing the remote device to receive a voltage far below the PSE output. As an example from measured data (telephone wire 0.5 mm spec, Class 12, PSE output 24 V):
| Transmission Distance | PD Received Voltage | PD Current | PD Actual Power |
|---|---|---|---|
| 400 m | 24 V | 0.03 A | 0.72 W |
| 600 m | 24 V | 0.013 A | 0.31 W |
| 800 m | 24 V | 0.005 A | 0.12 W |
| 1,000 m | 23.1 V | — | Unable to measure |
From the table above, it is clear that at 800 meters, the actual available power at the PD end has dropped to 0.12 W — far below the Class 12 theoretical maximum of 8.4 W. At 1,000 meters, the PD may even fail to start normally.
For this, engineers have several response strategies: first, select cable with larger conductor cross-section (smaller AWG number) to reduce loop resistance; second, choose Class 13–14 high-voltage systems (50–58 VDC), trading higher starting voltage for more adequate voltage drop headroom; third, perform rigorous Power Budget calculations during the initial system design stage to ensure power requirements at the furthest point remain feasible.
FAQ
Q1
Why does available PoDL power decrease as transmission distance increases?
Answer
Cable has inherent resistance (Ω/km), and current produces a voltage drop (I²R loss) during transmission. The longer the distance, the higher the cable impedance, the lower the voltage delivered to the remote PD, and the lower the actual available power. Therefore, in long-distance deployments, sufficient power margin must be reserved, or cable with larger conductor cross-section must be selected.
Q2
When using telephone wire (0.5 mm conductor) for PoDL, what distance limitations apply?
Answer
Telephone wire has a smaller conductor cross-section (approximately 0.5 mm), giving it higher resistance per unit length than dedicated SPE cable. Over long distances, resistive losses become more significant and may result in insufficient power at the remote PD. In practice, when using telephone wire, particular attention should be paid to the actual available power for the corresponding PoDL Class at the target distance, with testing and verification performed.
Q3
What methods can improve PoDL power delivery performance over long distances?
Answer
Primary improvement strategies include: selecting dedicated SPE cable with larger conductor cross-section (reducing resistance per unit length); choosing higher voltage PoDL Classes (such as Classes 13–15 at 50–58 V, where the higher voltage tolerates greater cable voltage drop); or shortening individual cable runs combined with relay power supply equipment for segmented power delivery.
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