installation · 4 min read

PoE NTP clock: requirements in practice

A PoE NTP clock makes installations far cheaper, but three pitfalls come up every month. PoE class, switch budget and cable length decide whether the clock keeps running stably.

Power over Ethernet (PoE) delivers network data and power simultaneously over a single cat5/cat6 cable to an NTP clock. No separate power supply, no electrician needed for every clock, installers love it. But three pitfalls regularly derail the project.

Which PoE classes exist for NTP clocks?

Standard	Max power	Cable class	Suitable for
IEEE 802.3af (PoE)	12.95 W	cat5e	Small NTDS clocks (2-digit), basic NTP clients
IEEE 802.3at (PoE+)	25.5 W	cat5e/6	Medium NTDS clocks (4-digit), bright display
IEEE 802.3bt (PoE++)	51–71.3 W	cat6/6a	Large NTDS (6+ digits), multicolor LUX26, double-sided variants

## Why does a budget mismatch between clock and switch occur?

Customer buys 20 NTDS44 clocks (8 W each). Switch is a 24-port PoE+ model with 370 W total PoE budget. On paper: 20 × 8 = 160 W, well within. In practice: the switch reserves the maximum 25.5 W per port allocation (not what the clock actually draws). 20 × 25.5 = 510 W. The switch refuses half of the clocks.

What to do: buy switches with LLDP power negotiation, those allocate exactly the requested wattage, no overhead.

When are PoE cables too long?

PoE guarantees up to 100 m of cable. At 100 m there is around 12% voltage drop, which sometimes is just enough to bring the clock below the required 44 V minimum. Result: the clock flickers in and out, the switch thinks there is a short circuit and cuts the port.

What to do: keep cables under 80 m for PoE+ clocks. If 100 m is unavoidable, use PoE++ (higher voltage).

What is the difference between passive PoE and IEEE-standard PoE?

Cheap brands supply "passive PoE", 12 V or 24 V over the spare pairs in cat5. That is not IEEE PoE. A Masterclock clock expects IEEE 802.3af/at/bt negotiation and refuses to start on a passive-PoE injector.

What to do: only use IEEE-certified PoE switches or injectors. Cisco, Aruba, Allied Telesis, MikroTik (mind the model), Ubiquiti UniFi are all suitable.

What PoE budget do multicolor and double-sided clocks need?

LUX26 multicolor and NTDS-DF models (Double Faced) clearly draw more power, often 25–40 W. Plan for PoE++ (802.3bt). An installation of 40 LUX26 on a 24-port switch calls for a new switch choice, not just a network-config tweak.

How does Daylight de-risk a PoE-rollout?

The most common site-survey question from our installers: "which switch is good?". We have a list of switches we have seen work in customer deployments. Call Daylight and we can recommend the right clock-switch combination before the order. Our installation checklist helps you prepare yourself.