Power: PoE, UPS and thermal

Three separate questions get called “power” on a low-voltage job, and they have different answers: how much power the switch has to deliver, how long the batteries hold up, and how much heat ends up in the room. This page gives the arithmetic behind each.

PoE budgets

Device draw is resolved per class against the IEEE 802.3 ceilings:

Class	Standard	Source-side maximum	Device-side maximum
Type 1	802.3af	15.4 W	12.95 W
Type 2	802.3at	30 W	25.5 W
Type 3	802.3bt	60 W	51 W
Type 4	802.3bt	100 W	71 W

The gap between the two columns is the cable loss the standard allows for; budgeting against the source-side number is what stops a switch being specified to its nameplate and then browning out.

Per-closet draw is summed from the devices actually placed on the plan, not from a typical-device assumption. Two headroom rules then apply to the switch recommendation:

• Power headroom — 20%. The required budget is the summed maximum draw multiplied by 1.2.
• Port headroom — the greater of two spare ports or 25%. Ports required is max(deviceCount + 2, ceil(deviceCount × 1.25)), so a small closet gets a floor of two spares and a large one scales.

A closet whose remaining headroom falls below 50 W is flagged rather than silently accepted.

UPS runtime

Runtime is the number most often quoted with false precision, so it is worth being exact about how it is produced.

A full-load runtime is interpolated from a runtime-by-capacity table drawn from manufacturer data, then scaled for the actual load using Peukert’s law with an exponent of 1.3:

runtime = fullLoadRuntime × (capacity / load) ^ 1.3

Peukert’s law is what stops runtime being treated as linear. A UPS at half load does not last exactly twice as long, and a spreadsheet that assumes it does will overstate hold-up time on every lightly loaded cabinet. The capacity-to-load ratio is capped at 5:1 so that a very lightly loaded unit is not credited with an implausible figure.

Battery sizing follows the IEEE Std 485 approach — amp-hours from load, efficiency, DC bus voltage and the required duration:

Ah = (load ÷ efficiency ÷ V_dc) × hours

Sizing carries a 25% headroom factor, and the resulting VA is rounded up to a standard supply rating rather than quoted as an exact figure no vendor sells.

This is a planning estimate and the deliverable says so. Runtime is presented with a tilde. The honest reason: real hold-up depends on the specific battery configuration in the specific unit, and no model that does not know which battery modules are fitted can promise minutes.

Thermal load

Heat is computed from the load, not from equipment nameplate cooling capacity — those are different numbers and conflating them is a common way to undersize a room.

BTU/hr = watts × 3.412141633
CFM    = BTU/hr ÷ (1.08 × altitudeFactor × ΔT)

The 1.08 coefficient assumes sea-level air density. Altitude is corrected as 1 − 0.035 × (feet ÷ 1000), floored so the derate cannot run away at extreme elevation. ASHRAE TC 9.9 thermal envelopes classify the resulting operating condition.

What this means in practice

The useful property is not any single formula — it is that the load driving all three comes from the same placed devices. Add eight cameras to a floor and the PoE budget, the UPS sizing and the heat load all move together, because they are reading the same design rather than three spreadsheets someone remembered to update.