PoE Budget Calculation: How Many Cameras Can One Switch Power?

"How many cameras will this switch power?" is the question behind more CCTV callbacks than any other. The answer is not the port count — it is the switch's total PoE budget divided by what the cameras actually draw, with margin for the conditions nobody puts in a datasheet.

The formula

Usable PoE budget ÷ (camera draw + margin) = number of cameras. Two adjustments matter: the budget is quoted at the port, before cable loss, and every camera draws more at night.

Typical camera draws

Camera class	Daytime draw	Night / IR draw	PoE class needed
2MP bullet, basic IR	3–5 W	4–6 W	802.3af
4MP dome, IR	5–7 W	7–10 W	802.3af
4MP bullet, long-range IR	7–9 W	10–13 W	802.3af / at
PTZ, 4–8MP	12–18 W	18–25 W	802.3at
Multi-sensor or 4K with heater	20–35 W	30–45 W	802.3at / bt

A worked example

Take a 240 W budget and eight 4MP domes rated 10 W at night: 8 × 10 W = 80 W, one third of the budget. That leaves 160 W for IR spikes, a later camera or two, and cable loss — comfortable. Ask the same switch to run twelve 4K cameras at 30 W and you need 360 W, which it cannot deliver: the honest answer is a larger switch or a second unit, not a longer cable.

Where budgets quietly fail

Nameplate versus measured draw: laboratory figures are typically 10–20% below real installation draw.
IR and heater spikes: night mode is the design case, not the exception.
Cable loss: thin or long cable can drop 10–15% of the delivered power before it reaches the camera.
Inrush at power-up: all cameras booting together is the moment a marginal budget trips.
Growth: the site always adds cameras before it adds switches.

When the numbers get close, the safer designs are a switch with a larger budget, a second PoE switch in the same cabinet, or PoE++ for the heavy cameras. RULE's PoE switch range and managed PoE models cover both cases.

How to check a datasheet in two minutes

Total PoE budget in watts — not "PoE on all ports".
Budget per port and the standard supported (af / at / bt).
Whether the budget is shared across all ports or split into groups, which matters on 24-port units.
Input power and UPS compatibility, since the budget is limited by the power supply, not the switch chip.

Tell us the camera models and cabinet plan; we will send back a power table with the recommended switch and the spare capacity left for expansion. Contact our team.

FAQ

Is PoE budget shared across all ports?

Yes. The total budget is drawn by all powered ports together, so two 25 W cameras use 50 W of it regardless of which ports they sit on.

Does a longer cable reduce available power?

It does. Resistance in the cable drops voltage and the camera draws more current to compensate. Keep runs under 100 m and prefer Cat6.

Should I count night mode in the budget?

Always. Design for the worst case: every camera with IR on, plus a cold start.

What is the difference between 802.3at and 802.3bt?

802.3at (PoE+) delivers up to 25.5 W at the port; 802.3bt (PoE++) delivers up to 60 W or 90 W for multi-sensor cameras, heaters and edge devices.

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