Load Calculations | Troy Story

In the first article of this series, we broke down Ohm’s Law and the power equation. We looked at the formulas, the wheel, and how knowing just two values lets you solve for the rest. That gave us the math, but formulas don’t mean much until you apply them to something real.

This time, I want to walk through one of my actual classroom designs here at Northwestern. Nothing hypothetical—this is how we build them and what my team supports every day.

Why Power Math Matters

Every circuit has a limit, and once you exceed it, things fail in unpredictable ways. Breakers trip, gear reboots, or something overheats just enough to cause issues later. In AV, we tend to cram a lot of equipment into a small rack and expect it to quietly run for ten hours a day, five days a week. Taking a few minutes to run the power math is how you make sure it does.

Doing proper load calculations helps you:

• Avoid nuisance trips and random reboots.

• Plan for growth without redoing circuits later.

• Size PDUs, UPS units, and sequencers correctly.

• Talk to facilities in numbers they understand.

A Real Classroom Example

Here’s what’s in one of our standard teaching spaces. The projector is on its own ceiling circuit, but everything else runs from a single 20-amp branch circuit that feeds the rack and the four auxiliary outlets on top of the lectern.

The rack includes a control processor, matrix switcher, a transmitter sending video to the projector, a camera receiver, a remote recorder for Panopto, a DSP, a 4-channel amplifier, an HDMI-to-USB converter, a PoE switch, a wireless presentation device, a document camera, two local monitors, and a Mac mini.

The Math

Here are the design wattages we use, based on manufacturer maximums.

That 614-watt figure for the Netgear switch comes straight from the spec sheet. It’s the maximum AC draw when fully loaded at its 480-watt PoE+ capacity. We include it in full because it represents the theoretical upper limit.

At 120 volts:

I = 1,305 ÷ 120 = 10.88 amps

So, the rack itself pulls about 11 amps at full load. On a 20-amp circuit, that’s a comfortable number.

Accounting for the Faculty Outlets

Now add the four auxiliary outlets on top of the lectern. These are meant for faculty and guest devices—usually a laptop or two, maybe a phone charger. To be thorough, we plan for a worst-case scenario of four laptops charging at 90 watts each, which totals 360 watts.

Combined load:

1,305 W (rack) + 360 W (outlets) = 1,665 W

At 120 volts:

I = 1,665 ÷ 120 = 13.88 amps

That’s still comfortably below the 16-amp continuous-load guideline for a 20-amp circuit. In reality, four laptops pulling 90 watts each just doesn’t happen. Most laptops only draw that kind of power when they’re nearly dead and charging fast. Real-world usage is closer to 150–200 watts total across all four outlets.

Since we designed these rooms to be consistent, the rack and lectern outlets intentionally share the same branch circuit. That decision simplifies installation, keeps costs down, and has proven reliable across hundreds of classrooms. The key is that we understand the load and verify that—even under worst-case conditions—we have plenty of headroom.

About That PoE Load

The PoE switch is another area where the math can look scarier than reality. The Netgear M4250 GSM4230PX has a 480-watt PoE+ budget and a 614-watt maximum AC draw. We standardized on that model across campus so every space uses the same hardware, but our standard classroom design doesn’t come close to needing that much PoE.

Most rooms only have a few powered devices: a touch panel, two beamforming mics, a PTZ camera, and a wireless presentation device. Altogether that totals around 150 watts of PoE draw—less than one-third of the switch’s capacity.

That means we’re never operating anywhere near the maximum power numbers. We designed for the worst case, but the actual load is light. That’s built-in headroom, and it’s exactly what you want in a mission-critical environment.

Why the Whole Power Picture Matters

It’s easy to think of AC and PoE as separate power systems, but they’re not. Everything ultimately ties back to the same branch circuit feeding the rack. Whether that wattage powers a DSP through a plug or a camera through Ethernet, it all starts at the same source.

To really understand your system, you have to look at the entire electrical picture—AC load, PoE delivery, and how they interact under real use. Once you do, the numbers start to make sense, and you can design confidently without overbuilding.

Pulling It Together

When we total everything, the rack and lectern combined draw around 14 amps on a 20-amp circuit under full load. In day-to-day operation, we’re closer to 10–11 amps. The amplifier only uses two channels, the switch never comes close to its PoE maximum, and the laptops aren’t all charging at once.

So while the math says we’re running at roughly two-thirds capacity, reality says we’re cruising with plenty of space to spare. Understanding that difference—between the calculated maximum and what actually happens in use—is the mark of a well-designed, reliable system.

Takeaways

• This design comes straight from one of my classrooms.

• Even when you use manufacturer maximums, a 20-amp circuit still has plenty of capacity for a full rack and lectern setup.

• Always include the PoE switch’s full AC draw in your load calculations, even if your design only uses part of the PoE budget.

• Plan for faculty outlet usage, but know that four laptops charging at full rate almost never happens.

• Standardizing on higher-capacity switches builds in PoE headroom and makes support across rooms easier.

• Reliability comes from understanding the entire power picture—both AC and PoE—and leaving real headroom in each.

Math might not be exciting, but it’s what keeps our classrooms running day in and day out without surprises.

In the next article, we’ll look at PoE vs. AC power best practices and how to decide which belongs where in your system.