A 30-day recording can answer the capacity question and show you what the new load is doing to the electrical system. Here is how to get both from one survey.
Every data center expansion starts with the same question. Do we have the capacity?
Operators are adding high-density and AI racks to halls that were designed for something lighter. The question lands on the facility engineer and on the electrical contractor doing the work. The original design calculation rarely settles it. The building has changed since it was drawn. The answer that holds up comes from measurement, and the National Electrical Code sets out how to take it.
What NEC 220.87 allows
Section 220.87, Determining Existing Loads, lets you size new load against what a service or feeder actually carries. You do not have to redo the full calculation. The method has a few conditions, and they are worth reading closely.
One year of demand data. Use the maximum demand data for a 1-year period, usually from utility metering.
Or a 30-day recording. If a year of data is not available, record the maximum demand continuously for at least 30 days. The code defines it as the highest average kilowatts reached and maintained for a 15-minute interval, taken with a recording ammeter or power meter on the highest loaded phase of the feeder or service.
Recorded under real conditions. The recording has to be taken while the building or space is occupied. It has to include, by measurement or calculation, the larger of the heating or cooling load, and other loads that are periodic or seasonal.
125 percent of what you found. Take the existing maximum demand at 125 percent and add the new load. The total cannot exceed the ampacity of the feeder or the rating of the service.
Proper protection. The feeder or service must have overcurrent protection in line with 240.4 and 230.90.
In a data center, cooling is where this goes wrong. A 30-day window in October may never see the chillers and CRAH units at their summer peak. Either time the recording so it captures the cooling peak, or document the cooling load by calculation as the code requires. Do not leave it out.
The summary above is for planning. Confirm the NEC edition your authority having jurisdiction enforces, and work from the code text.
Record more than kilowatts
A recording ammeter answers one question: how much. A Class A power quality analyzer on the same 30 days answers several more, and in a data center they matter.
Harmonics. UPS rectifiers, variable frequency drives on cooling equipment and server power supplies are all nonlinear loads. They draw distorted current, and adding racks adds more of it. IEEE 519 sets harmonic limits at the point of common coupling with the utility. If the utility asks, you want the data in hand.
Neutral current. On four-wire systems, triplen harmonics from single-phase power supplies add up in the neutral instead of cancelling. A neutral can carry more current than the phase conductors. Load calculations alone will not show that.
Sags, swells and transfers. Every utility event and every generator transfer during the window is timestamped, with waveforms. When a UPS goes to battery and nobody can say why, the record can.
Unbalance. Racks get added a circuit at a time. Over a few expansions the phases drift apart. The recording shows by how much.
Measurement class decides what that data is worth. IEC 61000-4-30 defines Class A, which fixes the methods, aggregation intervals and accuracy so that two Class A instruments on the same signal agree. If the recording has to stand up in front of an engineer of record, an inspector, the utility or an insurer, Class A is the standard to ask for.
One recording, two reports
The CHK Miro PQ45 is built for this survey. It is certified to IEC 61000-4-30 Class A and safety rated CAT IV 600 V. It is sealed to IP66 and powered from the circuit it is measuring, with an internal backup battery, so it can stay in the electrical room for the full 30 days. It has 8 GB of onboard memory, WiFi, and optional cellular, so you can check the recording without opening the gear.
CHK's CITRUS software produces an NEC 220.87 connected-load survey and an IEEE 519 harmonics report from the same recording. One install answers the capacity question and shows you what the load is doing to the system.
Then aim the thermal camera
The recording tells you where load and harmonic stress concentrate, and when. That is where and when to scan. Switchgear, busway, PDUs, remote power panels, UPS cabinets and breaker terminations all carry the new load, and any one of them running hot can take down a hall.
NFPA 70B, now a standard, builds maintenance intervals around equipment condition. In the 2026 edition, Table 9.3.2 sets infrared thermography on all equipment at 12 months for Conditions 1 and 2 and 6 months for Condition 3. Section 9.1.1 lists continuous monitoring and predictive techniques as a consideration when setting intervals. A power quality record and a thermal scan taken at the same load make a stronger condition file than either one alone.
ProgUSA carries FLIR and FOTRIC thermal imagers. In medium voltage gear, the FLIR Si2-PD acoustic imager locates partial discharge before it becomes a failure.
The rest of the build
Capacity and power quality are two of the tests a data center build needs. Grounding and bonding are another. We covered them in The Ground Test Data Centers Forget. UPS batteries are another, and our battery bank test range is built for them.
How ProgUSA helps
Since 2005, ProgUSA has supplied test equipment to utilities, NETA firms and industrial crews across the United States.
We have power quality engineers on staff. Buy a Miro PQ45 from us and you get access to them: help planning the recording window, help setting the instrument up so it produces the report you need, and help reading the results when something does not look right. We calibrate CHK instruments in the United States.
Planning an expansion? Request a quote or a demonstration. Call 407-332-8678 or email info@progusa.net.
FAQ
What does NEC 220.87 require to use measured load?
Maximum demand data for one year, or, if that is not available, the maximum demand recorded continuously for at least 30 days, defined as the highest average kilowatts maintained for 15 minutes, on the highest loaded phase. The existing demand is taken at 125 percent and the new load is added.
Can a power quality analyzer be used for an NEC 220.87 load study?
Yes. The code calls for a recording ammeter or power meter. A power quality analyzer records demand and also captures harmonics, events and unbalance over the same period. The CHK Miro PQ45 produces an NEC 220.87 report directly.
Why do harmonics matter in a data center?
UPS rectifiers, drives and server power supplies draw distorted current. Harmonics add heat to transformers and conductors, and on four-wire systems triplen harmonics add up in the neutral. IEEE 519 sets limits at the utility connection.