Power measurement and power quality

Electricians are increasingly asked not just to install circuits but to measure what they are doing: how much power a building draws and when, whether an existing service can take a new load, why equipment trips or fails, and whether a customer is paying penalties for demand or power factor. This lesson covers how real power is measured in single-phase and three-phase circuits, how utility meters measure energy and demand, how load studies support code decisions on adding load, the main types of power quality disturbances, and how to set up a power quality analyzer safely and interpret what it records.

Measuring real power

Real power is the average of instantaneous voltage times current (Lesson 4). A wattmeter, or the power function of a digital meter, measures it by sampling voltage and current simultaneously and averaging their product, which automatically accounts for phase angle and harmonics. Multiplying an RMS voltage reading by an RMS current reading gives apparent power in VA, not watts, unless the load is purely resistive.

A wattmeter has a voltage circuit, connected across the load, and a current circuit, carrying the load current or connected through a current clamp or current transformer. Correct polarity matters: reversing the current clamp gives a negative reading, which on a three-phase system can produce badly wrong totals. Clamps are marked with an arrow pointing toward the load.

Three-phase power: Blondel's theorem

Blondel's theorem states that the total power in a system with \(N\) wires can be measured with \(N - 1\) wattmeters, each with its current circuit in one line and its voltage circuit between that line and the remaining common wire. This gives two standard methods:

• Three-phase four-wire wye systems: three wattmeters (or one meter with three voltage and three current inputs), each measuring one phase from line to neutral. Total power is the sum of the three readings, whether or not the loads are balanced.
• Three-phase three-wire systems: two wattmeters, with current coils in lines A and C and voltage coils from A to B and from C to B. Total power is the algebraic sum of the two readings, for any load, balanced or not.

With the two-wattmeter method on a balanced load, the two readings are equal only at unity power factor. As the power factor falls, one reading rises and the other falls; below 0.5 power factor, one reading becomes negative and must be subtracted. The readings also give the power factor of a balanced load:

Modern multifunction meters and power analyzers implement these connections internally and report kW, kvar, kVA, power factor and energy per phase and in total. Their wiring configuration must be set to match the system (wye, delta, two-element or three-element), and checking that each phase shows a sensible power factor and positive power is the fastest way to catch a reversed clamp or a misassigned voltage lead.

Energy and demand metering

Utilities bill smaller customers for energy (kWh) only. Larger commercial and industrial customers are also billed for demand: the highest average power over a demand interval, typically 15 minutes (sometimes 5 or 30), during the billing period. The demand charge pays for the generation, transmission and distribution capacity reserved for the customer's peak, and it can be a third or more of the bill.

Some tariffs bill demand in kVA rather than kW, or add a penalty when power factor falls below a threshold such as 0.90 (Lesson 4), which is why power factor correction can pay for itself. Time-of-use and critical peak tariffs vary energy prices by hour. Interval data from smart meters, often available to the customer, show the load profile in 15-minute or hourly steps and are the first thing to examine in any energy or capacity question.

Revenue meters on larger services are transformer-rated: current transformers (and, on higher voltages, voltage transformers) reduce the quantities to levels the meter can handle (Lesson 8). The meter multiplies its readings by the transformer ratios. Electricians install the meter base, CT cabinet and wiring to the utility's specifications, and the CT secondary must never be opened under load, because an open CT secondary develops dangerous voltages.

Load studies for adding load

Before adding a large load (an EV charging installation, a new rooftop unit, a heat pump conversion) to an existing service or feeder, the electrician must show that the service can carry it. Calculated loads based on code demand factors are often conservative; measured demand can show more spare capacity.

NEC 220.87 allows the existing load to be determined from the maximum demand over the previous year (from utility data), or, if that is not available, from a recording of the maximum power demand over at least 30 days, taken at 15-minute intervals with the building occupied and including seasonal loads, multiplied by 125%. The new load is then added to determine whether the service is adequate.

Power quality disturbances

Power quality describes how closely the supply matches an ideal sine wave of constant amplitude and frequency. Disturbances are commonly classified (following IEEE 1159) as:

Sags are by far the most common cause of equipment problems in industrial plants: a fault on a distant utility line, cleared in a few cycles, can drop the voltage at the plant to 60% or 70% for 100 ms, enough to trip drives and drop out contactors whose coils release below about 70% voltage. The ITIC (CBEMA) curve describes the voltage tolerance expected of information technology equipment: it should ride through any interruption up to 20 ms and sags to 70% for up to 0.5 s.

Remedies depend on the disturbance: surge protective devices at the service and at sensitive equipment for transients; ride-through settings on drives, DC-powered or held-in contactor coils, and UPSs for sags; correct transformer taps and conductor sizing for sustained voltage problems; rebalancing for unbalance; and the harmonic measures of Lesson 3. Many problems blamed on the utility originate inside the building: a large motor starting on the same feeder, a loose neutral, or an undersized transformer.

Using a power quality analyzer

A power quality analyzer records voltages and currents on all phases and the neutral over days or weeks, logging RMS trends, demand, harmonics, events (sags, swells, transients) and waveforms around each event. Good practice when installing one:

• Treat the work as energized electrical work: obtain authorization, wear the arc-rated PPE required for the location, and use test leads and clamps rated for the measurement category.
• Check the measurement category: IEC 61010 categories describe where an instrument may be connected. CAT IV covers the service entrance and utility connection, CAT III the building distribution (panels, feeders, fixed equipment), and CAT II plug-connected loads. An instrument rated CAT III 600 V may be used on a 480 V panel but not at the service entrance of a large building, where transient energy is higher.
• Connect voltage leads with fused probes or clips to a suitable point, ideally a dedicated breaker or terminal; connect current clamps with arrows toward the load, on the correct phases.
• Configure the system type, nominal voltage, CT ranges and event thresholds, then verify phasor and power readings on the analyzer screen before leaving it.
• Record for long enough to capture the weekly cycle and the problem being investigated, at least seven days where possible.