Let's break down how demand meters work and address your specific concerns

1. Is the Measured Demand Instantaneous?
No. Utility demand meters do not measure instantaneous demand. They measure averaged demand over a specific interval, typically 15 or 30 minutes. This is called the "demand interval." The meter essentially calculates the average kW (or kVA) used during each interval and records the highest of these averages over the billing period.2. Unbalanced Load Handling Generally, meters are designed to accurately measure total power regardless of balance. Modern electronic meters use CTs (current transformers) on each phase and measure voltage on each phase. They then calculate the power for each phase individually and sum the three phases to get the total power. They don't just read the highest leg. This is especially important for a small town highway garage, which probably doesn't have balanced loads.3. kW vs. kVA kW (kilowatts) is most common for billing. Utilities often bill based on kW demand because it reflects the actual real power consumed, which drives the cost of fuel and power generation. kVA (kilovolt-amperes) considers reactive power (power used to establish magnetic fields in inductive loads like motors), but many utilities only bill for kVA demand if the customer's power factor is poor (usually below 0.9 or 0.85). Your utility explicitly stating kW is a strong indicator they are billing based on real power demand.4. Why the High Demand Reading?

Here are some reasons why the demand reading might be surprisingly high: Power Factor

Although it's often not directly billed, a poor power factor will indirectly increase kW demand. Inductive loads (motors in air compressors, lifts, door openers) contribute to reactive power. The utility must supply this reactive power, and while they might not bill you directly for kVA, the increased current flow from a low power factor will contribute to your kW demand.

Coincidence Factor

The coincidence factor is the ratio of the maximum simultaneous demand of a group of loads to the sum of the maximum demand of individual loads. Think about it this way: If everything could be on at once, it doesn't mean everything is on at once at its peak. A low coincidence factor means that different loads tend to peak at different times. However, it's still possible for enough loads to coincide and create a significant peak.

Simultaneous Load

Consider what loads could be running concurrently during a 15 or 30-minute interval: Air compressor cycling on and off. Truck lift in use. One or more large door openers activated. Welding equipment. Shop lighting. Electric heating (if applicable). Battery charging (vehicles).While each of these individually might not reach 89.6kW, a combination of them during the same demand interval could easily do so.

Heating/Cooling

Electric space heating or air conditioning can add a significant demand load. If the demand peak occurred during a very cold or very hot period, this is something to investigate.

Motor Starting

When motors start, they draw a much higher current than their running current. This inrush current can contribute significantly to the demand reading, especially for larger motors.

Meter Calibration/Malfunction

While less likely, it's possible the meter is malfunctioning.

Billing Period Anomalies

Perhaps there was a one-time event during that specific billing period that significantly increased demand.

Data Logging Interval Mismatch

The demand is not the load averaged over the billing period, it is the highest average over a short window (e.g. 15 minutes). A short period of high load can set the demand for the entire month.

Investigating the High Demand


Talk to the Utility

Ask the utility for the specific dates and times of the peak demand. They may be able to provide more detailed data, such as a load profile (a graph of demand over time).2.

Review Operations

Analyze the activity at the highway garage around the time of the peak demand. Did any unusual activities occur?3.

Consider Power Monitoring

Install a power meter or data logger to monitor your own electrical usage. This will give you real-time and historical data on kW, kVA, power factor, and other parameters. This is the best way to understand your actual load profile and identify the sources of high demand.4.

Power Factor Correction

If you have a lot of inductive loads and a low power factor, consider installing power factor correction capacitors. This can reduce your kW demand and potentially lower your bill. However, do not install these without a proper power quality study conducted by a qualified engineer.

Calculations and Rule of Thumb (Estimate)

While a 225A service at 208V, 3-phase could theoretically supply much more than 89.6kW, real-world factors often limit the actual capacity.

Theoretical Capacity

208V 225A sqrt(3) = 81,054 VA (or 81 kVA). With a power factor of 1, this would be 81 kW.

Derating

You shouldn't load a circuit breaker or panel to its absolute maximum. NEC guidelines typically limit continuous loading to 80% of the breaker rating. So, 225A 0.8 = 180A continuous.

More Realistic Capacity

208V 180A sqrt(3) 0.9 (power factor) ≈ 58.3 kW.This rough calculation shows that 89.6kW is high for that service size, especially given the type of loads you describe. It strongly suggests either a power factor problem, very simultaneous operation of major loads, or a potential issue with the meter or billing.

In summary, the utility's demand reading is unlikely to be instantaneous. It is averaged over a demand interval (usually 15-30 minutes) and measures total power across all phases. The best course of action is to investigate the potential causes of the high demand and consider power monitoring to gain a better understanding of your electrical usage. Flag for review