An electricity meter is sized by service class, socket rating, CT ratio, and accuracy class, not by nameplate kW. The selection flow runs: identify the service (120/208 V wye, 277/480 V wye, or 240-600 V delta), confirm continuous load current, pick a socket rating from the standard ladder (125, 200, 320, 400 A), and then choose an accuracy class matched to the metering objective [S1][S3].
Meter socket ratings are continuous ratings. 125 A is the de-facto residential standard size, with 200, 320, and 400 A as the next commercial steps, and most utilities move to CT metering once service capacity passes 400 A on plug-in bases or 600 A on self-contained bases [S3].
Service Class Determines the Meter Form
Residential service in the US is commonly 120 V, single-phase, three-wire with a center-tapped neutral, while light commercial runs 120/208 V three-phase four-wire wye and larger HVAC or inductive loads step up to 277/480 V wye [S1]. Industrial facilities with numerous large inductive loads typically receive three-phase three-wire delta service at 240-600 V or higher, and the meter inputs must be configured for each electrical service type [S1].
Frequency is standardized at 60 Hz in the United States, and the meter must be matched to phase count, wire count, neutral presence, voltage class, and circuit load capacity before any accuracy or comms question is even relevant [S1]. A building that mixes an administrative wing and a shop wing can carry two service classes on the same site, and each class gets its own meter spec [S1].
Socket Rating Ladder and the 125% Continuous Rule
Continuous load is the binding constraint: a 50 A continuous plus 45 A non-continuous load, when treated as a continuous load, must land on a 125 A meter base, not a 100 A unit, because the standard practice is to size the socket at 125% of continuous current [S3]. The standard residential meter socket sizes are 125 A, then 200, 320, and 400 A continuous; 100 A sockets are not a common stock item [S3].
The 320 A continuous rating is effectively the ceiling for plug-in style meter bases; above that, the form factor shifts to bolt-in meter bases that may be rated at 400 A or higher, and most US utilities switch to CT metering once service capacity exceeds 400 A on plug-in bases [S3]. Some utilities will not apply CT metering below 600 A, forcing designers to specify 320 A self-contained meter bases up to the 600 A boundary, which can crowd the meter can quickly on three-phase services [S3].
Current Transformer Ratio, Burden, and Accuracy
CTs step primary current down by a fixed ratio such as 100:5, so 100 A on the primary produces 5 A on the secondary that the meter actually measures, and the meter must be ordered to accept that secondary current plus the CT ratio [S1]. CTs come in multiple accuracy classes, and the meter plus CT pair must hold class across the full expected load range, not just at full scale, because a CT that is accurate at 800 A but unloaded at 100 A will under-register revenue and misreport kWh [S1][S3].
For tenant revenue billing, an ANSI C12.1-2024 accuracy class is the typical requirement; for spot measurement of an HVAC motor or a sub-panel, a lower class is usually acceptable, and the specifier should match the class to the metering objective, not to a single number on a data sheet [S1]. A common pitfall is to size the CT to peak load and then run the meter at 10% of scale for years: ANSI billing class instruments lose linearity at the low end, so undersizing the service or oversizing the CT both create revenue drift [S1][S3].
Self-Contained vs CT Metering at the Service Boundary
Self-contained meter bases carry full load current through the meter socket and are practical up to 320 A plug-in or 400 A bolt-in continuous; CT metering passes primary current through a CT and reads the proportional secondary, which is the only feasible method above roughly 600 A and is also used at lower currents when revenue exposure is high [S3]. Utility practice varies: some local utilities will not CT-meter below 600 A because they view the metering as less accurate at low current, while others will CT at 400 A to avoid large self-contained bases [S3].
For a buyer, the decision tree is: confirm the utility's CT threshold, then size the service entrance accordingly; if the utility CTs above 400 A, plan on a 400 A bolt-in base; if it CTs above 600 A, plan on a 320 A self-contained base and accept the wiring density, particularly on three-phase services [S3]. Designers should also check whether the utility owns the meter or the customer does, because that changes the procurement path and the calibration responsibility [S3].
Measurement Parameters and Comms That Actually Move the Spec
Advanced electricity meters vary widely in measurement parameters, and the specifier should list what is actually needed: kWh, kVAh, kVARh, demand kW, power factor, harmonic distortion, and per-phase data are common options, but each one adds cost and comms load [S1]. Communication protocols on advanced meters include BACnet, Modbus TCP/RTU, and DNP3 for SCADA, with Ethernet, RS-485, and pulse outputs as the typical physical layers, and the protocol must match the downstream EMS, BMS, or revenue system before the meter is ordered [S1].
For high-value semiconductor and data-center loads, a typical 2026 spec adds 0.2S class accuracy per IEC 62053-22 equivalents, Ethernet and Modbus TCP ports, and on-board data logging; the matching reference write-up for fab duty is covered in the semiconductor electricity meter spec guide, while broader system, class, CT, and comms selection logic is in the how to choose an electricity meter piece. For background on the meter itself, see the electricity meter reference page, and for related plant-side integration see the construction machinery and equipment reference for facility load planning.
Selection Criteria Comparison: Residential vs Light Commercial vs Industrial
For a residential 120/240 V service, the typical pick is a 200 A socket, 200:5 CTs on the service entrance if revenue is metered by CT, and a Class 200 (2%) meter; this covers whole-house loads up to about 16 kW continuous with the standard 125% continuous rule applied [S1][S3]. For light commercial 120/208 V wye or 277/480 V wye, the typical pick is a 320 A self-contained socket with a 0.5% class meter and RS-485 Modbus output for BMS tie-in [S1][S3]. For industrial 480 V delta or 600 V delta with large inductive loads, the typical pick is a 400 A bolt-in socket or a 600 V-class CT cabinet, with ANSI C12.1-2024 0.2% class accuracy, Ethernet and Modbus TCP comms, and per-phase harmonic data [S1][S3].
Who should NOT pick the 320 A self-contained route: any service where the utility forces CT metering above 400 A, or any three-phase service above roughly 400-500 A continuous, because the meter can will become physically unworkable; in that case, move to a 400 A bolt-in base or to CT metering on a dedicated cabinet [S3]. Who should NOT pick CT metering at low current: services below about 100 A continuous, because CT accuracy at the low end drifts and revenue may be undercounted, so stay on a self-contained base even if the service is large in nameplate [S3].
Verification, Standards, and Failure Modes
ANSI C12.1-2024 governs revenue-class meter accuracy in the US, and the specifier should confirm the meter and CT pair are jointly rated to the required class, not just the meter alone [S1]. Field verification should include a clamp-meter check of primary current against the metered secondary at three load points (low, mid, high) and a recorded burden check on the CT secondary loop; readings outside the CT class at the low end are the single most common source of revenue drift [S1][S3].
Two failure modes drive most field complaints: CTs that are too large for the actual load profile, which under-register at the typical operating point, and meter bases undersized for true continuous load, which trip the socket thermal limit and nuisance-fail in summer peaks [S3]. For the facility engineer, the corrective action is the same: re-rate the service entrance to the measured 125% continuous figure, and re-spec the CT to roughly 1.25x the measured peak load, not the nameplate peak [S3]. For an industrial buyer, the next verification node is to confirm the utility's CT threshold in writing before the meter spec is frozen, and to confirm the EMS protocol stack matches the meter's comms ports before purchase order release.
Spec-level background on the components involved: linear guide.