Revenue Metering: Forms, CT/PT, and Accuracy Class

9 min · difficulty 4/10

A new commercial service pulls about 600 A. Can a self-contained socket meter (typically rated to 200/320 A) take it directly, or does it need instrument transformers? Decide first.

It needs instrument transformers. A self-contained socket meter passes the full service current straight through the meter, and that approach tops out around 200 A class, roughly 320 A continuous. At 600 A you go transformer-rated: you install current transformers (CTs) that scale the line current down to a low standard value, so the meter itself reads a small current and never carries full load. At higher service voltages you add potential transformers (PTs) to scale the voltage down as well. The meter then computes real usage from the scaled-down signals and the known transformer ratios.

Self-contained versus transformer-rated metering Two cards compare how a meter measures service current. The left card, self-contained Form 2S, single-phase, shows the full service current passing as one thick line straight through a gold meter circle marked M from service to load; it serves up to about a 200 A class service, 320 A continuous. The right card, transformer-rated Form 9S or 16S, three-phase, shows the heavy service current passing through two current-transformer toroid rings marked CT, with only a small scaled secondary current running up a thin line to a 5 A class meter; it serves large services such as 600 A. A bottom note states that Class 200 is the CURRENT rating of 200 A while accuracy class 0.2 means within plus or minus 0.2 percent, where a lower number is more accurate. Self-contained vs transformer-rated metering how the meter sees the current: all of it, or a scaled-down copy Self-contained (Form 2S) single-phase service load M full current through meter up to ~200 A class 320 A continuous Transformer-rated (Form 9S / 16S) three-phase service load CT CT M scaled secondary to 5 A-class meter for large services e.g. 600 A Class 200 = CURRENT rating (200 A). Accuracy class 0.2 = within ±0.2% (lower = more accurate).
Small services meter the full current through a self-contained meter, while large services use current transformers to feed a scaled 5 A secondary to a transformer-rated meter.

The meter form is the socket and wiring configuration, and it tells you at a glance which world you are in. Form 2S is a self-contained single-phase meter: the conductors land on the meter’s own current terminals. Forms 9S and 16S are transformer-rated three-phase meters: they expect small CT and PT secondary signals, not the full line current. Order the wrong form and it will not fit the socket or the service.

Two ratings on the nameplate look similar and get mixed up, so keep them apart. The current rating says how much current the meter carries, written like Class 200, which means 200 A. The accuracy class says how close the reading is to true: a meter stamped 0.2 means it stays within plus or minus 0.2 percent. The smaller that number, the tighter the meter. A class 0.2 meter is more accurate than a class 0.5 meter.

That second rating is where money lives. On a small residential service a looser accuracy class is fine, but on a large commercial or industrial account the bill rides on every fraction of a percent, so utilities specify a tight accuracy class and high-grade metering CTs. Current rating tells you how much the meter can carry; accuracy class tells you how much you can trust the bill, and on big services that trust is worth real dollars.

When you spec or receive a meter, read three things in order: the form (does it match the service and the socket), the current path (self-contained or transformer-rated for the service size), and the accuracy class (tight enough for what this account is worth).

Every component on this list, conductor, switchgear, cutouts, regulators, carries its own lead time and supplier landscape. DistroForge report editions track capacity and lead times across the distribution equipment categories covered in this track.

Question 1 of 3

A new commercial service is sized for about 600 A. Which metering approach fits?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.