CT Meter: How Current-Transformer Metering Works and When You Need It

"CT meter" is what you hear when a service outgrows the ordinary plug-in meter. It looks similar from the yard; what runs through it, and how the bill is worked out, does not.

Short answer: a CT meter (a transformer-rated meter) does not carry your service current. Current transformers around the service conductors feed it a small current in a fixed ratio, such as 800:5, and the utility multiplies the reading back up, by 160 in that example. Utilities switch to CT metering when a service is larger than a self-contained socket meter handles, which for a 120/240 V home is commonly above 400 A. Your utility sets the exact cutoff.

Self-contained vs CT metering

Most homes have a self-contained meter: every amp the house uses passes through the meter's blades and the socket's jaws. That works up to the largest self-contained socket the utility accepts, commonly a 320 amp meter base on a 400 A service. Past that, the service conductors pass through current transformers instead, and only their small secondary current reaches the meter.

Where the cutoff sits

There is no national number; the utility's service requirements handbook sets it.

• 120/240 V residential: CT metering commonly starts above 400 A, once the service outgrows a Class 320 socket.
• Commercial and three-phase: some utilities draw the line lower. Some Western utilities, for example, go to CTs above 200 A on 480 V services.
• Exceptions: a few utilities use larger self-contained equipment instead of CTs for some service sizes.

Ask your utility where its cutoff is before anyone designs the service.

The parts of a CT installation

• CT cabinet (CT enclosure). A utility-sealed enclosure, usually beside the meter or on a pedestal, where the service conductors pass through the transformers. Some utilities call for a single enclosure that houses both the CT section and the meter socket.
• Current transformers. Either window (doughnut) CTs that a conductor passes through, or bar-type CTs with the line and load conductors bolted to each end. There is typically one on each ungrounded (hot) conductor.
• CT-rated meter socket. It looks like an ordinary socket from a distance but is wired with small conductors from the CTs and voltage taps. It is rated for that secondary circuit, commonly 20 A, not for the service.
• Test switch. Switches beside the meter that let the technician short the CT secondaries and open the voltage leads before testing or removing the meter. A CT secondary must never be left open while current flows in the primary, because the open circuit can develop a dangerous voltage. Some sockets short the CTs automatically when the meter is pulled instead.
• Secondary wiring. The run from the CTs to the socket, in its own conduit, wired the way the utility specifies.

Some utilities also specify whether the service disconnect goes ahead of the CTs or after them (cold or hot sequence). That comes from the handbook, not a choice made on site.

How the multiplier works

A CT ratio of 800:5 means 800 A in the service conductor produces 5 A in the secondary. The meter measures the secondary side, and the multiplier scales it back up.

Illustration only. Assumptions: 800:5 CTs, no voltage transformers (normal at 120/240 V), and a meter that displays the secondary-side reading.

• Multiplier: 800 ÷ 5 = 160
• At 400 A on the service conductors, the meter sees 400 ÷ 160 = 2.5 A
• If the register advances 25 kWh between reads, billed usage is 25 × 160 = 4,000 kWh

On services that also use voltage transformers, which is rare on a house, the multiplier is the CT ratio times the voltage-transformer ratio.

Two practical consequences:

• Reading your own meter. Many modern meters are programmed with the ratio, so the display already shows full usage; others show the raw reading and billing applies the multiplier. Ask the utility before you compare numbers.
• Errors get multiplied too. A wrong ratio in billing, a CT installed backwards, or a failed CT skews the bill by the same factor. An odd bill after a CT installation is worth a call to the utility's metering department.

The clamp-on CTs in a home energy monitor use the same principle, but they are your equipment on your side of the meter, and nothing is billed from them.

Who supplies what

Utility-specific, so treat this as a common pattern to confirm:

• Usually the owner's side, through the electrician: the CT cabinet, the CT-rated meter socket, the conduit between them, and the service conductors, chosen from the utility's approved-equipment list.
• Usually the utility's side: the meter and, in most territories, the CTs.
• Either side, depending on the utility: the test switch, the secondary wiring, and who mounts the CTs in the cabinet.

That is the usual ownership line on any service: the enclosures are the owner's, sealed and controlled by the utility, and the meter is the utility's. See who owns what on your electrical service.

When a house ends up with a CT meter

• A calculated load above what 400 A delivers. Large all-electric homes with resistance heat, a pool or spa, a big shop, several EV chargers, or an ADU on the same service. Residential services past 400 A are commonly 600 A or 800 A.
• Several panels. A service that size usually feeds several service disconnects, each supplying a panel. NEC 230.71 sets the rules for up to six disconnects on one service.

Most homes never get close. Check whether 400 A, or even 200 A, already covers the load: see 200-amp vs 400-amp service and run the home electrical load calculator. CT metering also rules out a meter collar adapter, which needs the full service current passing through the meter.

How to tell what you have

From the ground, without opening anything:

• Read the meter class through the glass. CL20 or CL10 means a transformer-rated (CT) meter. CL200 or CL320 means self-contained.
• Look for a second enclosure. A larger gray cabinet near the meter, carrying utility seals and connected to the meter by conduit, is usually the CT cabinet.
• Check the bill. A multiplier other than 1 points to CT metering, although a programmed meter may show 1.
• Ask the utility. It has the meter type and CT ratio on record.

Never open the CT cabinet or the meter socket, and never pull the meter. Both sit on the line side, with no breaker of yours ahead of them, and an opened CT secondary is a hazard of its own.

Why is this electrician work?

The CT cabinet carries the full service current on the line side, where only the utility can de-energize it. The secondary wiring, meter form and test switch follow utility drawings, and the meter is typically set only after the electrical inspection passes. It is licensed, permitted, utility-coordinated work.

Standards and code reference

• NEC 90.2: installations under the exclusive control of an electric utility, including associated metering, are outside the NEC's scope, so the utility's handbook governs the CTs and meter
• NEC 230.82: equipment permitted on the supply side of the service disconnect, including meters, meter sockets and instrument transformers
• NEC 230.71: rules for up to six service disconnecting means on one service
• NEC 230.42: service-entrance conductor ampacity from the calculated load
• NEC 110.3(B): listed equipment installed according to its listing and instructions
• IEEE C57.13: instrument transformers, including CT ratios and accuracy classes
• ANSI C12 series: utility metering standards, including meter class and form

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• 320 amp meter base
• 200-amp vs 400-amp service
• Meter socket and service entrance explained
• Service upgrade vs panel replacement
• Size the service: home electrical load calculator

More in our electrical panel guides.