100 Amp Meter Base: Where It Still Fits and Why Upgrades Replace It

A 100 amp meter base is what many older houses still have on the outside wall. It works at its size; the real question is usually whether it survives an upgrade.

Short answer: a 100 amp meter base is a meter socket rated to carry 100 amps continuously, the size that matches a Class 100 meter and a service of up to 100 A. You still find them on older homes, small services and separately metered outbuildings. It cannot carry a 200 A service, so any 100 to 200 A upgrade replaces the meter base, and utilities commonly install 200 A sockets even on new 100 A services.

What is a 100 amp meter base?

It is the enclosure the utility's meter plugs into, rated for 100 A continuously. Some are labeled 100/125 A; the label inside the socket gives the exact ratings. Older meters on these services often read "CL100" on the nameplate, for Class 100, a meter class from the ANSI C12 metering standards.

The socket rating is a ceiling. NEC 230.66 requires a meter socket to be listed and rated for the voltage and ampacity of the service it is on, so a 100 A socket can serve a 100 A service or a smaller one, never a larger one.

The meter belongs to the utility; the socket is normally your equipment, sealed and controlled by the utility, and some utilities furnish the socket for the electrician to install. Where that line sits on your service is in who owns what on your electrical service. The same cover, bypass and entry options exist as on larger sockets, explained in 200 amp meter base.

Where do 100 amp meter bases still appear?

• Older homes whose 100 A service was never upgraded, sometimes behind a newer breaker panel.
• Small services: small houses, cabins, and condo or apartment units with their own meter positions on a shared meter bank.
• Outbuildings with separate meters: a barn, shop, detached garage, well pump or irrigation service that the utility supplies directly instead of through the house. The alternative, a feeder from the house panel, is covered in feeding a detached building.
• Temporary construction power, where the job and the utility set the size.

For a new one-family house, 100 A is also the floor: NEC 230.79(C) requires a service disconnect rated at least 100 A, 3-wire. Smaller services, such as 60 A, survive only on older homes. Most other buildings have a lower floor of 60 A under 230.79(D).

Why do utilities use 200 A sockets on 100 A services?

Because a 200 A socket covers every standard home service up to 200 A, and a socket rated above the service is allowed.

• Meters. Utilities commonly use Class 200 meters on standard residential services, whatever the service size up to 200 A.
• Code. NEC 230.66 only requires the socket to be rated for the service, so a 200 A socket on a 100 A disconnect is simply a 100 A service.
• Later upgrades. With a 200 A socket already in place, a future upgrade still needs new conductors and a new disconnect, but the socket may stay if the utility accepts it.
• Utility minimums. Some utilities set 200 A as the minimum socket for new or replaced residential services.

These are common practices, not universal rules. Check your utility's service requirements before buying any meter equipment.

Upgrading from 100 to 200 amps means a new meter base

A 100 A socket is rated below a 200 A service, so NEC 230.66 rules it out. The upgrade replaces the socket (or swaps it for a meter-main), the service-entrance conductors, and the service disconnect, and it brings a grounding review, a permit, an inspection, and a utility disconnect and reconnect. If the existing socket turns out to be rated 200 A already, it may stay, as long as the utility accepts it. How the replacement itself runs is in meter base replacement.

Whether you need the upgrade at all is a load-calculation question, worked through in 100A vs 200A electrical panel.

To confirm what you have before collecting quotes, see how to tell if you have 100 or 200 amp service.

Because the upgrade replaces service equipment, NEC 230.67 applies: since the 2020 edition, a service supplying a dwelling unit needs a Type 1 or Type 2 surge protective device, including where service equipment is replaced. Ask for it in the quote. Where the new disconnect is an indoor main-breaker panel, a Type 2 unit that mounts beside any brand of load center, such as the Siemens BoltShield FSPD140, is one option; your electrician and inspector decide the type and location.

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The bigger picture of what changes is in service upgrade vs panel replacement.

How are the conductors sized?

By the electrician, from the load calculation under NEC 230.42. For 120/240 V single-phase dwelling services rated 100 to 400 A, NEC 310.12 permits conductors with at least 83% of the service rating when they supply the entire dwelling load.

Illustration only. Assumptions: one set of conductors, 75°C terminations (NEC 110.14(C)), no more than three current-carrying conductors together, and 30°C (86°F) ambient with no correction.

One size down fails each case: 3 AWG aluminum carries 75 A, short of 83 A. Change any assumption and the answer changes, and on an underground service the conductors up to the socket are often the utility's. The full chart is on the 100 amp wire size page. Use it as a sanity check on a quote, not as a spec.

How to tell what you have

From the ground, without opening anything:

• Read the meter nameplate through the glass. "CL100" means a Class 100 meter. "CL200" does not prove the socket is 200 A, because utilities commonly put Class 200 meters on 100 A services.
• Check the main breaker and the cable from the meter. A 100 A main fed by 2 AWG aluminum is the usual 100 A service.
• Ask the utility for the service size on record, or have an electrician read the socket label, which is inside the sealed enclosure.

Do not pull the meter, break the seal, or remove the socket cover. The line side is always energized.

Why is this electrician work?

Everything in the meter base is on the line side: no breaker of yours protects it, and only the utility can de-energize it. Replacing one means utility coordination, a permit, conductors torqued to the manufacturer's specification, and a grounding electrode system checked against the new service. It is licensed, permitted and inspected work.

Standards and code reference

• NEC 110.14(C): termination temperature limits
• NEC 230.2: one service per building, with listed exceptions
• NEC 230.42: service-entrance conductor ampacity from the calculated load
• NEC 230.66: meter sockets listed and rated for the voltage and ampacity of the service
• NEC 230.67: Type 1 or Type 2 surge protective device on services supplying dwelling units, including where service equipment is replaced (2020 NEC onward)
• NEC 230.79(C) and (D): minimum service disconnect ratings: 100 A for one-family dwellings, 60 A for most other installations
• NEC 310.12: 83% allowance for 120/240 V single-phase dwelling services from 100 to 400 A
• NEC Table 310.16: conductor ampacities
• UL 414: meter sockets; ANSI C12 series: utility metering standards, including meter class

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

• 200 amp meter base
• 100A vs 200A electrical panel
• Meter socket and service entrance explained
• Feeding a detached building
• Size the service: home electrical load calculator

More in our electrical panel guides.