320 Amp Meter Base: What Class 320 Means and When You Need One

"320 amp meter base" confuses people because nobody sells 320-amp service. The number is a meter rating, and it lines up with 400 amps by design.

Short answer: a 320 amp meter base is a Class 320 meter socket, rated to carry 320 amps continuously and commonly labeled 400 A non-continuous. Because 320 is exactly 80% of 400, it is the socket utilities commonly specify for what is sold as 400-amp residential service, usually feeding two 200 A panels or a meter-main with two 200 A breakers. Your utility decides which socket it will accept, and many require a lever bypass at this size.

What does "Class 320" mean?

"Class" is the meter's continuous current rating. A Class 200 meter and socket are built for up to 200 A continuously; a Class 320 for up to 320 A. Class designations come from the ANSI C12 metering standards that utilities follow, and the socket itself is a listed product built to UL 414.

The same equipment goes by several names: 320 amp meter base, Class 320 meter socket, 320/400 meter socket, or 400 amp meter base. The meter that plugs into it shows its class on the nameplate, typically as "CL320".

Keep the socket rating apart from the service rating, which is set by the service disconnects, the conductors, and what the utility supplies. NEC 230.66 requires the socket to be listed and rated for the voltage and ampacity of the service it is on, but the socket alone does not set how much the house can draw.

Why does Class 320 equal 400 amps?

It comes from the continuous-load rule. A continuous load is one whose maximum current is expected to last three hours or more (NEC Article 100). A standard breaker is sized at 125% of the continuous load, which is the same as loading it to no more than 80% of its rating. NEC 210.20(A) and 215.3 apply this to branch circuits and feeders, and 230.42(A)(1) applies the 125% factor to service-entrance conductors.

Apply it to a 400 A service:

• 400 A × 0.80 = 320 A maximum continuous load
• The other direction: 320 A × 1.25 = 400 A

The same arithmetic holds for the most common setup, two 200 A breakers:

• 200 A × 0.80 = 160 A continuous per breaker
• 160 A × 2 = 320 A

So the socket's continuous rating matches the most continuous current a standard-rated 400 A service can deliver, and it handles up to 400 A for non-continuous peaks. That is why one piece of equipment is called both "320" and "400".

Common Class 320 configurations

These are common, not universal. Your utility and electrician set the actual design.

• Class 320 meter-main with two 200 A breakers. Meter and both main breakers share one outdoor enclosure, and each breaker feeds its own indoor panel. Those panels sit downstream of the service disconnects, so their neutrals are isolated. The trade-offs of that layout are in meter main combo vs separate meter and panel.
• Class 320 socket feeding two 200 A main-breaker panels. The socket has two sets of load-side terminals, and each panel's main breaker serves as a service disconnect.

Two 200 A panels are easier to source than one 400 A panel, and one can sit near the loads it serves, such as an ADU or workshop. The breakers add up to 400 A, but that sum is not the capacity that matters: the calculated load against the service and the socket's 320 A continuous rating is.

What your utility decides

The meter socket is where your equipment meets the utility's, and the utility approves it.

• Approved equipment. Many utilities publish an approved-meter-socket list or a service requirements handbook. A socket that isn't on it may not get a meter set.
• Lever bypass. A lever bypass lets the utility pull the meter for testing or replacement without cutting power to the house. It is commonly required on Class 320 sockets; check your utility.
• Where socket metering stops. For services larger than a self-contained socket meter handles, utilities typically switch to current-transformer (CT) metering with a separate CT cabinet, which is a different installation. Ask the utility where its cutoff is.
• Supply capacity. The transformer and service drop or lateral must support the larger service, on the utility's timeline.

Do you need a Class 320 socket at all?

Only if the load calculation says the house needs more than a 200 A service. Most single-family homes, including many all-electric ones, do not. The full trade-off is in 200-amp vs 400-amp service, and electrical panel sizes explained shows where each service size typically lands. Run your own numbers with the home electrical load calculator.

Moving to a Class 320 socket is a service upgrade, not a panel swap: new socket, new conductors, a grounding review, and utility coordination. See service upgrade vs panel replacement.

The upgrade also brings in surge protection: since the 2020 edition, NEC 230.67 requires a Type 1 or Type 2 surge protective device on every service supplying a dwelling unit, including when service equipment is replaced, so ask the electrician to include one in the upgrade quote. The Leviton P2120-B is a Type 1 unit made for service-entrance installs and meter-adjacent mounting, the fit for a Class 320 meter-main outside.

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Where the service disconnects are indoor main-breaker panels instead, a Type 2 unit that mounts beside any brand of load center, such as the Siemens BoltShield FSPD140, is the alternative. Your electrician and inspector decide which type and location suit the equipment.

How are the service conductors sized?

By the electrician, from the load calculation and the service rating. NEC 230.42 requires service-entrance conductors to carry the calculated load: noncontinuous load plus 125% of continuous load. For 120/240 V single-phase dwelling services rated 100 to 400 A, NEC 310.12 permits conductors with an ampacity of at least 83% of the service rating when they supply the entire dwelling load.

Illustration only. Assumptions: a 400 A single-phase dwelling service, one set of conductors supplying the whole load, 75°C terminations (NEC 110.14(C)), no more than three current-carrying conductors in the raceway, and 30°C (86°F) ambient with no correction.

• 400 A × 0.83 = 332 A minimum ampacity
• Smallest Table 310.16 conductor at 75°C that meets 332 A: 400 kcmil copper (335 A) or 600 kcmil aluminum (340 A)

Change any assumption and the answer changes. Many installations use parallel conductor sets, the utility often owns the conductors up to the meter on an underground service, and the conductors from the socket to each 200 A disconnect supply only part of the load, so they are sized separately. Use the example 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. "CL200" means a Class 200 meter; "CL320" means Class 320.
• Count the main breakers. A meter-main with two large breakers, or two indoor panels each with a 200 A main, usually points to a Class 320 setup.
• Ask the utility. It has your service size and meter class on record.

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 at the meter base is on the line side: no breaker of yours protects it, and only the utility can de-energize it. A Class 320 installation also means utility approval, a permit, heavy conductors torqued to the manufacturer's specification, and a grounding electrode system reviewed for the larger service. It is licensed, permitted, and inspected work.

Standards and code reference

• NEC Article 100 — continuous load: maximum current expected to continue for 3 hours or more
• NEC 210.20(A) and 215.3 — overcurrent devices sized at 125% of continuous load, the source of the 80% relationship
• NEC 230.42 — service-entrance conductor ampacity from the calculated load, continuous load at 125%
• NEC 230.66 — service equipment listing; meter sockets listed and rated for the service voltage and ampacity
• 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 310.12 — 83% allowance for 120/240 V single-phase dwelling services from 100 to 400 A
• NEC Table 310.16 — conductor ampacities
• NEC 110.14(C) — termination temperature limits
• 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

• Meter main combo vs separate meter and panel
• How to do a home electrical load calculation
• Overhead vs underground service
• Meter socket and service entrance explained
• Run the numbers: home electrical load calculator

More in our electrical panel guides.