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inspect electric meters & meter bases for signs of arcing and damage; how to estimate the electrical service size

  • ELECTRIC METERS & METER BASES - CONTENTS: Inspection of Electrical meters or "electric meters" - can electric meters be examined or used to determine the electrical service capacity at a building? How to determine the size, capacity, or ampacity of electrical service at a building. Evidence of arcing and burns at electric meter mounting base. Photos of electrical arcing, burns, and heat damage at electrical meter base connections
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Electric meter inspection: this article discusses the visual inspection of electrical meters & meter bases & explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the service entry cables, electric meter and meter base, electrical service panel, main switch, & other details.

We also discuss electrical arc burns at the meter base.

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ELECTRIC METERS & METER BASES - Inspecting Electric Meters & Meter Bases - Another Consideration When Determining Service Ampacity

Electric meter (C) Daniel FriedmanVisual inspection of the electric meter and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are discussed. Photographs and sketches illustrate electrical panels, meter bases, and electric meters. One of the most frequently asked questions at home inspection conferences and education seminars is "How do I determine the service amperage?"

This question is no surprise, as even a casual inspection of the components of a building electrical service can turn up inconsistencies among service entry cable size, electrical meter rating, electrical panel rating and the main power disonnect ampacity.

Watch out: building inspectors learn quickly enough that just reading off the amperage number on the main power disconnect or5 "main breaker" is not an assurance that we have correctly named the building electrical service ampacity.

And discussion among various experts and one of the original reviewers (Douglas Hansen) quite aptly raised questions about the accuracy of estimating service size in volts or amps by inspecting modern electrical meters. While some of the cases illustrated here remain accurate, these methods may be unreliable when examining modern electrical meters and meter bases.

It's not as difficult as one may think to get a reasonable handle on the electrical service capacity at a building without sophisticated analysis. But there are some pitfalls, and the process itself is dangerous.

The photos and text below will assist in identifying a variety of electrical meters and service panels of various ages and capacities.

An article in the Winter, in 1992 an ASHI Technical Journal produce & edited b DF discussed the procedure and considerations for inspectors to determine the service ampacity of an electrical system. That article indicated that the safe and proper service amperage available at a property is set by the smallest of the service entrance conductor, the main disconnect fuse or circuit breaker, and the rated capacity of the panel itself. These are also issues of concern for the electrical contractor installing the service. ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E. Here information has been updated through March 2014. .

A Broad View of Service Ampacity Determination Must Also Consider Electric Meters & Meter Bases

The concerns of inspectors must be broad in scope. Inspectors must look at the entire system, some of which may be dictated by the electric utility, and may not be a concern of the wiring installer or National Electrical Code. [National Electrical Code, article 90-2(b)(5)]

Click for LARGER PHOTOBecause of this, there is one additional item that must be considered when determining service ampacity. This item is the rating of the electrical meter base and the meter itself. Many times there will be no sure way of determining this rating on a visual inspection, since the rating sticker would typically be inside the meter base and would require pulling the meter off to see it. However, there are some guidelines and "rules of thumb" that can be used.

Electric Meter Bases as Indicators of Electrical Service Ampacity

Warning: Looking at meter bases and meters to guess at ampacity is questioned by some experts as too unreliable.

Systems that have been installed for the last 20 years or so will typically have a rectangular meter base of various sizes as shown in the rectangular-base meter at left. These are usually compatible with 200 Amp services, although undoubtedly there are exceptions to this.

NOTE: Did you notice the measuring tape marking the 3" gap between the siding "J" channel and the top of the meter in our photo shown at above left? This electric meter had been pulled downwards by backfill settlement which pushed down the horizontal portion of the buried conduit bringing the service laterals to the building.

We [DF] considered this a reportable defect as it was possible that the buried conduit was broken and subject to water entry from this condition and since the wires entering the building panel from the meter box have were disturbed.

Obsolete electrical meter (C) Daniel FriedmanHowever M. Cramer points out that most conduit is not continuous, so breaking it is not really an issue aside from damage to conductors underground.

In 1976, an Underwriters Laboratory (UL) ruling required that all new meter bases be "continuous rated for 200 Amps," otherwise only 70% of the actual rating would be considered usable, (i.e., a 200 Amp meter base that was not "continuous rated" could only be used on a 160 Amp or smaller service). Inspectors can probably assume that a rectangular meter base is compatible with the system it is serving, unless a major change or a new panel has been installed.

The meter in our photo at left is obsolete.

Square meter bases such as the unit being pointed to with great caution by an ASHI Headquarters staff member on a field trip in the photo at below-left were normally rated at 100 Amps. Later generations of them were sometimes rated at 125 Amps.

Older electrical systems that were installed 30 or more years ago may be served by a square meter base with a round meter mounted on top of the square base as seen in the next photo at below-center - often called an "A-base" meter.

Still older electrical metering systems used the round meter base shown at below right. Round meter bases originally were rated for 60 Amps when installed 50 or more years ago. (Later generations of round meter bases were rated at 100 Amps.)

These meters can have a NMC main service wire running to and from them, or the wire may be installed in conduit.


[Click any image for a larger, more detailed view.]


The photo just above is an exciting collection of main switch, meter base, round meter above the square base, and a four-fuse "main" panel.

What do you think the ampacity was of this electrical meter, main switch, and fuse panel system?

Electric Meters as Indicators of Service Ampacity

Electric meter (C) Daniel FriedmanThe meter itself is also a consideration when determining the service ampacity of an electrical system.

Most modern electrical meters for single family homes have the designation "CL200" somewhere on their face, which indicates they are rated for up to a 200 Amp service. The CL200 rating may not limit these to 200 amps. Cramer reports finding larger services with CL200 meter bases.

Inspectors will also occasionally find a "CL10" meter which is a transformer-rated meter for large houses with larger electrical systems or two separate main panels.

Some older meters have other designations such as "15 Amps" on their face. This was their test rating. These meters are only usable on systems up to 100 Amps.

Some older meters were also designated as "30 Amps" on their face and these are compatible with 200 Amp services.

For example, one may occasionally find an older house with an upgraded 200 Amp service that still has an old 15 Amp meter plugged into a new meter base.

This meter is not really compatible with the system and should be replaced. Since the responsibility for the meter varies throughout the country, inspectors should contact the local electric utility for their policies and procedures concerning meters.

INSPECTING Electrical METERS & Electric Meter BASES - Inspecting Electric Meters and Electric Meter Bases

Electrical inspectors need to examine the meter and meter base and should take them into account when determining service ampacity. The elements to consider are:

  1. The size/rating of the service entrance conductors.
  2. The rating of the panel.
  3. The rating of the main breaker, if any.
  4. The rating of the meter base.
  5. The rating of the meter.

The weakest or smallest of these five items will determine the service ampacity of the system. This inspection will also determine whether or not all parts of the system are compatible with each other and proper for the installation. There are other good reasons to look at the meter base and meter during your inspection. Occasionally one may discover other meter defects or concerns:

Rusty electric meter base (C) Daniel Friedman

  1. Moisture or water inside the meter face and/or rust on the meter base, such as leaks at the service entry cable or other meter base components. Note the rust at the bottom corners of the electrical meter box in our photo (left).
  2. Missing knockouts on the meter base - openings into the electrical meter base.
  3. Broken or cracked conduit at the electrical meter or meter hubs
  4. Loose connections of the meter base, including meter pulling away from the building wall and loose mounting screws
  5. Inadequate fastening of the meter base and/or mast to the building.
  6. Evidence of tampering at the electrical meter, such as a locking ring seal that has been cut and not replaced by the electrical utility company.
  7. Evidence of burning or arcing at the electric meter connection mounts - see Overheating, Arcing, Corrosion at Electric Meter Mounting Bases below. This condition is unlikely to be visible unless the meter is actually removed from its mounting base.
    Watch out: this step should be performed by a licensed electrician. It is not within the scope of a visual home inspection.

The building inspector's responsibility goes beyond that of the electrical contractor and the local electric utility. Because of this, the inspection of the meter and meter base is an important part of your electrical system inspection, and must be done carefully. This will help to protect your client's interests and will help fully inform them about electrical system defects and the proper service ampacity of the house they are considering to buy.

Beware of Overheating, Arcing, Corrosion at Electric Meter Mounting Bases

Experienced Electrical Worker Warns of Electric Meter Base Arcing & Burns

Our photos (below) show a simple electrical meter base and meter connection mounts where arcing was not present. Field photos of electric meter damage or arcing are wanted. CONTACT us.

Electric meter base no arcing (C) Daniel Friedman Electric meter base no arcing (C) Daniel Friedman

There's one important point about electrical meter sockets I'd like to make. I've been in the electrical trade since 1976, and have seen a lot of interesting situations over the years. One that I feel needs to be addressed is on existing installations, involving arcing at the meter stabs.

I live in California, and So. Cal. Edison loves their "vandal proof" meter locking rings, so in order to do a "proper" service entrance inspection, we have to call a service tech out to unlock the meter ring, they always pitch a "bad time", about coming all the way out to unlock a ring, but it's my reputation and the customer's safety that are both on the line.

More than once, but not really-often I've discovered electrical arcing damage to the electrical meter mounting stabs. Arcing and burning in this location can only be seen by removing the meter. When I find one of these electrical meter mount arc-burn cases, the Edison guy is always amazed.

Not too many sparky's pull the electrical meters to inspect the meter connections, but I recommend it.

And pulling all the breakers off the bus and checking for discoloration or arcing on the bus bars, and breaker clips is a good practice to live by, Especially Zinsco panels (see ZINSCO / SYLVANIA HAZARDS), and especially 30 and 40 amp breakers. - Kirk Schwoebel

Photographs of Arcing Damage & Heat Damage at Connectors in Electric Meter Bases

The electrical meter arcing damage photographs below were provided courtesy of Robert McBride.

At below left the photograph of an electrical meter base shows severe arcing burns at the connecting jaws. At below right you will note (arrow) arc burns at the upper left connector in the photograph. The contributor indicated that this arcing burn occurred because the meter was inserted into the meter base while the system was under load.

Electric meter base no arcing (C) Daniel Friedman Electric meter base no arcing (C) Daniel Friedman

Below the McBride photo of a burned electrical meter base connection shows heat damage at the electric meter mounting base due to loose connections.

Keep in mind that even minor electrical arcing such as shown in the photograph at right can be the beginning of a more serious problem, failure, or electrical fire.

Electric meter base no arcing (C) Daniel FriedmanThat's because electrical resistance created by the flashover burned materials at the connection increases heating and future arcing or burn risks at that same connector.

The articles from which some of this online material originated appeared first in the ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson, and the ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E., with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors

Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.

Also see the US DOE publication "How to Read Residential Electric and Natural Gas Meters".

Reader Question: Special Smart Meter Mount Arcing Problems in Retrofit Electrical Meter Programs?

4 Sept 2015 THOMAS said:
I have seen comments to the effect that meter base jaws are sized to fit analog meters only, and the blades of SmartMeters are thinner and therefore more subject to arcing. This seems very unlikely to me, but it is often seen in blog comments. Is there any truth at all to this?

Reply: risk of arcing at electrical meter mounting bayonets is addressed by inspection & testing; patent disclosure research.

Interesting question, Thomas.

A scholarly search for research found plenty of research citing the properties of smart electrical meters and no research or papers discussing mounting clip problems. One would *think* that for cost reasons if a replacement meter is to mount on an existing meter base the new meter's connectors would be sized to fit the existing clips.

In fact loose electrical meter connections is a concern with ANY electrical meter, particularly if the meter has been pulled and replaced as that process might spread the receiving meter connectors. That this is a known concern for all meters is reflected by patent disclosures such as I cite below.

What I infer from this is that there is probably not a special problem with under-sized smart meter bayonets but there might be occurrences of poor connections in removing and plugging in ANY electrical meter, including smart meters being installed to replace older conventional units.

Electric meter socket jaws tension testing device

Meter bayonet tool for testing tension of electrical meter socket jaw tension - Patent US 20140157907 Johnson 2014

Electric Meters that Run Backwards

Reader Question: Duncan and General Electric 3 phase meters running sometimes forward and sometimes backwards

8/21/14 Anonymous said:

I have a Duncan and General Electric 3 phase meters running sometimes forward and sometimes backwards. These are not smart meters. I've checked voltages, neutrals and connections. Can anyone help answer or have a remedy for these two meters. The other meters fed from the same source are running fine.

Reply: US DOI document on electric meter testing gives details about meter creep and backwards-running electric meters

If you are not sending electrical power backwards into the grid - say from a local energy source - then I suspect the meter is defective OR more likely your meter is experiencing what I quote below: 3-phase, 3-wire, 2-element Meters.(Figure 11)

An inherent characteristic of this metering circuit lends itself to checking by means of shifting connections. On loads having a lagging power factor greater than 50-percent, both elementsrotate forward, whereas on less than 50-percent power factor, one element tends to rotate backward, and will do so if the potential circuit of the other element is disconnected.

A power factor of 50-percent or less is not normally encountered in loaded power circuits. Loads such as an induction motor running with no load or a very light load, or a synchronous motor partly loaded and underexcited, will draw current at less than 50-percent power factor. Also, a generator operated at part load with strong excitation (field) can be made to supply power at less than 50- percent power factor. – “Watt-Hour Meter Maintenance and Testing”, Vol. 3-10, Dec 2000, Facilities Engineering Branch, Denver Office, Denver CO, US Department of the Interior, Bureau of Reclamation, retrieved 8/21/14, original source:

8.3 [Electric Meter] Creep.

The disk of a meter may move, either forward or backward when all load is disconnected. A meter in service is considered to creep when, with all load wires disconnected and test voltage applied, the moving element makes one revolution in 10 minutes or less. (ANSI Standards, C12-1965). Meter disks usually have holes or slots punched in them to stop creep when the holes or slots reach a position directly under the potential coil pole. Observation of creep should, therefore be based on at least one complete revolution.

The document we cite lists eight reasons for this "backwards creep" in some electrical meters

(1) Light-load adjustment used to compensate for instead of removing friction. Examine for friction. If the friction has since disappeared, or is found and eliminated, the meter will be found fast on light load under test, and the creep will disappear
when the meter is adjusted at light load.

(2) Short-circuited turns in potential coil. In this case the meter will be found inaccurat e at low power factor. Replace coil or entire electromagnet.

(3) Vibration. Remove cause of vi- bration. Move meter.

(4) Stray fields either internal or external.

(5) Too high voltage which has the same effect as overcompensation of light-load adjustment.

(6) The potential circuit being connected on the load side of the meter.

(7) Short-circuited turns in current coils. Replace coil or entire electromagnet.

(8) Mechanical disarrangement of the electromagnetic circuit.

A high-resistance short or ground in the customer's circuit can cause a turning of the rotating element which may be mistaken for creeping; therefore, residence wiring should be isolated from the meter when checking for creep.

Although the definition of creep permits one revolution in 10 minutes or less, the serviceman should persevere in eliminating any tendency of a meter to creep. – “Watt-Hour Meter Maintenance and Testing”, Vol. 3-10, Dec 2000, Facilities Engineering Branch, Denver Office, Denver CO, US Department of the Interior, Bureau of Reclamation, retrieved 8/21/14, original source:

Articles on Determination of Ampacity & Voltage at Building Electrical Services


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